So Many Cells!
This baby girl 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, 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.
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
The diagram below represents the cell cycle of an eukaryotic 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. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.
Figure 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.
Interphase of the eukaryotic cell cycle can be subdivided into the following three phases, which are represented in the figure above.
- 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 the cell 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 divisionare made during G2, including microtubules used in the mitotic spindle.
Before a eukaryotic cell divides, all the DNA in the cell’s multiple chromosomes is replicated. Its organelles are also duplicated. This happens in 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.
A resting phase where the cell has left the cycle and has stopped dividing.
1st growth phase
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 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 the figure below. There are a number of main checkpoints.
Figure 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.
- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.
- The S checkpoint determines if the DNA has been replicated properly.
- The mitosis checkpoint ensures that all the chromosomes are properly aligned before the cell is allowed to divide.
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. 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 atumor (see the image below). 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 non functional 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.
Figure 4. These cells are cancer cells, growing out of control and forming a tumor.
Feature: Human Biology in the News
Figure 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.
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 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!
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.
If you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot's The Immortal Life of Henrietta Lacks (or watch the movie if it is available). You can also watch this documentary about Henrietta Lacks and her immortal cells:
- The cell cycle is a repeating series of events that include growth, DNA synthesis, and cell division. The cycle is more complicated in eukaryotic than prokaryotic cells.
- In a eukaryotic cell, the cell cycle has two major phases: interphase and mitotic phase. During interphase, the cell grows, performs routine life processes, and prepares to divide. The mitotic phase of the cell cycle in an eukaryotic cell occurs in two major steps: mitosis, when the nucleus divides; and cytokinesis, when the cytoplasm divides and two daughter cells form.
- 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.
- Cancer is a disease that occurs when the cell cycle is no longer regulated, for example, because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.
- What are the two main phases of the cell cycle in a eukaryotic cell?
- Describe the three phases of interphase in a eukaryotic cell.
- Explain how the cell cycle is controlled.
- How is cancer related to the cell cycle?
- Explain why cell division is more complex in eukaryotic than prokaryotic cells.
- What are the two major steps of cell division in a eukaryotic cell?
- In which phase of the eukaryotic cell cycle do cells typically spend most of their lives?
- 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.
- If there is damage to a gene that encodes for a cell cycle regulatory protein, what do you think might happen? Explain your answer.
- True or False. A cell fully splits into two daughter cells during part of interphase.
- True or False. Cells go into G0 if they do not pass the G1 checkpoint.
- True or False. Cytokinesis occurs in both prokaryotes and eukaryotes.
- In which phase within interphase does the cell make final preparations to divide?
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?
Watch the video below to learn more about cell cycle checkpoints.
The video below discusses the cell cycle and how it relates to cancer.
Credit: By National Library of Australia from Canberra, Australia [No restrictions], via Wikimedia Commons;
License: CC BY-NC 3.0
Source: CK-12 Foundation
License: CC BY-NC 3.0
Source: CK-12 Foundation
License: CC BY-NC 3.0
Credit: Ed Uthman, MD;
License: CC BY-NC 3.0
Credit: Harvard University, public domain;
License: CC BY-NC 3.0