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15.2: Cell Cycle and Cell Division

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    So Many Cells!

    The baby in Figure \(\PageIndex{1}\) has a lot of growing to do before they are as big as their mom. Most of their growth will be the result of cell division. By the time the baby is an adult, their 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 and 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.

    Mother child joining faces and happy.
    Figure \(\PageIndex{1}\): Baby and mother

    The Cell Cycle

    Cell division is 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 or eukaryotic. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no nucleus, and few other organelles. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus and many other organelles. All of these cell parts must be duplicated and then separated when the cell divides. Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle is a repeating series of events that include growth, DNA synthesis, and cell division. The cell cycle in prokaryotes is quite simple: the cell grows, its DNA replicates, and the cell divides. This form of division in prokaryotes is called asexual reproduction. In eukaryotes, the cell cycle is more complicated.

    Eukaryotic Cell Cycle

    Figure \(\PageIndex{2}\) represents the cell cycle of a eukaryotic cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) 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. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.


    The Interphase of the eukaryotic cell cycle can be subdivided into the following phases (Figure \(\PageIndex{2}\)).

    • Growth Phase 1 (G1): The cell spends most of its life in the first gap (sometimes referred to as growth) phase, G1. During this phase, a cell undergoes rapid growth and performs its routine functions. During this phase, the biosynthetic and metabolic activities of the cell occur at a high rate. The synthesis of amino acids and hundreds of thousands or millions of proteins that are required by the cell occurs during this phase. Proteins produced include those needed for DNA replication. If a cell is not dividing, the cell enters the G0 phase from this phase.
    • G0 phase: The G0 phase is a resting phase where the cell has left the cycle and has stopped dividing. Non-dividing cells in multicellular eukaryotic organisms enter G0 from G1. These cells may remain in G0 for long periods of time, even indefinitely, such as with neurons. Cells that are completely differentiated may also enter G0. Some cells stop dividing when issues of sustainability or viability of their daughter cells arise, such as with DNA damage or degradation, a process called cellular senescence. Cellular senescence occurs when normal diploid cells lose the ability to divide, normally after about 50 cell divisions.
    • Synthesis Phase (S): Dividing cells enter the Synthesis (S) phase from G1. For two genetically identical daughter cells to be formed, the cell’s DNA must be copied through DNA replication. When the DNA is replicated, both strands of the double helix are used as templates to produce two new complementary strands. These new strands then hydrogen bond to the template strands and two double helices form. During this phase, the amount of DNA in the cell has effectively doubled, though the cell remains in a diploid state.
    • Growth Phase 2 (G2): The second gap (growth) (G2) phase is a shortened growth period in which many organelles are reproduced or manufactured. Parts necessary for mitosis and cell division are made during G2, including microtubules used in the mitotic spindle.
    Cell Cycle
    Figure \(\PageIndex{2}\): Eukaryotic Cell Cycle. The First Gap (G1), Synthesis (S), and Second Gap (G2) phases make up interphase (I). The mitotic phase (yellow M) includes mitosis (purple M) and cytokinesis. During cytokinesis two cells result. Some cells do not divide and they enter into G0 phage.

    Mitotic Phase

    Before a eukaryotic cell divides, all the DNA in the cell’s multiple chromosomes is replicated. Its organelles are also duplicated. This happens in the interphase. Then, when the cell divides (mitotic phase), it occurs in two major steps, called mitosis and cytokinesis, both of which are described in greater detail in the concept Mitotic Phase: Mitosis and Cytokinesis.

    • The first step in the mitotic phase of a eukaryotic cell is mitosis, a multi-phase process in which the nucleus of the cell divides. During mitosis, the nuclear envelope (membrane) breaks down and later reforms. The chromosomes are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.
    • The second major step is cytokinesis. This step, which occurs in prokaryotic cells as well, is when the cytoplasm divides and two daughter cells form.
    Table \(\PageIndex{2}\): Cell Cycle Summary
    State Name Description
    Quiescent Senescent Resting phase (G0) A resting phase where the cell has left the cycle and has stopped dividing.

    1st growth phase (G1)

    Synthesis phase (S)

    2ndgrowth phase (G2)

    Cells increase in size in G1. Cells perform their normal activities.

    DNA replication occurs during this phase.

    The cell will continue to grow and many organelles will divide during their phase.

    Cell division Mitosis (M) Cell growth stops at this stage. Mitosis divides the nucleus into two nuclei, followed by cytokinesis which divides the cytoplasm. Two genetically identical daughter cells result.

    Control of the Cell Cycle

    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 \(\PageIndex{3}\). There are a number of main checkpoints:

    1. The G1 checkpoint: just before entry into the S phase, makes the key decision of whether the cell big enough to divide. If the cell is not big enough, it goes into the resting period (G0)
    2. DNA synthesis Checkpoint: The S checkpoint determines if the DNA has been replicated properly.
    3. The mitosis checkpoint: This checkpoint ensures that all the chromosomes are properly aligned before the cell is allowed to divide.
    Cell Cycle Checkpoints
    Figure \(\PageIndex{3}\): Checkpoints in the eukaryotic cell cycle ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.

    Cancer and the Cell Cycle

    Cancer is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell’s DNA becomes damaged. This results in mutations in the genes that regulate the cell cycle. Damage can occur due to exposure to hazards such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. They may form a mass of abnormal cells called a tumor (see Figure \(\PageIndex{4}\)). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death. When uncontrolled cell division happens in the bone marrow, abnormal and nonfunctional blood cells are produced because the division is happening before the cell is ready for division. In these types of cancer, there is not any evident tumor.

    Cut our colon with polyps and cancerous tumor
    Figure \(\PageIndex{4}\): Gross appearance of an opened large intestine specimen containing an invasive colorectal carcinoma and two adenomatous polyps. The cancer cells have grown out of control and forming a tumor.
    Feature: Human Biology in the News

    Henrietta Lacks sought treatment for her cancer at Johns Hopkins University Hospital 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 took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, 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 Lacks cells — called HeLa cells for her name — are still alive today. In fact, there are currently many billions of HeLa cells in laboratories around the world!

    Fluorescently dyed HaLe immortal cells
    Figure \(\PageIndex{5}\): Fluorescently dyed HeLa Cells. The woman named Henrietta Lacks 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.

    Why Henrietta Lacks' 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 research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. For example, Jonas Salk 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.

    You would think that Henrietta Lacks' name would be well known in medical history for her unparalleled contributions to biomedical research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot published a nonfiction book about Henrietta Lacks, named The Immortal Life of Henrietta Lacks. Based on a decade of research, the book is riveting, and it 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.

    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. However, there is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.


    1. What are the two main phases of the cell cycle in a eukaryotic cell?
    2. Describe the three phases of interphase in a eukaryotic cell.
    3. Explain how the cell cycle is controlled.
    4. How is cancer-related to the cell cycle?
    5. What are the two major steps of cell division in a eukaryotic cell?
    6. In which phase of the eukaryotic cell cycle do cells typically spend most of their lives?
    7. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.
    8. If there is damage to a gene that encodes for a cell cycle regulatory protein, what do you think might happen? Explain your answer.
    9. True or False. Cells go into G0 if they do not pass the G1 checkpoint.
    10. In which phase within interphase does the cell make final preparations to divide?
      1. Mitosis
      2. Cytokinesis
      3. G2
      4. S
    11. 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?

    Explore More

    The video below discusses the cell cycle and how it relates to cancer.


    1. Woman holding baby by M.T ElGassier, via Unsplash license
    2. Cell cycle by Zephyris, CC BY-SA 3.0 Wikimedia Commons
    3. Cell cycle checkpoints by Lumen Learning, CC BY 4.0
    4. Colon cancer by Emmanuelm, CC BY 3.0 via Wikimedia Commons
    5. HeLa cells by National Institutes of Health (NIH), public domain via Wikimedia Commons
    6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

    This page titled 15.2: Cell Cycle and Cell Division is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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