The\u00a0Cell Cycle<\/h1>\r\n<\/div>\r\nCell division is just one of several stages that a cell goes through during its lifetime. The\u00a0[pb_glossary id=\"5643\"]cell cycle<\/strong>[\/pb_glossary]\u00a0is a repeating series of events that includes growth,\u00a0DNA\u00a0synthesis, and cell division. The cell cycle in\u00a0[pb_glossary id=\"1572\"]prokaryotes[\/pb_glossary]\u00a0is quite simple: the cell grows, its DNA replicates, and the cell divides. In [pb_glossary id=\"1573\"]eukaryotes[\/pb_glossary], the cell cycle is more complicated.\r\nEukaryotic Cell Cycle<\/h1>\r\nThe diagram in Figure 4.12.2 represents the cell cycle of a [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as [pb_glossary id=\"1941\"]interphase[\/pb_glossary]<\/strong>. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.\r\n\r\n[caption id=\"attachment_1940\" align=\"aligncenter\" width=\"438\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cell-Cycle-2.png\")
Figure 4.12.2 Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The First Gap (G1), Synthesis, and Second Gap (G2) phases make up interphase (I). The mitotic phase includes mitosis and cytokinesis. After the mitotic phase, two cells result.<\/em>[\/caption]\r\n\r\n\r\nInterphase<\/h2>\r\n<\/div>\r\nThe [pb_glossary id=\"1941\"]interphase[\/pb_glossary] of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.\r\n\r\n \t- Growth Phase 1 (G1):<\/strong>\u00a0During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes\u00a0proteins\u00a0needed for\u00a0DNA\u00a0replication and copies some of its\u00a0organelles\u00a0in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.<\/li>\r\n \t
- Synthesis Phase (S):<\/strong>\u00a0During this phase, the cell\u2019s\u00a0DNA\u00a0is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.<\/li>\r\n \t
- Growth Phase 2 (G2):<\/strong>\u00a0During this phase, the cell makes final preparations to divide. For example, it makes additional\u00a0proteins\u00a0and\u00a0organelles. This phase is also known as gap phase 2.<\/li>\r\n<\/ul>\r\n
Control of the Cell Cycle<\/h2>\r\nIf the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.\r\n\r\n[caption id=\"attachment_1943\" align=\"aligncenter\" width=\"842\"]![\"\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cell-Cycle-Checkpoints-2.png\")
Figure 4.12.3 Eukaryotic Cell Cycle - Checkpoints. <\/em>[\/caption]\r\n\r\n\r\n\r\nCheckpoints in the [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] [pb_glossary id=\"5643\"]cell cycle[\/pb_glossary] ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.\r\n\r\n<\/div>\r\n\r\n \t- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\r\n \t
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\r\n \t
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\r\n<\/ul>\r\n
Cancer and the Cell Cycle<\/h2>\r\n[pb_glossary id=\"5605\"]Cancer<\/strong>[\/pb_glossary]\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s [pb_glossary id=\"277\"]DNA[\/pb_glossary] becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0[pb_glossary id=\"1984\"]tumor<\/strong>[\/pb_glossary] (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.\r\n\r\n[caption id=\"attachment_1985\" align=\"alignnone\" width=\"500\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Cancer-cells-forming-a-tumour-2.jpg\")
Figure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em>[\/caption]\r\n\r\n\r\n\r\nCell Division<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"5633\"]Cell division<\/strong>[\/pb_glossary]\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] or [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary].\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.\r\n\r\nBefore a eukaryotic cell divides, all of the [pb_glossary id=\"277\"]DNA[\/pb_glossary] in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary] and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em>\r\n\r\n \t- The first step in the division of a eukaryotic cell is\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary], a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary] (membrane) breaks down and later reforms. The [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\r\n \t
- The second major step is\u00a0[pb_glossary id=\"1988\"]cytokinesis<\/strong>[\/pb_glossary]. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1990\" align=\"alignleft\" width=\"235\"]![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/4446362464_9810a71ebb_o-2.jpg\")
Figure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em>[\/caption]\r\n\r\nHenrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks's doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing... and dividing and dividing and dividing. Copies of Henrietta's cells\u00a0\u2014\u00a0called [pb_glossary id=\"1991\"]HeLa cells[\/pb_glossary], for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!\r\n\r\nWhy Henrietta's cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.\r\n\r\nYou would think that Henrietta's name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.\r\n\r\nIronically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.\r\n\r\nIf you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot's\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=22lGbAVWhro\r\nThe immortal cells of Henrietta Lacks - Robin Bulleri, TED-Ed, 2016.<\/p>\r\n\r\n
\r\n\r\n4.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5643\"]cell cycle[\/pb_glossary] is a repeating series of events that includes growth, DNA synthesis, and [pb_glossary id=\"5633\"]cell division[\/pb_glossary].\u00a0The cycle\u00a0is more complicated in [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] than [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells.<\/li>\r\n \t
- In a eukaryotic cell, the cell cycle has two major phases: mitotic phase and [pb_glossary id=\"1941\"]interphase[\/pb_glossary]. During mitotic phase, first the nucleus and then the cytoplasm divide. During interphase, the cell grows, performs routine life processes, and prepares to divide.<\/li>\r\n \t
- The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. There are a number of main checkpoints in the regulation of the cell cycle.<\/li>\r\n \t
- [pb_glossary id=\"5605\"]Cancer[\/pb_glossary] is a disease that occurs when the cell cycle is no longer regulated, often because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a [pb_glossary id=\"1984\"]tumor[\/pb_glossary].<\/li>\r\n \t
- The cell division phase of the cell cycle in a eukaryotic cell occurs in two major steps:\u00a0[pb_glossary id=\"1987\"]mitosis[\/pb_glossary]\u00a0\u2014 when the nucleus divides \u2014 and [pb_glossary id=\"1988\"]cytokinesis[\/pb_glossary], when the cytoplasm divides and two daughter cells form.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"482\"]<\/li>\r\n \t
- Explain why cell division is more complex in eukaryotic than prokaryotic cells.<\/li>\r\n \t
- Using a technique called flow cytometry, scientists can distinguish between cells with the normal amount of DNA and those that contain twice the normal amount of DNA as they go through the cell cycle. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.<\/li>\r\n \t
- What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.12 Explore More<\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=QVCjdNxJreE\r\nThe Cell Cycle (and cancer) [Updated], The Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.12.1<\/strong>\r\n\r\nMom and baby<\/a> by\u00a0Taiying Lu<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.12.2<\/strong>\r\n\r\nCell Cycle<\/a>\u00a0by LadyofHats; CK-12 Foundation<\/a> is used under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n![\"\"](\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/logo_ck12.png\")
\u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span>![\"CK-12](\"https:\/\/www.ck12info.org\/wp-content\/uploads\/2016\/05\/icon_licence.png\" "\\"CK-12")
<\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.3<\/strong>\r\n\r\nCell Cycle Checkpoints<\/a> by LadyofHats; CK-12 Foundation<\/a> is used and adapted by Christine Miller under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n \u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span><\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.4<\/strong>\r\n\r\nCancer cells forming a tumour<\/a> by Ed Uthman, MD on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.12.5<\/strong>\r\n\r\nHenrietta Lacks<\/a>\u00a0by Oregon State University<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters.\u00a0 (2018, March 20). The cell cycle (and cancer) [Updated]. YouTube. https:\/\/www.youtube.com\/watch?v=QVCjdNxJreE&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, February 8). The immortal cells of Henrietta Lacks - Robin Bulleri. YouTube. https:\/\/www.youtube.com\/watch?v=22lGbAVWhro&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 23). Henrietta Lacks. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Henrietta_Lacks&oldid=964020268<\/p>\r\nWikipedia contributors. (2020, May 11). Howard W. Jones. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Howard_W._Jones&oldid=956033806<\/p>\r\nWikipedia contributors. (2020, July 1). George Otto Gey. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=George_Otto_Gey&oldid=965394045<\/p>\r\nWikipedia contributors. (2020, July 6). Johns Hopkins Hospital. In ,Wikipedia.<\/em>\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Johns_Hopkins_Hospital&oldid=966348552<\/span><\/p>\r\nWikipedia contributors. (2020, June 28). Jonas Salk. In Wikipedia<\/em>.\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Jonas_Salk&oldid=964883129<\/p>\r\nWikipedia contributors. (2020, April 14). Rebecca Skloot. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Rebecca_Skloot&oldid=950837115<\/p>\r\nWikipedia contributors. (2020, February 21). The immortal life of Henrietta Lacks. In Wikipedia. <\/em>https:\/\/en.wikipedia.org\/w\/index.php?title=The_Immortal_Life_of_Henrietta_Lacks&oldid=941942679<\/p>\r\n \r\n\r\n<\/div>","rendered":" <\/p>\n![\"Image](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Mom-and-baby-2-scaled.jpg\")
Figure 4.12.1 Mother and growing baby girl. <\/em><\/figcaption><\/figure>\nSo Many Cells!<\/h1>\n
This baby girl (Figure 4.12.1) has a lot of growing to do before she’s as big as her mom. Most of her growth will be the result of cell division. By the time she is an adult, her body will consist of trillions of cells. Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells, causing them to grow out of control. In fact, this is how cancer cells cause illness. In this concept, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.<\/p>\n
\nThe\u00a0Cell Cycle<\/h1>\n<\/div>\n
Cell division is just one of several stages that a cell goes through during its lifetime. The\u00a0cell cycle<\/strong><\/a>\u00a0is a repeating series of events that includes growth,\u00a0DNA\u00a0synthesis, and cell division. The cell cycle in\u00a0prokaryotes<\/a>\u00a0is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes<\/a>, the cell cycle is more complicated.<\/p>\nEukaryotic Cell Cycle<\/h1>\n
The diagram in Figure 4.12.2 represents the cell cycle of a eukaryotic<\/a> cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase<\/strong>. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.<\/p>\nFigure 4.12.2 Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The First Gap (G1), Synthesis, and Second Gap (G2) phases make up interphase (I). The mitotic phase includes mitosis and cytokinesis. After the mitotic phase, two cells result.<\/em><\/figcaption><\/figure>\n\nInterphase<\/h2>\n<\/div>\n
The interphase of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.<\/p>\n
\n- Growth Phase 1 (G1):<\/strong>\u00a0During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes\u00a0proteins\u00a0needed for\u00a0DNA\u00a0replication and copies some of its\u00a0organelles\u00a0in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.<\/li>\n
- Synthesis Phase (S):<\/strong>\u00a0During this phase, the cell\u2019s\u00a0DNA\u00a0is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.<\/li>\n
- Growth Phase 2 (G2):<\/strong>\u00a0During this phase, the cell makes final preparations to divide. For example, it makes additional\u00a0proteins\u00a0and\u00a0organelles. This phase is also known as gap phase 2.<\/li>\n<\/ul>\n
Control of the Cell Cycle<\/h2>\n
If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.<\/p>\nFigure 4.12.3 Eukaryotic Cell Cycle – Checkpoints. <\/em><\/figcaption><\/figure>\n\nCheckpoints in the eukaryotic<\/a> cell cycle<\/a> ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.<\/p>\n<\/div>\n\n- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\n
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\n
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\n<\/ul>\n
Cancer and the Cell Cycle<\/h2>\n
Cancer<\/strong><\/a>\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s DNA<\/a> becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0tumor<\/strong><\/a> (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.<\/p>\nFigure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em><\/figcaption><\/figure>\n\nCell Division<\/span><\/p>\n<\/div>\nCell division<\/strong><\/a>\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic<\/a> or eukaryotic<\/a>.\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.<\/p>\nBefore a eukaryotic cell divides, all of the DNA<\/a> in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its organelles<\/a>\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0mitosis<\/strong><\/a> and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em><\/p>\n\n- The first step in the division of a eukaryotic cell is\u00a0mitosis<\/strong><\/a>, a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the nuclear envelope<\/a> (membrane) breaks down and later reforms. The chromosomes<\/a>\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\n
- The second major step is\u00a0cytokinesis<\/strong><\/a>. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\nFigure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em><\/figcaption><\/figure>\nHenrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks’s doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing… and dividing and dividing and dividing. Copies of Henrietta’s cells\u00a0\u2014\u00a0called HeLa cells<\/a>, for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!<\/p>\nWhy Henrietta’s cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.<\/p>\nYou would think that Henrietta’s name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.<\/p>\nIronically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.<\/p>\n
If you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot’s\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:<\/p>\n
Figure 4.12.2 Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The First Gap (G1), Synthesis, and Second Gap (G2) phases make up interphase (I). The mitotic phase includes mitosis and cytokinesis. After the mitotic phase, two cells result.<\/em>[\/caption]\r\n\r\nInterphase<\/h2>\r\n<\/div>\r\nThe [pb_glossary id=\"1941\"]interphase[\/pb_glossary] of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.\r\n\r\n \t- Growth Phase 1 (G1):<\/strong>\u00a0During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes\u00a0proteins\u00a0needed for\u00a0DNA\u00a0replication and copies some of its\u00a0organelles\u00a0in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.<\/li>\r\n \t
- Synthesis Phase (S):<\/strong>\u00a0During this phase, the cell\u2019s\u00a0DNA\u00a0is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.<\/li>\r\n \t
- Growth Phase 2 (G2):<\/strong>\u00a0During this phase, the cell makes final preparations to divide. For example, it makes additional\u00a0proteins\u00a0and\u00a0organelles. This phase is also known as gap phase 2.<\/li>\r\n<\/ul>\r\n
Control of the Cell Cycle<\/h2>\r\nIf the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.\r\n\r\n[caption id=\"attachment_1943\" align=\"aligncenter\" width=\"842\"] Figure 4.12.3 Eukaryotic Cell Cycle - Checkpoints. <\/em>[\/caption]\r\n\r\n\r\n\r\nCheckpoints in the [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] [pb_glossary id=\"5643\"]cell cycle[\/pb_glossary] ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.\r\n\r\n<\/div>\r\n\r\n \t- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\r\n \t
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\r\n \t
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\r\n<\/ul>\r\n
Cancer and the Cell Cycle<\/h2>\r\n[pb_glossary id=\"5605\"]Cancer<\/strong>[\/pb_glossary]\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s [pb_glossary id=\"277\"]DNA[\/pb_glossary] becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0[pb_glossary id=\"1984\"]tumor<\/strong>[\/pb_glossary] (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.\r\n\r\n[caption id=\"attachment_1985\" align=\"alignnone\" width=\"500\"] Figure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em>[\/caption]\r\n\r\n\r\n\r\nCell Division<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"5633\"]Cell division<\/strong>[\/pb_glossary]\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] or [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary].\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.\r\n\r\nBefore a eukaryotic cell divides, all of the [pb_glossary id=\"277\"]DNA[\/pb_glossary] in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary] and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em>\r\n\r\n \t- The first step in the division of a eukaryotic cell is\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary], a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary] (membrane) breaks down and later reforms. The [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\r\n \t
- The second major step is\u00a0[pb_glossary id=\"1988\"]cytokinesis<\/strong>[\/pb_glossary]. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1990\" align=\"alignleft\" width=\"235\"] Figure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em>[\/caption]\r\n\r\nHenrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks's doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing... and dividing and dividing and dividing. Copies of Henrietta's cells\u00a0\u2014\u00a0called [pb_glossary id=\"1991\"]HeLa cells[\/pb_glossary], for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!\r\n\r\nWhy Henrietta's cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.\r\n\r\nYou would think that Henrietta's name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.\r\n\r\nIronically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.\r\n\r\nIf you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot's\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=22lGbAVWhro\r\nThe immortal cells of Henrietta Lacks - Robin Bulleri, TED-Ed, 2016.<\/p>\r\n\r\n
\r\n\r\n4.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5643\"]cell cycle[\/pb_glossary] is a repeating series of events that includes growth, DNA synthesis, and [pb_glossary id=\"5633\"]cell division[\/pb_glossary].\u00a0The cycle\u00a0is more complicated in [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] than [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells.<\/li>\r\n \t
- In a eukaryotic cell, the cell cycle has two major phases: mitotic phase and [pb_glossary id=\"1941\"]interphase[\/pb_glossary]. During mitotic phase, first the nucleus and then the cytoplasm divide. During interphase, the cell grows, performs routine life processes, and prepares to divide.<\/li>\r\n \t
- The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. There are a number of main checkpoints in the regulation of the cell cycle.<\/li>\r\n \t
- [pb_glossary id=\"5605\"]Cancer[\/pb_glossary] is a disease that occurs when the cell cycle is no longer regulated, often because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a [pb_glossary id=\"1984\"]tumor[\/pb_glossary].<\/li>\r\n \t
- The cell division phase of the cell cycle in a eukaryotic cell occurs in two major steps:\u00a0[pb_glossary id=\"1987\"]mitosis[\/pb_glossary]\u00a0\u2014 when the nucleus divides \u2014 and [pb_glossary id=\"1988\"]cytokinesis[\/pb_glossary], when the cytoplasm divides and two daughter cells form.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"482\"]<\/li>\r\n \t
- Explain why cell division is more complex in eukaryotic than prokaryotic cells.<\/li>\r\n \t
- Using a technique called flow cytometry, scientists can distinguish between cells with the normal amount of DNA and those that contain twice the normal amount of DNA as they go through the cell cycle. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.<\/li>\r\n \t
- What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.12 Explore More<\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=QVCjdNxJreE\r\nThe Cell Cycle (and cancer) [Updated], The Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.12.1<\/strong>\r\n\r\nMom and baby<\/a> by\u00a0Taiying Lu<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.12.2<\/strong>\r\n\r\nCell Cycle<\/a>\u00a0by LadyofHats; CK-12 Foundation<\/a> is used under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n \u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span><\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.3<\/strong>\r\n\r\nCell Cycle Checkpoints<\/a> by LadyofHats; CK-12 Foundation<\/a> is used and adapted by Christine Miller under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n \u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span><\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.4<\/strong>\r\n\r\nCancer cells forming a tumour<\/a> by Ed Uthman, MD on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.12.5<\/strong>\r\n\r\nHenrietta Lacks<\/a>\u00a0by Oregon State University<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters.\u00a0 (2018, March 20). The cell cycle (and cancer) [Updated]. YouTube. https:\/\/www.youtube.com\/watch?v=QVCjdNxJreE&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, February 8). The immortal cells of Henrietta Lacks - Robin Bulleri. YouTube. https:\/\/www.youtube.com\/watch?v=22lGbAVWhro&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 23). Henrietta Lacks. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Henrietta_Lacks&oldid=964020268<\/p>\r\nWikipedia contributors. (2020, May 11). Howard W. Jones. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Howard_W._Jones&oldid=956033806<\/p>\r\nWikipedia contributors. (2020, July 1). George Otto Gey. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=George_Otto_Gey&oldid=965394045<\/p>\r\nWikipedia contributors. (2020, July 6). Johns Hopkins Hospital. In ,Wikipedia.<\/em>\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Johns_Hopkins_Hospital&oldid=966348552<\/span><\/p>\r\nWikipedia contributors. (2020, June 28). Jonas Salk. In Wikipedia<\/em>.\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Jonas_Salk&oldid=964883129<\/p>\r\nWikipedia contributors. (2020, April 14). Rebecca Skloot. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Rebecca_Skloot&oldid=950837115<\/p>\r\nWikipedia contributors. (2020, February 21). The immortal life of Henrietta Lacks. In Wikipedia. <\/em>https:\/\/en.wikipedia.org\/w\/index.php?title=The_Immortal_Life_of_Henrietta_Lacks&oldid=941942679<\/p>\r\n \r\n\r\n<\/div>","rendered":" <\/p>\nFigure 4.12.1 Mother and growing baby girl. <\/em><\/figcaption><\/figure>\nSo Many Cells!<\/h1>\n
This baby girl (Figure 4.12.1) has a lot of growing to do before she’s as big as her mom. Most of her growth will be the result of cell division. By the time she is an adult, her body will consist of trillions of cells. Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells, causing them to grow out of control. In fact, this is how cancer cells cause illness. In this concept, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.<\/p>\n
\nThe\u00a0Cell Cycle<\/h1>\n<\/div>\n
Cell division is just one of several stages that a cell goes through during its lifetime. The\u00a0cell cycle<\/strong><\/a>\u00a0is a repeating series of events that includes growth,\u00a0DNA\u00a0synthesis, and cell division. The cell cycle in\u00a0prokaryotes<\/a>\u00a0is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes<\/a>, the cell cycle is more complicated.<\/p>\nEukaryotic Cell Cycle<\/h1>\n
The diagram in Figure 4.12.2 represents the cell cycle of a eukaryotic<\/a> cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase<\/strong>. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.<\/p>\nFigure 4.12.2 Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The First Gap (G1), Synthesis, and Second Gap (G2) phases make up interphase (I). The mitotic phase includes mitosis and cytokinesis. After the mitotic phase, two cells result.<\/em><\/figcaption><\/figure>\n\nInterphase<\/h2>\n<\/div>\n
The interphase of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.<\/p>\n
\n- Growth Phase 1 (G1):<\/strong>\u00a0During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes\u00a0proteins\u00a0needed for\u00a0DNA\u00a0replication and copies some of its\u00a0organelles\u00a0in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.<\/li>\n
- Synthesis Phase (S):<\/strong>\u00a0During this phase, the cell\u2019s\u00a0DNA\u00a0is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.<\/li>\n
- Growth Phase 2 (G2):<\/strong>\u00a0During this phase, the cell makes final preparations to divide. For example, it makes additional\u00a0proteins\u00a0and\u00a0organelles. This phase is also known as gap phase 2.<\/li>\n<\/ul>\n
Control of the Cell Cycle<\/h2>\n
If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.<\/p>\nFigure 4.12.3 Eukaryotic Cell Cycle – Checkpoints. <\/em><\/figcaption><\/figure>\n\nCheckpoints in the eukaryotic<\/a> cell cycle<\/a> ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.<\/p>\n<\/div>\n\n- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\n
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\n
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\n<\/ul>\n
Cancer and the Cell Cycle<\/h2>\n
Cancer<\/strong><\/a>\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s DNA<\/a> becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0tumor<\/strong><\/a> (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.<\/p>\nFigure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em><\/figcaption><\/figure>\n\nCell Division<\/span><\/p>\n<\/div>\nCell division<\/strong><\/a>\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic<\/a> or eukaryotic<\/a>.\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.<\/p>\nBefore a eukaryotic cell divides, all of the DNA<\/a> in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its organelles<\/a>\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0mitosis<\/strong><\/a> and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em><\/p>\n\n- The first step in the division of a eukaryotic cell is\u00a0mitosis<\/strong><\/a>, a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the nuclear envelope<\/a> (membrane) breaks down and later reforms. The chromosomes<\/a>\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\n
- The second major step is\u00a0cytokinesis<\/strong><\/a>. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\nFigure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em><\/figcaption><\/figure>\nHenrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks’s doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing… and dividing and dividing and dividing. Copies of Henrietta’s cells\u00a0\u2014\u00a0called HeLa cells<\/a>, for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!<\/p>\nWhy Henrietta’s cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.<\/p>\nYou would think that Henrietta’s name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.<\/p>\nIronically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.<\/p>\n
If you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot’s\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:<\/p>\n
Control of the Cell Cycle<\/h2>\r\nIf the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.\r\n\r\n[caption id=\"attachment_1943\" align=\"aligncenter\" width=\"842\"] Figure 4.12.3 Eukaryotic Cell Cycle - Checkpoints. <\/em>[\/caption]\r\n\r\n\r\n\r\nCheckpoints in the [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] [pb_glossary id=\"5643\"]cell cycle[\/pb_glossary] ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.\r\n\r\n<\/div>\r\n\r\n \t- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\r\n \t
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\r\n \t
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\r\n<\/ul>\r\n
Cancer and the Cell Cycle<\/h2>\r\n[pb_glossary id=\"5605\"]Cancer<\/strong>[\/pb_glossary]\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s [pb_glossary id=\"277\"]DNA[\/pb_glossary] becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0[pb_glossary id=\"1984\"]tumor<\/strong>[\/pb_glossary] (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.\r\n\r\n[caption id=\"attachment_1985\" align=\"alignnone\" width=\"500\"] Figure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em>[\/caption]\r\n\r\n\r\n\r\nCell Division<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"5633\"]Cell division<\/strong>[\/pb_glossary]\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] or [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary].\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.\r\n\r\nBefore a eukaryotic cell divides, all of the [pb_glossary id=\"277\"]DNA[\/pb_glossary] in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary] and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em>\r\n\r\n \t- The first step in the division of a eukaryotic cell is\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary], a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary] (membrane) breaks down and later reforms. The [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\r\n \t
- The second major step is\u00a0[pb_glossary id=\"1988\"]cytokinesis<\/strong>[\/pb_glossary]. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1990\" align=\"alignleft\" width=\"235\"] Figure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em>[\/caption]\r\n\r\nHenrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks's doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing... and dividing and dividing and dividing. Copies of Henrietta's cells\u00a0\u2014\u00a0called [pb_glossary id=\"1991\"]HeLa cells[\/pb_glossary], for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!\r\n\r\nWhy Henrietta's cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.\r\n\r\nYou would think that Henrietta's name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.\r\n\r\nIronically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.\r\n\r\nIf you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot's\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=22lGbAVWhro\r\nThe immortal cells of Henrietta Lacks - Robin Bulleri, TED-Ed, 2016.<\/p>\r\n\r\n
\r\n\r\n4.12 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5643\"]cell cycle[\/pb_glossary] is a repeating series of events that includes growth, DNA synthesis, and [pb_glossary id=\"5633\"]cell division[\/pb_glossary].\u00a0The cycle\u00a0is more complicated in [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] than [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells.<\/li>\r\n \t
- In a eukaryotic cell, the cell cycle has two major phases: mitotic phase and [pb_glossary id=\"1941\"]interphase[\/pb_glossary]. During mitotic phase, first the nucleus and then the cytoplasm divide. During interphase, the cell grows, performs routine life processes, and prepares to divide.<\/li>\r\n \t
- The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. There are a number of main checkpoints in the regulation of the cell cycle.<\/li>\r\n \t
- [pb_glossary id=\"5605\"]Cancer[\/pb_glossary] is a disease that occurs when the cell cycle is no longer regulated, often because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a [pb_glossary id=\"1984\"]tumor[\/pb_glossary].<\/li>\r\n \t
- The cell division phase of the cell cycle in a eukaryotic cell occurs in two major steps:\u00a0[pb_glossary id=\"1987\"]mitosis[\/pb_glossary]\u00a0\u2014 when the nucleus divides \u2014 and [pb_glossary id=\"1988\"]cytokinesis[\/pb_glossary], when the cytoplasm divides and two daughter cells form.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n4.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"482\"]<\/li>\r\n \t
- Explain why cell division is more complex in eukaryotic than prokaryotic cells.<\/li>\r\n \t
- Using a technique called flow cytometry, scientists can distinguish between cells with the normal amount of DNA and those that contain twice the normal amount of DNA as they go through the cell cycle. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.<\/li>\r\n \t
- What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n4.12 Explore More<\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=QVCjdNxJreE\r\nThe Cell Cycle (and cancer) [Updated], The Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.12.1<\/strong>\r\n\r\nMom and baby<\/a> by\u00a0Taiying Lu<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.12.2<\/strong>\r\n\r\nCell Cycle<\/a>\u00a0by LadyofHats; CK-12 Foundation<\/a> is used under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n \u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span><\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.3<\/strong>\r\n\r\nCell Cycle Checkpoints<\/a> by LadyofHats; CK-12 Foundation<\/a> is used and adapted by Christine Miller under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n \u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span><\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.4<\/strong>\r\n\r\nCancer cells forming a tumour<\/a> by Ed Uthman, MD on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.12.5<\/strong>\r\n\r\nHenrietta Lacks<\/a>\u00a0by Oregon State University<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters.\u00a0 (2018, March 20). The cell cycle (and cancer) [Updated]. YouTube. https:\/\/www.youtube.com\/watch?v=QVCjdNxJreE&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, February 8). The immortal cells of Henrietta Lacks - Robin Bulleri. YouTube. https:\/\/www.youtube.com\/watch?v=22lGbAVWhro&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 23). Henrietta Lacks. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Henrietta_Lacks&oldid=964020268<\/p>\r\nWikipedia contributors. (2020, May 11). Howard W. Jones. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Howard_W._Jones&oldid=956033806<\/p>\r\nWikipedia contributors. (2020, July 1). George Otto Gey. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=George_Otto_Gey&oldid=965394045<\/p>\r\nWikipedia contributors. (2020, July 6). Johns Hopkins Hospital. In ,Wikipedia.<\/em>\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Johns_Hopkins_Hospital&oldid=966348552<\/span><\/p>\r\nWikipedia contributors. (2020, June 28). Jonas Salk. In Wikipedia<\/em>.\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Jonas_Salk&oldid=964883129<\/p>\r\nWikipedia contributors. (2020, April 14). Rebecca Skloot. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Rebecca_Skloot&oldid=950837115<\/p>\r\nWikipedia contributors. (2020, February 21). The immortal life of Henrietta Lacks. In Wikipedia. <\/em>https:\/\/en.wikipedia.org\/w\/index.php?title=The_Immortal_Life_of_Henrietta_Lacks&oldid=941942679<\/p>\r\n \r\n\r\n<\/div>","rendered":" <\/p>\nFigure 4.12.1 Mother and growing baby girl. <\/em><\/figcaption><\/figure>\nSo Many Cells!<\/h1>\n
This baby girl (Figure 4.12.1) has a lot of growing to do before she’s as big as her mom. Most of her growth will be the result of cell division. By the time she is an adult, her body will consist of trillions of cells. Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells, causing them to grow out of control. In fact, this is how cancer cells cause illness. In this concept, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.<\/p>\n
\nThe\u00a0Cell Cycle<\/h1>\n<\/div>\n
Cell division is just one of several stages that a cell goes through during its lifetime. The\u00a0cell cycle<\/strong><\/a>\u00a0is a repeating series of events that includes growth,\u00a0DNA\u00a0synthesis, and cell division. The cell cycle in\u00a0prokaryotes<\/a>\u00a0is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes<\/a>, the cell cycle is more complicated.<\/p>\nEukaryotic Cell Cycle<\/h1>\n
The diagram in Figure 4.12.2 represents the cell cycle of a eukaryotic<\/a> cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase<\/strong>. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.<\/p>\nFigure 4.12.2 Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The First Gap (G1), Synthesis, and Second Gap (G2) phases make up interphase (I). The mitotic phase includes mitosis and cytokinesis. After the mitotic phase, two cells result.<\/em><\/figcaption><\/figure>\n\nInterphase<\/h2>\n<\/div>\n
The interphase of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.<\/p>\n
\n- Growth Phase 1 (G1):<\/strong>\u00a0During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes\u00a0proteins\u00a0needed for\u00a0DNA\u00a0replication and copies some of its\u00a0organelles\u00a0in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.<\/li>\n
- Synthesis Phase (S):<\/strong>\u00a0During this phase, the cell\u2019s\u00a0DNA\u00a0is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.<\/li>\n
- Growth Phase 2 (G2):<\/strong>\u00a0During this phase, the cell makes final preparations to divide. For example, it makes additional\u00a0proteins\u00a0and\u00a0organelles. This phase is also known as gap phase 2.<\/li>\n<\/ul>\n
Control of the Cell Cycle<\/h2>\n
If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.<\/p>\nFigure 4.12.3 Eukaryotic Cell Cycle – Checkpoints. <\/em><\/figcaption><\/figure>\n\nCheckpoints in the eukaryotic<\/a> cell cycle<\/a> ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.<\/p>\n<\/div>\n\n- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\n
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\n
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\n<\/ul>\n
Cancer and the Cell Cycle<\/h2>\n
Cancer<\/strong><\/a>\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s DNA<\/a> becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0tumor<\/strong><\/a> (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.<\/p>\nFigure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em><\/figcaption><\/figure>\n\nCell Division<\/span><\/p>\n<\/div>\nCell division<\/strong><\/a>\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic<\/a> or eukaryotic<\/a>.\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.<\/p>\nBefore a eukaryotic cell divides, all of the DNA<\/a> in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its organelles<\/a>\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0mitosis<\/strong><\/a> and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em><\/p>\n\n- The first step in the division of a eukaryotic cell is\u00a0mitosis<\/strong><\/a>, a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the nuclear envelope<\/a> (membrane) breaks down and later reforms. The chromosomes<\/a>\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\n
- The second major step is\u00a0cytokinesis<\/strong><\/a>. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\nFigure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em><\/figcaption><\/figure>\nHenrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks’s doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing… and dividing and dividing and dividing. Copies of Henrietta’s cells\u00a0\u2014\u00a0called HeLa cells<\/a>, for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!<\/p>\nWhy Henrietta’s cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.<\/p>\nYou would think that Henrietta’s name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.<\/p>\nIronically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.<\/p>\n
If you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot’s\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:<\/p>\n
- \r\n \t
- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\r\n \t
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\r\n \t
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\r\n<\/ul>\r\n
Cancer and the Cell Cycle<\/h2>\r\n[pb_glossary id=\"5605\"]Cancer<\/strong>[\/pb_glossary]\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s [pb_glossary id=\"277\"]DNA[\/pb_glossary] becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0[pb_glossary id=\"1984\"]tumor<\/strong>[\/pb_glossary] (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.\r\n\r\n[caption id=\"attachment_1985\" align=\"alignnone\" width=\"500\"]
Figure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em>[\/caption]\r\n\r\n\r\n\r\nCell Division<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"5633\"]Cell division<\/strong>[\/pb_glossary]\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] or [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary].\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.\r\n\r\nBefore a eukaryotic cell divides, all of the [pb_glossary id=\"277\"]DNA[\/pb_glossary] in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its [pb_glossary id=\"5557\"]organelles[\/pb_glossary]\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary] and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em>\r\n- \r\n \t
- The first step in the division of a eukaryotic cell is\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary], a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary] (membrane) breaks down and later reforms. The [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\r\n \t
- The second major step is\u00a0[pb_glossary id=\"1988\"]cytokinesis<\/strong>[\/pb_glossary]. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\r\n<\/ul>\r\n
\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_1990\" align=\"alignleft\" width=\"235\"]
Figure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em>[\/caption]\r\n\r\nHenrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks's doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing... and dividing and dividing and dividing. Copies of Henrietta's cells\u00a0\u2014\u00a0called [pb_glossary id=\"1991\"]HeLa cells[\/pb_glossary], for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!\r\n\r\nWhy Henrietta's cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.\r\n\r\nYou would think that Henrietta's name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.\r\n\r\nIronically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.\r\n\r\nIf you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot's\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=22lGbAVWhro\r\nThe immortal cells of Henrietta Lacks - Robin Bulleri, TED-Ed, 2016.<\/p>\r\n\r\n
\r\n\r\n 4.12 Summary<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- The [pb_glossary id=\"5643\"]cell cycle[\/pb_glossary] is a repeating series of events that includes growth, DNA synthesis, and [pb_glossary id=\"5633\"]cell division[\/pb_glossary].\u00a0The cycle\u00a0is more complicated in [pb_glossary id=\"1573\"]eukaryotic[\/pb_glossary] than [pb_glossary id=\"1572\"]prokaryotic[\/pb_glossary] cells.<\/li>\r\n \t
- In a eukaryotic cell, the cell cycle has two major phases: mitotic phase and [pb_glossary id=\"1941\"]interphase[\/pb_glossary]. During mitotic phase, first the nucleus and then the cytoplasm divide. During interphase, the cell grows, performs routine life processes, and prepares to divide.<\/li>\r\n \t
- The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. There are a number of main checkpoints in the regulation of the cell cycle.<\/li>\r\n \t
- [pb_glossary id=\"5605\"]Cancer[\/pb_glossary] is a disease that occurs when the cell cycle is no longer regulated, often because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a [pb_glossary id=\"1984\"]tumor[\/pb_glossary].<\/li>\r\n \t
- The cell division phase of the cell cycle in a eukaryotic cell occurs in two major steps:\u00a0[pb_glossary id=\"1987\"]mitosis[\/pb_glossary]\u00a0\u2014 when the nucleus divides \u2014 and [pb_glossary id=\"1988\"]cytokinesis[\/pb_glossary], when the cytoplasm divides and two daughter cells form.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n\r\n
\r\n 4.12 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- [h5p id=\"482\"]<\/li>\r\n \t
- Explain why cell division is more complex in eukaryotic than prokaryotic cells.<\/li>\r\n \t
- Using a technique called flow cytometry, scientists can distinguish between cells with the normal amount of DNA and those that contain twice the normal amount of DNA as they go through the cell cycle. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.<\/li>\r\n \t
- What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 4.12 Explore More<\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/www.youtube.com\/watch?v=QVCjdNxJreE\r\nThe Cell Cycle (and cancer) [Updated], The Amoeba Sisters, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 4.12.1<\/strong>\r\n\r\nMom and baby<\/a> by\u00a0Taiying Lu<\/a> on Unsplash<\/a> is used under the Unsplash License<\/a> (https:\/\/unsplash.com\/license).\r\n\r\nFigure 4.12.2<\/strong>\r\n\r\nCell Cycle<\/a>\u00a0by LadyofHats; CK-12 Foundation<\/a> is used under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n
\u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span><\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.3<\/strong>\r\n\r\nCell Cycle Checkpoints<\/a> by LadyofHats; CK-12 Foundation<\/a> is used and adapted by Christine Miller under a CC BY-NC 3.0<\/span><\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/3.0\/) license.\r\n\r\n \u00a9<\/span>CK-12 Foundation<\/a> Licensed under\u00a0<\/span><\/a>\u00a0\u2022\u00a0<\/span>Terms of Use<\/a>\u00a0\u2022\u00a0<\/span>Attribution<\/a>\r\n\r\nFigure 4.12.4<\/strong>\r\n\r\nCancer cells forming a tumour<\/a> by Ed Uthman, MD on Wikimedia Commons is released into the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 4.12.5<\/strong>\r\n\r\nHenrietta Lacks<\/a>\u00a0by Oregon State University<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\nReferences<\/h2>\r\n
Amoeba Sisters.\u00a0 (2018, March 20). The cell cycle (and cancer) [Updated]. YouTube. https:\/\/www.youtube.com\/watch?v=QVCjdNxJreE&feature=youtu.be<\/p>\r\n
TED-Ed. (2016, February 8). The immortal cells of Henrietta Lacks - Robin Bulleri. YouTube. https:\/\/www.youtube.com\/watch?v=22lGbAVWhro&feature=youtu.be<\/p>\r\n
Wikipedia contributors. (2020, June 23). Henrietta Lacks. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Henrietta_Lacks&oldid=964020268<\/p>\r\n
Wikipedia contributors. (2020, May 11). Howard W. Jones. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Howard_W._Jones&oldid=956033806<\/p>\r\n
Wikipedia contributors. (2020, July 1). George Otto Gey. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=George_Otto_Gey&oldid=965394045<\/p>\r\n
Wikipedia contributors. (2020, July 6). Johns Hopkins Hospital. In ,Wikipedia.<\/em>\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Johns_Hopkins_Hospital&oldid=966348552<\/span><\/p>\r\n
Wikipedia contributors. (2020, June 28). Jonas Salk. In Wikipedia<\/em>.\u00a0 https:\/\/en.wikipedia.org\/w\/index.php?title=Jonas_Salk&oldid=964883129<\/p>\r\n
Wikipedia contributors. (2020, April 14). Rebecca Skloot. In Wikipedia<\/em>. https:\/\/en.wikipedia.org\/w\/index.php?title=Rebecca_Skloot&oldid=950837115<\/p>\r\n
Wikipedia contributors. (2020, February 21). The immortal life of Henrietta Lacks. In Wikipedia. <\/em>https:\/\/en.wikipedia.org\/w\/index.php?title=The_Immortal_Life_of_Henrietta_Lacks&oldid=941942679<\/p>\r\n \r\n\r\n<\/div>","rendered":"
<\/p>\n
Figure 4.12.1 Mother and growing baby girl. <\/em><\/figcaption><\/figure>\n So Many Cells!<\/h1>\n
This baby girl (Figure 4.12.1) has a lot of growing to do before she’s as big as her mom. Most of her growth will be the result of cell division. By the time she is an adult, her body will consist of trillions of cells. Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells, causing them to grow out of control. In fact, this is how cancer cells cause illness. In this concept, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.<\/p>\n
\nThe\u00a0Cell Cycle<\/h1>\n<\/div>\n
Cell division is just one of several stages that a cell goes through during its lifetime. The\u00a0cell cycle<\/strong><\/a>\u00a0is a repeating series of events that includes growth,\u00a0DNA\u00a0synthesis, and cell division. The cell cycle in\u00a0prokaryotes<\/a>\u00a0is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes<\/a>, the cell cycle is more complicated.<\/p>\n
Eukaryotic Cell Cycle<\/h1>\n
The diagram in Figure 4.12.2 represents the cell cycle of a eukaryotic<\/a> cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase<\/strong>. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.<\/p>\n
Figure 4.12.2 Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The First Gap (G1), Synthesis, and Second Gap (G2) phases make up interphase (I). The mitotic phase includes mitosis and cytokinesis. After the mitotic phase, two cells result.<\/em><\/figcaption><\/figure>\n \nInterphase<\/h2>\n<\/div>\n
The interphase of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.<\/p>\n
- \n
- Growth Phase 1 (G1):<\/strong>\u00a0During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes\u00a0proteins\u00a0needed for\u00a0DNA\u00a0replication and copies some of its\u00a0organelles\u00a0in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.<\/li>\n
- Synthesis Phase (S):<\/strong>\u00a0During this phase, the cell\u2019s\u00a0DNA\u00a0is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.<\/li>\n
- Growth Phase 2 (G2):<\/strong>\u00a0During this phase, the cell makes final preparations to divide. For example, it makes additional\u00a0proteins\u00a0and\u00a0organelles. This phase is also known as gap phase 2.<\/li>\n<\/ul>\n
Control of the Cell Cycle<\/h2>\n
If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.<\/p>\n
Figure 4.12.3 Eukaryotic Cell Cycle – Checkpoints. <\/em><\/figcaption><\/figure>\n \nCheckpoints in the eukaryotic<\/a> cell cycle<\/a> ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.<\/p>\n<\/div>\n
- \n
- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.<\/li>\n
- The S checkpoint determines if the DNA has been replicated properly.<\/li>\n
- The mitosis\u00a0checkpoint ensures that all the\u00a0chromosomes\u00a0are properly aligned before the cell is allowed to divide.<\/li>\n<\/ul>\n
Cancer and the Cell Cycle<\/h2>\n
Cancer<\/strong><\/a>\u00a0is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell\u2019s DNA<\/a> becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a\u00a0tumor<\/strong><\/a> (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.<\/p>\n
Figure 4.12.4 These cells are cancer cells, growing out of control and forming a tumor.<\/em><\/figcaption><\/figure>\n \nCell Division<\/span><\/p>\n<\/div>\n
Cell division<\/strong><\/a>\u00a0is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic<\/a> or eukaryotic<\/a>.\u00a0Cell division is simpler in\u00a0prokaryotes\u00a0than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no\u00a0nucleus, and few other\u00a0organelles. Eukaryotic cells, in contrast, have multiple\u00a0chromosomes\u00a0contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.<\/p>\n
Before a eukaryotic cell divides, all of the DNA<\/a> in the cell\u2019s multiple\u00a0chromosomes\u00a0is replicated. Its organelles<\/a>\u00a0are also duplicated.\u00a0Cell division occurs\u00a0in two major steps, called\u00a0mitosis<\/strong><\/a> and cytokinesis, both of which are described in greater detail in Chapter 5<\/a>.<\/em><\/p>\n
- \n
- The first step in the division of a eukaryotic cell is\u00a0mitosis<\/strong><\/a>, a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the nuclear envelope<\/a> (membrane) breaks down and later reforms. The chromosomes<\/a>\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\n
- The second major step is\u00a0cytokinesis<\/strong><\/a>. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\n<\/ul>\n
\nFeature: Human Biology in the News<\/h1>\n<\/div>\n
Figure 4.12.5 The woman in this mid-1900s photo was named Henrietta Lacks. When she died in 1951 of an unusual form of cervical cancer, she was just 31 years old. A poor, African American tobacco farmer and mother of five, she (or at least her cells) would eventually be called immortal.<\/em><\/figcaption><\/figure>\n Henrietta Lacks<\/a> sought treatment for her cancer at Johns Hopkins University Hospital<\/a> at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks’s doctor, Howard Jones<\/a>, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey<\/a>, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing… and dividing and dividing and dividing. Copies of Henrietta’s cells\u00a0\u2014\u00a0called HeLa cells<\/a>, for her name (He<\/span>nrietta La<\/span>cks) \u2014\u00a0are still alive today. In fact, there are currently\u00a0 billions of HeLa cells in laboratories around the world!<\/p>\n
Why Henrietta’s cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical\u00a0research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk<\/a>, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.<\/p>\n
You would think that Henrietta’s name would be well known in medical history for her unparalleled contributions to biomedical\u00a0research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot<\/a> published a nonfiction book, The Immortal Life of Henrietta Lacks.<\/em> Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.<\/p>\n
Ironically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.<\/p>\n
If you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot’s\u00a0The Immortal Life of Henrietta Lacks<\/em><\/a> (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:<\/p>\n
- The second major step is\u00a0cytokinesis<\/strong><\/a>. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\n<\/ul>\n
- The first step in the division of a eukaryotic cell is\u00a0mitosis<\/strong><\/a>, a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the nuclear envelope<\/a> (membrane) breaks down and later reforms. The chromosomes<\/a>\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\n
- Synthesis Phase (S):<\/strong>\u00a0During this phase, the cell\u2019s\u00a0DNA\u00a0is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.<\/li>\n
- Growth Phase 1 (G1):<\/strong>\u00a0During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes\u00a0proteins\u00a0needed for\u00a0DNA\u00a0replication and copies some of its\u00a0organelles\u00a0in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.<\/li>\n
- The second major step is\u00a0[pb_glossary id=\"1988\"]cytokinesis<\/strong>[\/pb_glossary]. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.<\/li>\r\n<\/ul>\r\n
- The first step in the division of a eukaryotic cell is\u00a0[pb_glossary id=\"1987\"]mitosis<\/strong>[\/pb_glossary], a multi-phase process in which the\u00a0nucleus\u00a0of the cell divides. During\u00a0mitosis, the [pb_glossary id=\"5785\"]nuclear envelope[\/pb_glossary] (membrane) breaks down and later reforms. The [pb_glossary id=\"5619\"]chromosomes[\/pb_glossary]\u00a0are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.<\/li>\r\n \t
![Cancer cells forming a tumour<\/a> by Ed Uthman, MD on Wikimedia Commons is released into the](\"https:\/\/commons.wikimedia.org\/wiki\/File:Tubulovillous_Polyp_of_the_Colon_1.jpg\"){kind=link}