It is impossible to work fast and get high quality results. The body faces this challenge when it makes new cells. If it produces cells too fast, it will likely make errors. Not fast enough, and body functions may suffer. In the end, being careful is better than being fast, because errors can lead to cancer.

Why this is important…When the cell cycle is not properly regulated it can result in cancer. Cancer touches almost everyone’s life in some way. At least a third of people will get cancer in their lifetime. Thankfully, many of these cases are treatable. Cancer is different from the contagious diseases we usually think about. It is the result of errors in the inner workings of cells, specifically how cells divide.

Foundations

As we reach adulthood, most of our cells stop dividing. Cells of your muscles, brain and heart haven’t produced new cells since you were very young. Instead of continuing with the cell cycle these cells are content to focus on their job. However, some parts of the body, need to make new cells constantly. Examples of body parts that make new cells frequently include:

skin and hair
lining of the digestive system
bone marrow (where new blood is made)

Since making new cells is costly and needs to be done correctly, this process is carefully regulated with lots of “supervision,” both from within the cell and from its neighboring cells. We have several genes that supervise this process. If something goes wrong with those regulatory genes, cancer is a likely result.

Key terms

Checkpoints – control mechanisms during the cell cycle during which the process of cell division can be stopped in order to correct errors or make up for deficiencies

G₀ phase – resting state when cells are no longer preparing to divide

Apoptosis – process of organized cell death. This happens to cells that are no longer needed or have genetic errors.

Stem cell – cell that is able to divide repeatedly in order to produce new cells

Cancer – disease caused by uncontrolled division of abnormal cells

Details

Stem cells give actively dividing parts of the body the ability to keep making new cells. These cells are programmed to keep producing new cells. Cell production is regulated by chemical signals from hormones and surrounding cells. This coordination helps them produce new cells at just the right pace.

As cells divide, they go through their own internal regulation – a series of checkpoints to make sure the process is working correctly. This ensures that cells don’t divide before they are ready. It also makes sure they catch any problems before new cells inherit them. If problems are too serious and the cell cannot correct it, it always has the option of dying. This may seem drastic, but it prevents disease by stopping errors. When cells die this way, they go through organized steps to “die gracefully” which we call apoptosis.

Explore the following interactive webpage to learn about the checkpoints. Biointeractive: The Eukaryotic Cell Cycle and Cancer. Read about each of the checkpoints, then click on the link in the middle of the circle – Cell Cycle Regulators and Cancer – to read about specific signals. Find answers to the following questions:

What are the 4 checkpoints? What aspects of the cell is checked during these stages?
What is the purpose of p53 and Rb genes?

At each checkpoint, cells respond to the signals that tell them to speed up or slow down cell division. These signals can come from other cells or within the cell itself. The “slow down” signals are especially important. They signal that something is wrong within the cell. If the cell ignores the signal and divides anyway, these errors will be passed down to all of the daughter cells (Figure 2-7). If the problem is an error in the DNA (that is, a mutation), the instructions the daughter cells receive will be faulty. The effects of inheriting faulty genes depends a lot on how important those genes are to that cell.

Diagram of cells dividing - laid out in pyramid. Red star indicates one cell had a mutation which is carried to all cells made from it.

FIGURE 2-7. Mitosis error. An error in once cell, if not corrected, can be passed down to all daughter cells. (“Mitosis error” by J. Buegge is licensed under CC BY-SA 4.0)

Errors to the genes that regulate the cell cycle can result in cancer. These genes, like p53, Rb and BRCA1, help cells catch errors to the DNA after it is copied. When these genes malfunction, cells keep dividing without catching errors. The daughter cells become increasingly abnormal as errors accumulate. The end result is often cancer, which is an abnormal group of cells that is dividing rapidly. These cells usually don’t respond to signals that help regulate their division.

Stop & Think #4: Based on the information about checkpoint genes, how could a person inherit a greater risk of cancer?

Skin is particularly susceptible to cancer. It has two major risk factors:

It already divides frequently
It is exposed to harsh environmental factors, like ultra-violet light and chemicals, that can cause mutations to DNA.

Stop & Think #5: Based on the factors that make skin susceptible to cancer, what other parts of the body are likely to have greater risk of cancer?

Stop & Think #6: Chemotherapy drugs often target cell division. Stopping cell division in cancer helps control the tumor growth, but also stops normal cell division in other parts of the body. How would that cause some of the common side-effects of chemotherapy – hair loss, nausea, and anemia (from too few blood cells)?

Learning objectives

Describe how the cell cycle is regulated.
Describe the role of stem cells in the body and what areas of the body have stem cells.
Explain how problems with cell cycle checkpoint genes can lead to cancer.

Stop & Think Questions Explained

4. A person with two normal genes (alleles) could have one damaged and still function normally. However, if a person inherited one abnormal regulatory gene then any damage to the normal allele would result in an unregulated cell cycle.

5. Any area where cells divide frequently are at higher risk for cancer. Examples include: lining of digestive system and bone marrow.

6. Chemotherapy results in hair loss because hair grows by mitosis; if mitosis stops then hair falls out. Nausea is caused by the lining of the digestive tract getting worn out and irritated. Anemia would result from slowing of blood cell production.