Sexual Reproduction<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5807\"]Reproduction[\/pb_glossary]<\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0[pb_glossary id=\"2468\"]Sexual reproduction[\/pb_glossary] <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called [pb_glossary id=\"6023\"]gametes[\/pb_glossary]\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0[pb_glossary id=\"2484\"]haploid[\/pb_glossary]<\/strong>\u00a0(or [pb_glossary id=\"2485\"]1N[\/pb_glossary])\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0[pb_glossary id=\"2486\"]meiosis[\/pb_glossary], which is described in detail below. The process in which two gametes unite is called\u00a0[pb_glossary id=\"6009\"]fertilizat<\/strong>i<\/strong>on<\/strong>[\/pb_glossary]. The fertilized cell that results is referred to as a\u00a0[pb_glossary id=\"2471\"]zygote[\/pb_glossary]<\/strong>. A zygote is a\u00a0[pb_glossary id=\"2103\"]diploid[\/pb_glossary]<\/strong>\u00a0(or [pb_glossary id=\"2103\"]2N[\/pb_glossary]) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.\r\n\r\n[caption id=\"attachment_2488\" align=\"aligncenter\" width=\"555\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Human-Life-Cycle-2.png\")
Figure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em>[\/caption]\r\n\r\n\r\nMeiosis<\/h1>\r\n<\/div>\r\nThe process that produces haploid gametes is called meiosis.\u00a0[pb_glossary id=\"2486\"]Meiosis[\/pb_glossary]<\/strong> is a type of cell division in which the number of [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary] is reduced by half. It occurs only in certain special cells of an organism. During meiosis, [pb_glossary id=\"5609\"]homologous (paired) chromosomes[\/pb_glossary] separate, and four [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.\r\n\r\n[caption id=\"attachment_2491\" align=\"aligncenter\" width=\"587\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/MajorEventsInMeiosis_variant_int-2.png\")
Figure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\nAs you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0[pb_glossary id=\"5559\"]DNA\u00a0replicates[\/pb_glossary] during [pb_glossary id=\"1941\"]interphase[\/pb_glossary]. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.\r\nMeiosis I- Increasing genetic variation<\/h2>\r\nThe phases of Meiosis I are:\r\n\r\n \t- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\r\n \t
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\r\n \t
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\r\n \t
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_2493\" align=\"aligncenter\" width=\"1052\"]
![\"Illustrates](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Meiosis-1-2.png\")
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em>[\/caption]\r\nMeiosis II- Halfing the DNA<\/h2>\r\nThe phases of Meiosis II are:\r\n\r\n \t- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\r\n \t
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\r\n \t
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\r\n \t
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\r\n<\/ol>\r\n\r\n\r\n\r\n[caption id=\"attachment_2498\" align=\"aligncenter\" width=\"1000\"]
![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Meiosis-2-1-2.png\")
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em>[\/caption]\r\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\r\n<\/div>\r\n\"It takes two to tango\" might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:\r\n\r\n[caption id=\"attachment_2501\" align=\"alignright\" width=\"518\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Corssing-Over-by-OpenStax-2.jpg\")
Figure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em>[\/caption]\r\n\r\n \t- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0[pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary] <\/strong>is the exchange of genetic material between non-sister chromatids of [pb_glossary id=\"5609\"]homologous chromosomes[\/pb_glossary]. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\r\n \t
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary]<\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\r\n \t
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it's obviously another source of genetic variation in offspring.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2503\" align=\"alignleft\" width=\"512\"]
![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Independent_assortment.svg_-2.png\")
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em>[\/caption]\r\n\r\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong>\r\n\r\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VzDMG7ke69g\r\nMeiosis (updated), Amoeba Sisters, 2017.<\/p>\r\n\r\n
Gametogenesis<\/h1>\r\n<\/div>\r\nAt the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of\u00a0fertilization. The\u00a0development\u00a0of haploid cells into gametes is called\u00a0gametogenesis<\/strong>. It\u00a0differs between males and females.\r\n\r\n \t- A gamete produced by a male is called a\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]<\/strong>,<\/strong>\u00a0and the process that produces a mature\u00a0sperm\u00a0is\u00a0called\u00a0[pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary]<\/strong>. During this process, a sperm cell grows a tail and gains the ability to \u201cswim,\u201d like the human sperm cell shown in Figure 5.12.8.<\/li>\r\n \t
- A gamete produced by a female is called an[pb_glossary id=\"5849\"]egg or ovum,<\/strong>[\/pb_glossary]\u00a0and the process that produces a mature egg is called\u00a0[pb_glossary id=\"2509\"]oogenesis[\/pb_glossary]<\/strong>, during which just one functional egg is produced. The other three haploid cells that result from meiosis are called polar bodies, and they disintegrate. The single egg is a very large cell, as you can see from the human egg also shown in Figure 5.12.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2510\" align=\"aligncenter\" width=\"391\"]
![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Fertilization-2.jpg\")
Figure 5.12.8 A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph?<\/em>[\/caption]\r\n\r\n\r\n\r\n5.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n<\/p>","rendered":"
<\/p>\n![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/loly-galina-JrHhKeQBM9A-unsplash-scaled-3.jpg\")
Figure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\nAll in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\nFigure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n\nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\nFigure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\nMeiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
\n- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\nMeiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
\n- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n\n\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\nFigure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n\n- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
Meiosis<\/h1>\r\n<\/div>\r\nThe process that produces haploid gametes is called meiosis.\u00a0[pb_glossary id=\"2486\"]Meiosis[\/pb_glossary]<\/strong> is a type of cell division in which the number of [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary] is reduced by half. It occurs only in certain special cells of an organism. During meiosis, [pb_glossary id=\"5609\"]homologous (paired) chromosomes[\/pb_glossary] separate, and four [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.\r\n\r\n[caption id=\"attachment_2491\" align=\"aligncenter\" width=\"587\"] Figure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em>[\/caption]\r\n\r\n\r\n\r\n \r\n\r\n<\/div>\r\nAs you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0[pb_glossary id=\"5559\"]DNA\u00a0replicates[\/pb_glossary] during [pb_glossary id=\"1941\"]interphase[\/pb_glossary]. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.\r\nMeiosis I- Increasing genetic variation<\/h2>\r\nThe phases of Meiosis I are:\r\n\r\n \t- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\r\n \t
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\r\n \t
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\r\n \t
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_2493\" align=\"aligncenter\" width=\"1052\"] Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em>[\/caption]\r\n
Meiosis II- Halfing the DNA<\/h2>\r\nThe phases of Meiosis II are:\r\n\r\n \t- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\r\n \t
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\r\n \t
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\r\n \t
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\r\n<\/ol>\r\n\r\n\r\n\r\n[caption id=\"attachment_2498\" align=\"aligncenter\" width=\"1000\"] Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em>[\/caption]\r\n
Sexual Reproduction and\u00a0Genetic Variation<\/h1>\r\n<\/div>\r\n\"It takes two to tango\" might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:\r\n\r\n[caption id=\"attachment_2501\" align=\"alignright\" width=\"518\"] Figure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em>[\/caption]\r\n\r\n \t- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0[pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary] <\/strong>is the exchange of genetic material between non-sister chromatids of [pb_glossary id=\"5609\"]homologous chromosomes[\/pb_glossary]. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\r\n \t
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary]<\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\r\n \t
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it's obviously another source of genetic variation in offspring.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2503\" align=\"alignleft\" width=\"512\"] Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em>[\/caption]\r\n\r\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong>\r\n\r\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VzDMG7ke69g\r\n
Meiosis (updated), Amoeba Sisters, 2017.<\/p>\r\n\r\n
Gametogenesis<\/h1>\r\n<\/div>\r\nAt the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of\u00a0fertilization. The\u00a0development\u00a0of haploid cells into gametes is called\u00a0gametogenesis<\/strong>. It\u00a0differs between males and females.\r\n\r\n \t- A gamete produced by a male is called a\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]<\/strong>,<\/strong>\u00a0and the process that produces a mature\u00a0sperm\u00a0is\u00a0called\u00a0[pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary]<\/strong>. During this process, a sperm cell grows a tail and gains the ability to \u201cswim,\u201d like the human sperm cell shown in Figure 5.12.8.<\/li>\r\n \t
- A gamete produced by a female is called an[pb_glossary id=\"5849\"]egg or ovum,<\/strong>[\/pb_glossary]\u00a0and the process that produces a mature egg is called\u00a0[pb_glossary id=\"2509\"]oogenesis[\/pb_glossary]<\/strong>, during which just one functional egg is produced. The other three haploid cells that result from meiosis are called polar bodies, and they disintegrate. The single egg is a very large cell, as you can see from the human egg also shown in Figure 5.12.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2510\" align=\"aligncenter\" width=\"391\"] Figure 5.12.8 A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph?<\/em>[\/caption]\r\n\r\n\r\n
\r\n5.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n<\/p>","rendered":"
<\/p>\nFigure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\nAll in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\nFigure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n\nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\nFigure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\nMeiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
\n- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\nMeiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
\n- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n\n\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\nFigure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n\n- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
Meiosis I- Increasing genetic variation<\/h2>\r\nThe phases of Meiosis I are:\r\n\r\n \t- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\r\n \t
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\r\n \t
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\r\n \t
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_2493\" align=\"aligncenter\" width=\"1052\"] Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em>[\/caption]\r\n
Meiosis II- Halfing the DNA<\/h2>\r\nThe phases of Meiosis II are:\r\n\r\n \t- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\r\n \t
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\r\n \t
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\r\n \t
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\r\n<\/ol>\r\n\r\n\r\n\r\n[caption id=\"attachment_2498\" align=\"aligncenter\" width=\"1000\"] Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em>[\/caption]\r\n
Sexual Reproduction and\u00a0Genetic Variation<\/h1>\r\n<\/div>\r\n\"It takes two to tango\" might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:\r\n\r\n[caption id=\"attachment_2501\" align=\"alignright\" width=\"518\"] Figure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em>[\/caption]\r\n\r\n \t- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0[pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary] <\/strong>is the exchange of genetic material between non-sister chromatids of [pb_glossary id=\"5609\"]homologous chromosomes[\/pb_glossary]. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\r\n \t
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary]<\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\r\n \t
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it's obviously another source of genetic variation in offspring.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2503\" align=\"alignleft\" width=\"512\"] Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em>[\/caption]\r\n\r\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong>\r\n\r\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VzDMG7ke69g\r\n
Meiosis (updated), Amoeba Sisters, 2017.<\/p>\r\n\r\n
Gametogenesis<\/h1>\r\n<\/div>\r\nAt the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of\u00a0fertilization. The\u00a0development\u00a0of haploid cells into gametes is called\u00a0gametogenesis<\/strong>. It\u00a0differs between males and females.\r\n\r\n \t- A gamete produced by a male is called a\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]<\/strong>,<\/strong>\u00a0and the process that produces a mature\u00a0sperm\u00a0is\u00a0called\u00a0[pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary]<\/strong>. During this process, a sperm cell grows a tail and gains the ability to \u201cswim,\u201d like the human sperm cell shown in Figure 5.12.8.<\/li>\r\n \t
- A gamete produced by a female is called an[pb_glossary id=\"5849\"]egg or ovum,<\/strong>[\/pb_glossary]\u00a0and the process that produces a mature egg is called\u00a0[pb_glossary id=\"2509\"]oogenesis[\/pb_glossary]<\/strong>, during which just one functional egg is produced. The other three haploid cells that result from meiosis are called polar bodies, and they disintegrate. The single egg is a very large cell, as you can see from the human egg also shown in Figure 5.12.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2510\" align=\"aligncenter\" width=\"391\"] Figure 5.12.8 A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph?<\/em>[\/caption]\r\n\r\n\r\n
\r\n5.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n<\/p>","rendered":"
<\/p>\nFigure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\nAll in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\nFigure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n\nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\nFigure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\nMeiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
\n- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\nMeiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
\n- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n\n\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\nFigure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n\n- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em>[\/caption]\r\nMeiosis II- Halfing the DNA<\/h2>\r\nThe phases of Meiosis II are:\r\n\r\n \t- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\r\n \t
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\r\n \t
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\r\n \t
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\r\n<\/ol>\r\n\r\n\r\n\r\n[caption id=\"attachment_2498\" align=\"aligncenter\" width=\"1000\"] Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em>[\/caption]\r\n
Sexual Reproduction and\u00a0Genetic Variation<\/h1>\r\n<\/div>\r\n\"It takes two to tango\" might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:\r\n\r\n[caption id=\"attachment_2501\" align=\"alignright\" width=\"518\"] Figure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em>[\/caption]\r\n\r\n \t- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0[pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary] <\/strong>is the exchange of genetic material between non-sister chromatids of [pb_glossary id=\"5609\"]homologous chromosomes[\/pb_glossary]. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\r\n \t
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary]<\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\r\n \t
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it's obviously another source of genetic variation in offspring.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2503\" align=\"alignleft\" width=\"512\"] Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em>[\/caption]\r\n\r\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong>\r\n\r\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VzDMG7ke69g\r\n
Meiosis (updated), Amoeba Sisters, 2017.<\/p>\r\n\r\n
Gametogenesis<\/h1>\r\n<\/div>\r\nAt the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of\u00a0fertilization. The\u00a0development\u00a0of haploid cells into gametes is called\u00a0gametogenesis<\/strong>. It\u00a0differs between males and females.\r\n\r\n \t- A gamete produced by a male is called a\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]<\/strong>,<\/strong>\u00a0and the process that produces a mature\u00a0sperm\u00a0is\u00a0called\u00a0[pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary]<\/strong>. During this process, a sperm cell grows a tail and gains the ability to \u201cswim,\u201d like the human sperm cell shown in Figure 5.12.8.<\/li>\r\n \t
- A gamete produced by a female is called an[pb_glossary id=\"5849\"]egg or ovum,<\/strong>[\/pb_glossary]\u00a0and the process that produces a mature egg is called\u00a0[pb_glossary id=\"2509\"]oogenesis[\/pb_glossary]<\/strong>, during which just one functional egg is produced. The other three haploid cells that result from meiosis are called polar bodies, and they disintegrate. The single egg is a very large cell, as you can see from the human egg also shown in Figure 5.12.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2510\" align=\"aligncenter\" width=\"391\"] Figure 5.12.8 A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph?<\/em>[\/caption]\r\n\r\n\r\n
\r\n5.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n<\/p>","rendered":"
<\/p>\nFigure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\nAll in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\nFigure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n\nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\nFigure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\nMeiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
\n- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\nMeiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
\n- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n\n\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\nFigure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n\n- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em>[\/caption]\r\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\r\n<\/div>\r\n\"It takes two to tango\" might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:\r\n\r\n[caption id=\"attachment_2501\" align=\"alignright\" width=\"518\"] Figure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em>[\/caption]\r\n\r\n \t- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0[pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary] <\/strong>is the exchange of genetic material between non-sister chromatids of [pb_glossary id=\"5609\"]homologous chromosomes[\/pb_glossary]. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\r\n \t
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary]<\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\r\n \t
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it's obviously another source of genetic variation in offspring.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2503\" align=\"alignleft\" width=\"512\"] Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em>[\/caption]\r\n\r\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong>\r\n\r\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VzDMG7ke69g\r\n
Meiosis (updated), Amoeba Sisters, 2017.<\/p>\r\n\r\n
Gametogenesis<\/h1>\r\n<\/div>\r\nAt the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of\u00a0fertilization. The\u00a0development\u00a0of haploid cells into gametes is called\u00a0gametogenesis<\/strong>. It\u00a0differs between males and females.\r\n\r\n \t- A gamete produced by a male is called a\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]<\/strong>,<\/strong>\u00a0and the process that produces a mature\u00a0sperm\u00a0is\u00a0called\u00a0[pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary]<\/strong>. During this process, a sperm cell grows a tail and gains the ability to \u201cswim,\u201d like the human sperm cell shown in Figure 5.12.8.<\/li>\r\n \t
- A gamete produced by a female is called an[pb_glossary id=\"5849\"]egg or ovum,<\/strong>[\/pb_glossary]\u00a0and the process that produces a mature egg is called\u00a0[pb_glossary id=\"2509\"]oogenesis[\/pb_glossary]<\/strong>, during which just one functional egg is produced. The other three haploid cells that result from meiosis are called polar bodies, and they disintegrate. The single egg is a very large cell, as you can see from the human egg also shown in Figure 5.12.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2510\" align=\"aligncenter\" width=\"391\"] Figure 5.12.8 A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph?<\/em>[\/caption]\r\n\r\n\r\n
\r\n5.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n<\/p>","rendered":"
<\/p>\nFigure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\nAll in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\nFigure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n\nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\nFigure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\nMeiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
\n- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\nMeiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
\n- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n\n\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\nFigure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n\n- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em>[\/caption]\r\n\r\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong>\r\n\r\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=VzDMG7ke69g\r\nMeiosis (updated), Amoeba Sisters, 2017.<\/p>\r\n\r\n
Gametogenesis<\/h1>\r\n<\/div>\r\nAt the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of\u00a0fertilization. The\u00a0development\u00a0of haploid cells into gametes is called\u00a0gametogenesis<\/strong>. It\u00a0differs between males and females.\r\n\r\n \t- A gamete produced by a male is called a\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]<\/strong>,<\/strong>\u00a0and the process that produces a mature\u00a0sperm\u00a0is\u00a0called\u00a0[pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary]<\/strong>. During this process, a sperm cell grows a tail and gains the ability to \u201cswim,\u201d like the human sperm cell shown in Figure 5.12.8.<\/li>\r\n \t
- A gamete produced by a female is called an[pb_glossary id=\"5849\"]egg or ovum,<\/strong>[\/pb_glossary]\u00a0and the process that produces a mature egg is called\u00a0[pb_glossary id=\"2509\"]oogenesis[\/pb_glossary]<\/strong>, during which just one functional egg is produced. The other three haploid cells that result from meiosis are called polar bodies, and they disintegrate. The single egg is a very large cell, as you can see from the human egg also shown in Figure 5.12.8.<\/li>\r\n<\/ul>\r\n[caption id=\"attachment_2510\" align=\"aligncenter\" width=\"391\"] Figure 5.12.8 A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph?<\/em>[\/caption]\r\n\r\n\r\n
\r\n5.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n<\/p>","rendered":"
<\/p>\nFigure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\nAll in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\nFigure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n\nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\nFigure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\nMeiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
\n- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\nMeiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
\n- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n\n\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\nFigure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n\n- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
Figure 5.12.8 A human sperm is a tiny cell with a tail. A human egg is much larger. Both cells are mature haploid gametes that are capable of fertilization. What process is shown in this photograph?<\/em>[\/caption]\r\n\r\n5.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n<\/p>","rendered":"
<\/p>\nFigure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\nAll in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\nFigure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n\nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\nFigure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n\n <\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\nMeiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
\n- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\nMeiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
\n- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n\n\n
Figure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\nSexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\nFigure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n\n- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\nWith all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\nAll of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
- \r\n \t
- In [pb_glossary id=\"2468\"]sexual reproduction[\/pb_glossary], two parents produce [pb_glossary id=\"6023\"]gametes[\/pb_glossary] that unite in the process of\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0to form a single-celled [pb_glossary id=\"2471\"]zygote[\/pb_glossary]. Gametes are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] cells with one copy of each of the 23 chromosomes, and the zygote is a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] cell with two\u00a0copies of each of the 23 chromosomes.<\/li>\r\n \t
- [pb_glossary id=\"2486\"]Meiosis[\/pb_glossary] is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).<\/li>\r\n \t
- Meiosis is followed by\u00a0[pb_glossary id=\"6025\"]gametogenesis[\/pb_glossary], the process during which the haploid daughter cells change into mature gametes. Males produce gametes called [pb_glossary id=\"2506\"]sperm[\/pb_glossary] in a process known as [pb_glossary id=\"2507\"]spermatogenesis[\/pb_glossary], and females produce gametes called [pb_glossary id=\"5849\"]eggs[\/pb_glossary] in the process known as [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/li>\r\n \t
- Sexual reproduction produces genetically unique offspring. [pb_glossary id=\"2500\"]Crossing-over[\/pb_glossary], [pb_glossary id=\"2502\"]independent alignment[\/pb_glossary], and the random union of gametes work together to result in an amazing range of potential genetic variation.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 5.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- [h5p id=\"506\"]<\/li>\r\n \t
- Explain how sexual reproduction happens\u00a0at the cellular level.<\/li>\r\n \t
- Summarize what happens during Meiosis.<\/li>\r\n \t
- Compare and contrast gametogenesis in males and females.<\/li>\r\n \t
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction.<\/li>\r\n \t
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?<\/li>\r\n \t
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis.<\/li>\r\n \t
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 5.12 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=emb_logo\r\nMeiosis: Where the Sex Starts - Crash Course Biology #13, CrashCourse, 2012.<\/p>\r\nhttps:\/\/www.youtube.com\/watch?v=zrKdz93WlVk\r\n
Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\nAttributions<\/span>\r\n\r\n<\/div>\r\nFigure 5.12.1<\/strong>\r\n\r\nFamily portrait<\/a> by loly galina<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 5.12.2<\/strong>\r\n\r\nHuman Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc-sa\/4.0\/) license.\r\n\r\nFigure 5.12.3<\/strong>\r\n\r\nMajorEventsInMeiosis_variant_int<\/a> by Patr\u00edciaR<\/a>\u00a0(internationalization) on Wikimedia Commons is used and <\/span>adapted by Christine Miller. This image<\/span>\u00a0in the <\/span>public domain.<\/a> (Original image from NCBI<\/a>; original vector version by Jakov.<\/a>)<\/span>\r\n\r\nFigure 5.12.4<\/strong>\r\n\r\nMeiosis 1\/ Meiosis Stages<\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.5<\/strong>\r\n\r\nMeiosis 2\/ Meiosis Stages <\/a> by Ali Zifan<\/a><\/span> on Wikimedia Commons is used and adapted by Christine Miller under a\u00a0 CC BY-SA 4.0<\/span><\/a>\u00a0 (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.<\/span>\r\n\r\nFigure 5.12.6<\/strong>\r\n\r\nCrossover\/ Figure 17 02 01<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 5.12.7<\/strong>\r\n\r\nIndependent_assortment<\/a> by Mtian20<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 5.12.8<\/strong>\r\n\r\nsperm fertilizing egg<\/a> by AndreaLaurel<\/a> on Flickr<\/a> is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0\/) license.\r\n
References<\/h2>\r\n
Amoeba Sisters. (2017, July 11). Meiosis (updated). YouTube. https:\/\/www.youtube.com\/watch?v=VzDMG7ke69g&feature=youtu.be<\/p>\r\n
Amoeba Sisters. (2018, May 31). Mitosis vs meiosis comparison. YouTube. https:\/\/www.youtube.com\/watch?v=zrKdz93WlVk&feature=youtu.be<\/p>\r\n
CrashCourse, (2012, April 23). Meiosis: Where the sex starts - Crash Course Biology #13. YouTube. https:\/\/www.youtube.com\/watch?v=qCLmR9-YY7o&feature=youtu.be<\/p>\r\n
OpenStax CNX. (2016, May 27). Figure 1 Crossover may occur at different locations on the chromosome. <\/span>In <\/span>OpenStax, Biology <\/em>(Section 17.2). http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.<\/span><\/p>\r\n
<\/p>","rendered":"
<\/p>\n
Figure 5.12.1 Family resemblance. <\/em><\/figcaption><\/figure>\n All in the Family<\/h1>\n
This family photo (Figure 5.12.1) clearly illustrates an important point: children in a family resemble their parents and each other, but the children never look exactly the same, unless they are identical twins. Each of the daughters in the photo have inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex \u2014 sexual reproduction, that is.<\/p>\n
\nSexual Reproduction<\/h1>\n<\/div>\n
Reproduction<\/a><\/strong>\u00a0is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. Like many other organisms, human beings reproduce sexually.\u00a0Sexual reproduction<\/a> <\/strong>involves two parents. As you can see from Figure 5.12.2, in sexual reproduction, parents produce reproductive (sex) cells \u2014 called gametes<\/a>\u00a0<\/strong>\u2014 that unite to form an offspring. Gametes are\u00a0haploid<\/a><\/strong>\u00a0(or 1N<\/a>)\u00a0cells.\u00a0This means they contain one copy of each chromosome in the\u00a0nucleus.\u00a0Gametes are produced by a type of\u00a0cell division\u00a0called\u00a0meiosis<\/a>, which is described in detail below. The process in which two gametes unite is called\u00a0fertilizat<\/strong>i<\/strong>on<\/strong><\/a>. The fertilized cell that results is referred to as a\u00a0zygote<\/a><\/strong>. A zygote is a\u00a0diploid<\/a><\/strong>\u00a0(or 2N<\/a>) cell,\u00a0which means it contains two copies of each chromosome. Thus, it has twice the number of\u00a0chromosomes\u00a0as a gamete.<\/p>\n
Figure 5.12.2 Sexual reproduction involves the production of haploid gametes by meiosis, followed by fertilization and the formation of a diploid zygote. The number of chromosomes in a gamete is represented by the letter N. Why does the zygote have 2N, or twice as many, chromosomes?<\/em><\/figcaption><\/figure>\n \nMeiosis<\/h1>\n<\/div>\n
The process that produces haploid gametes is called meiosis.\u00a0Meiosis<\/a><\/strong> is a type of cell division in which the number of chromosomes<\/a> is reduced by half. It occurs only in certain special cells of an organism. During meiosis, homologous (paired) chromosomes<\/a> separate, and four haploid<\/a> cells form that have only one chromosome from each pair. The diagram (Figure 5.12.3) gives an overview of meiosis.<\/p>\n
Figure 5.12.3 Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells. Notice the exchange of genetic material that occurs prior to the first cell division.<\/em><\/figcaption><\/figure>\n \n<\/p>\n<\/div>\n
As you can see in\u00a0the meiosis diagram, two cell divisions occur during the overall process,\u00a0producing\u00a0a total of four haploid<\/a> cells from one parent cell. The two cell divisions are called meiosis I and meiosis II.\u00a0Meiosis I begins after\u00a0DNA\u00a0replicates<\/a> during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from\u00a0mitosis, the division of the\u00a0nucleus\u00a0that takes place during routine\u00a0cell division\u00a0of eukaryotic cells.<\/p>\n
Meiosis I- Increasing genetic variation<\/h2>\n
The phases of Meiosis I are:<\/p>\n
- \n
- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Anaphase I:<\/strong> Spindle fibres shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.<\/li>\n
- Telophase I and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.<\/li>\n<\/ol>\n
Figure 5.12.4 Meiosis I is critical in creating genetic diversity in resulting gametes. Crossing over, in Prophase I and independent alignment in Metaphase I ensure that each resulting gamete is unique.<\/em><\/figcaption><\/figure>\n Meiosis II- Halfing the DNA<\/h2>\n
The phases of Meiosis II are:<\/p>\n
- \n
- Prophase II:<\/strong>\u00a0The nuclear envelope breaks down, and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Anaphase II:<\/strong>\u00a0Sister chromatids separate and move to opposite poles.<\/li>\n
- Telophase II and Cytokinesis:<\/strong>\u00a0The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.<\/li>\n<\/ol>\n
\n\nFigure 5.12.5 In Meiosis II, dyads are separated to create four unique haploid cells.<\/em><\/figcaption><\/figure>\n Sexual Reproduction and\u00a0Genetic Variation<\/h1>\n<\/div>\n
“It takes two to tango” might be a euphemism for sexual reproduction. Requiring\u00a0two<\/em>\u00a0individuals to produce offspring, however, is also the main drawback of this way of reproducing, because it requires extra steps \u2014 and often a certain amount of luck \u2014 to successfully reproduce with a partner. On the other hand, sexual reproduction greatly increases the potential for\u00a0genetic variation\u00a0in offspring,\u00a0which\u00a0increases the likelihood that the resulting offspring will have genetic advantages. In fact, each offspring produced is almost guaranteed to be genetically unique, differing from both parents and from any other offspring. Sexual reproduction increases genetic variation in a number of ways:<\/p>\n
Figure 5.12.6 Crossing over results in exchange of sections of DNA between homologous pairs of chromosomes.<\/em><\/figcaption><\/figure>\n - \n
- When homologous chromosomes pair up during meiosis I, crossing-over can occur.\u00a0Crossing-over<\/a> <\/strong>is the exchange of genetic material between non-sister chromatids of homologous chromosomes<\/a>. It results in new combinations of genes on each chromosome. This is called recombination. You can see how it happens in the figure to the right.<\/li>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is a matter of chance, and it’s obviously another source of genetic variation in offspring.<\/li>\n<\/ul>\n
Figure 5.12.7 Independent alignment greatly increases the genetic diversity among gametes produced.\u00a0 Depending on how the homologous pairs align (with paternal or maternal DNA on the left or right side) determines which mix of genes will end up in each of the four unique haploid gametes produced.<\/em><\/figcaption><\/figure>\n With all of this recombination of genes, there is a need for a new set of vocabulary.\u00a0 Remember, that sister chromatids are two\u00a0identical<\/em> pieces of DNA connected at a centromere.\u00a0 Once crossing over has occured, we can no longer call them sister chromatids since they are no longer identical; we term them dyads.\u00a0 <\/strong>In addition, once crossing over has occurred, the pair of homologous chromosomes can be referred to as\u00a0tetrads.\u00a0\u00a0<\/strong><\/p>\n
All of these mechanisms\u00a0\u2014\u00a0crossing over, independent assortment, and the random union of gametes\u00a0\u2014\u00a0work together to result in an amazing\u00a0range\u00a0of potential genetic variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!<\/p>\n
- When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent alignment<\/a><\/strong>. It results in gametes that have unique combinations of chromosomes.\u00a0 You can see how it happens in Figure 5.12.7.<\/li>\n
- Metaphase II:<\/strong> Spindle fibres line up the sister chromatids of each chromosome along the equator of the cell.<\/li>\n
- Metaphase I:<\/strong> Spindle fibres attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell, randomly aligning in a process called independent alignment.\u00a0 The significance of independent alignment is discussed below. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.<\/li>\n
- Prophase I:<\/strong>\u00a0The nuclear envelope begins to break down, and the\u00a0chromosomes\u00a0condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of\u00a0mitosis\u00a0and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed below.<\/li>\n
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