{"id":4501,"date":"2019-06-24T12:46:30","date_gmt":"2019-06-24T12:46:30","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/4-13-mitosis-and-cytokinesis-3\/"},"modified":"2023-11-30T17:55:23","modified_gmt":"2023-11-30T17:55:23","slug":"4-13-mitosis-and-cytokinesis-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/4-13-mitosis-and-cytokinesis-3\/","title":{"raw":"4.13\u00a0Mitosis and Cytokinesis","rendered":"4.13\u00a0Mitosis and Cytokinesis"},"content":{"raw":" \r\n

Divide and Split<\/h1>\r\n[caption id=\"attachment_1996\" align=\"alignright\" width=\"400\"]\"Image Figure 4.13.1 A cell in anaphase of mitosis. <\/em>[\/caption]\r\n\r\nCan you guess what the colourful image in Figure 4.13.1 represents? It shows a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell during the process of [pb_glossary id=\"5633\"]cell division[\/pb_glossary]. In particular, the image shows the cell in a part of cell division called anaphase, where the [pb_glossary id=\"277\"]DNA[\/pb_glossary] is being pulled to opposite ends of the cell. Normally, DNA is located in the [pb_glossary id=\"5797\"]nucleus[\/pb_glossary] of most human cells. The nucleus divides before the cell itself splits in two, and before the nucleus divides, the cell\u2019s DNA is replicated (or copied). There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To answer that question, you first need to know more about DNA and the forms it takes.\r\n
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The Forms of\u00a0DNA<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_2000\" align=\"alignright\" width=\"380\"]\"Diagram Figure 4.13.2 Forms of DNA. <\/em>[\/caption]\r\n\r\nExcept when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called\u00a0[pb_glossary id=\"1997\"]chromatin<\/strong>[\/pb_glossary].\u00a0Only once a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a\u00a0[pb_glossary id=\"5619\"]chromosome<\/strong>[\/pb_glossary],<\/strong>\u00a0like the one shown\u00a0below.\r\n\r\n[caption id=\"attachment_2001\" align=\"alignleft\" width=\"344\"]\"Labelled Figure 4.13.3 Diagram of a chromosome showing that in a chromosome with the typical \"X\" shape, it is comprised of two identical pieces of DNA, each called a chromatid.<\/em>[\/caption]\r\n\r\nMost\u00a0cells\u00a0in the\u00a0human body\u00a0have\u00a0two\u00a0pairs of 23 different\u00a0chromosomes, for a total of 46 chromosomes.\u00a0Cells\u00a0that have two\u00a0pairs of\u00a0chromosomes\u00a0are called diploid.\u00a0Because DNA has already replicated when it coils into a chromosome, each chromosome actually consists of two identical\u00a0structures called\u00a0[pb_glossary id=\"1999\"]sister<\/strong>\u00a0chromatids<\/strong>[\/pb_glossary].<\/strong>\u00a0Sister chromatids are joined together at a region called a [pb_glossary id=\"2030\"]centromere[\/pb_glossary].\r\n\r\n \r\n
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Mitosis<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_2003\" align=\"alignright\" width=\"414\"]\"Diagram Figure 4.13.4 Mitosis is the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells. What happens during mitosis?<\/em>[\/caption]\r\n\r\nThe process in which the\u00a0nucleus\u00a0of a eukaryotic cell divides is called\u00a0mitosis.<\/strong>\u00a0During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. This is shown in the figure\u00a0below.\r\n\r\nMitosis actually occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase.\r\n

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Prophase<\/h2>\r\n[caption id=\"attachment_2009\" align=\"alignleft\" width=\"75\"]\"\" Figure 4.13.5 Mitotic prophase.<\/em>[\/caption]\r\n\r\nThe first and longest phase of mitosis is\u00a0[pb_glossary id=\"2006\"]prophase<\/strong>[\/pb_glossary]. During prophase, [pb_glossary id=\"1997\"]chromatin[\/pb_glossary] condenses into\u00a0[pb_glossary id=\"5619\"]chromosomes[\/pb_glossary], and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the [pb_glossary id=\"5565\"]centrioles[\/pb_glossary]\u00a0near the\u00a0nucleus\u00a0begin to separate and move to opposite poles of the cell.\u00a0Centrioles<\/strong> are small organelles found only in eukaryotic cells. They help ensure that the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The spindle consists of fibres made of microtubules.\r\n\r\n[caption id=\"attachment_2005\" align=\"aligncenter\" width=\"541\"]\"Diagram Figure 4.13.6 Diagram of a cell in prophase of mitosis. <\/em>[\/caption]\r\n\r\n
\r\n\r\n \r\n\r\n \r\n\r\nMetaphase<\/span>\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_2011\" align=\"alignleft\" width=\"89\"]\"\" Figure 4.13.7 Metaphase.<\/em>[\/caption]\r\n\r\nDuring\u00a0[pb_glossary id=\"2029\"]metaphase[\/pb_glossary]<\/strong>, spindle fibres attach to the centromere of each pair of sister chromatids. As you can see in Figure 4.13.7, the sister chromatids line up at the equator (or center) of the cell. The spindle fibres ensure that sister chromatids will separate and go to different daughter cells when the cell divides.\r\n\r\n[caption id=\"attachment_2010\" align=\"aligncenter\" width=\"543\"]\"Diagram Figure 4.13.8 Diagram showing the metaphase of mitosis. <\/em>[\/caption]\r\n

Anaphase<\/h2>\r\n[caption id=\"attachment_2032\" align=\"alignleft\" width=\"114\"]\"\" Figure 4.13.9 Mitotic anaphase.<\/em>[\/caption]\r\n\r\nDuring\u00a0[pb_glossary id=\"2031\"]anaphase[\/pb_glossary]<\/strong>, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibres. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole. At the end of anaphase, each pole of the cell has a complete set of chromosomes.\r\n\r\n[caption id=\"attachment_2033\" align=\"aligncenter\" width=\"541\"]\"Image Figure 4.13.10 Diagram showing eukaryotic cell in anaphase of cell cycle. <\/em>[\/caption]\r\n

Telophase<\/h2>\r\n[caption id=\"attachment_2035\" align=\"alignleft\" width=\"113\"]\"\" Figure 4.13.11 Mitotic telophase.<\/em>[\/caption]\r\n\r\nDuring\u00a0[pb_glossary id=\"2037\"]telophase[\/pb_glossary]<\/strong>, the chromosomes begin to uncoil and form chromatin. This prepares the genetic material for directing the metabolic activities of the new cells. The spindle also breaks down, and new nuclear envelopes form.\r\n\r\n[caption id=\"attachment_2036\" align=\"aligncenter\" width=\"540\"]\"Telophase Figure 4.13.12 Diagram showing telophase in mitosis. <\/em>[\/caption]\r\n\r\n
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Cytokinesis<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_5597\" align=\"alignleft\" width=\"128\"]\"\" Figure 4.13.13 Mitotic cytokinesis.<\/em>[\/caption]\r\n\r\nCytokinesis<\/strong>\u00a0is the final stage of\u00a0cell division. During cytokinesis, the cytoplasm splits in two and the cell divides, as shown\u00a0below. In\u00a0animal cells, the\u00a0plasma membrane\u00a0of the parent cell pinches inward along the cell\u2019s equator until two daughter cells form.\u00a0Thus, the goal of\u00a0mitosis and cytokinesis\u00a0is now complete, because one parent cell\u00a0has given\u00a0rise to two daughter cells. The daughter cells have the same chromosomes as the parent cell.\r\n
\r\n\r\n[caption id=\"attachment_2038\" align=\"aligncenter\" width=\"542\"]\"Cytokinesis Figure 4.13.14 Diagram showing the final step in cell division: cytokinesis. <\/em>[\/caption]\r\n\r\n
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4.13 Summary<\/span><\/h1>\r\n<\/header>\r\n
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