CANCER: A DISEASE OF THE CELL CYCLE

Adapted from The American Cancer Society (cancer.org) and “Cancer Cells vs. Normal Cells” by Dr. Lynne Eldridge What Is Cancer?

Cancer is the general name for a group of more than 100 diseases. Although there are many kinds of cancer, all cancers start because abnormal cells grow out of control. Untreated cancers can cause serious illness and death. How Common is Cancer?

About half of all men and one-third of all women in the US will develop cancer during their lifetimes.

The risk of developing most types of cancer can be reduced by changes in a person’s lifestyle, for instance, by staying away from tobacco, limiting time in the sun, and being physically active and eating healthy foods. Normal Cells in the Body

The body is made up of trillions of living cells. Normal body cells grow, divide to make new cells, and die in an orderly way. During the early years of a person’s life, normal cells divide faster to allow the person to grow. After the person becomes an adult, most cells divide only to replace worn-out or dying cells or to repair injuries. How Cancer Starts

Cancer starts when cells in a part of the body start to grow out of control. Cancer cell growth is different from normal cell growth. Instead of dying, cancer cells continue to grow and form new, abnormal cells. Cancer cells can also invade (grow into) other tissues, something that normal cells can’t do. Growing out of control and invading other tissues are what makes a cell a cancer cell.

Cells become cancer cells because of DNA (deoxyribonucleic acid) damage. DNA is in every cell and it directs all its actions. In a normal cell, when DNA is damaged the cell either repairs the damage or dies. In cancer cells, the damaged DNA is not repaired, but the cell doesn’t die like it should. Instead, the cell goes on making new cells that the body doesn’t need. These new cells all have the same damaged DNA as the first abnormal cell does.

People can inherit abnormal DNA (it’s passed on from their parents), but most often DNA damage is caused by mistakes that happen while the normal cell is reproducing or by something in the environment. Sometimes the cause of the DNA damage may be something obvious like cigarette smokie or sun exposure. But it’s rare to know exactly what caused any one person’s cancer.

In most cases, the cancer cells form a tumor. Over time, the tumors can replace normal tissue, crowd it, or push it aside. This is what causes the person to become ill. How Cancer Spreads

Cancer cells often travel to other parts of the body where they can grow and form new tumors. This happens when the cancer cells get into the body’s bloodstream or lymph vessels. The process of cancer spreading is called metastasis. No matter where a cancer may spread, it’s always named based on the place where it started. For example, colon cancer that has spread to the liver is called metastatic colon cancer, not liver cancer. In this case, cancer cells taken from the liver would be the same as those in the colon. They would be treated in the same ways too.

Cancer Cells vs Normal Cells

• Growth – Normal cells stop growing (reproducing) when enough cells are present. For example, if cells are being produced to repair a cut in the skin, when the repair work is done, cells are no longer reproduced to fill in the hole. In contrast, cancer cells don’t stop growing when there are enough cells present. This continued growth often results in a tumor (a cluster of cancer cells) being formed.

• Rate of growth – Normal cells spend a significant portion of their life cycle in interphase, carrying out normal cell processes before dividing. Cancer cells spend less time in interphase and reproduce rapidly before the cells have had a chance to mature.

• Communication – Cancer cells don’t interact with other cells as normal cells do. Normal cells respond to signals sent from other nearby cells that say, essentially, “you’ve reached your boundary.” When normal cells “hear” these signals they stop growing. Cancer cells do not respond to these signals.

• Cell repair and cell death – Normal cells are either repaired or die (undergo apoptosis) when they are damaged or get old. Cancer cells are either not repaired or do not undergo apoptosis.

• Ability to Metastasize (Spread) – Normal cells stay in the area of the body where they belong. For example, lung cells remain in the lungs. Cancer cells, because they lack the adhesion molecules that cause the cells to “stick” to nearby cells, are able to travel via the bloodstream and lymphatic system to other regions of the body. Once they arrive in a new region (such as lymph nodes, the lungs, the liver, or the bones) they begin to grow, often forming tumors far removed from the original tumor.

• Appearance – Under a microscope, normal cells and cancer cells may look quite different. In contrast to normal cells, cancer cells often exhibit much more variability in cell size – some are larger than normal and some are smaller than normal. In addition, cancer cells often have an abnormal shape, both of the cell, and of the nucleus (the “brain” of the cell.) The nucleus appears both larger and darker than normal cells. The reason for the darkness is that the nucleus of cancer cells contains excess DNA. Up close, cancer cells often have an abnormal number of chromosomes that are arranged is a disorganized fashion.

Reading: Cancer - A Disease of the Cell Cycle Period: Name: Date:

1. Please read the article. As you read, highlight ways in which cancer cells are DIFFERENT from normal cells. 2. Check your understanding of the article by sorting the slips of paper at your table with your partner. Some slips describe characteristics of normal cells, while others describe characteristics of cancer cells. 3. Cancer is often called “a disease of the cell cycle.” With your partner, brainstorm specific ideas from the article that support this idea. Then, decide on the top 5 pieces of evidence that explain & support this idea.

BRAINSTORMING SPACE: TOP 5 PIECES OF EVIDENCE: 1.

2.

3.

4.

5.

4. When you are finished, please read the article on the back. It is about a treatment for cancer called chemotherapy. Answer the questions at the end. (You may discuss your thinking with your partner, but make sure your final answers are written in your own words.) Treating Cancer: How Chemotherapy Works Adapted from: American Cancer Society (www.cancer.org)

To understand how chemotherapy works, it helps to understand the normal life cycle of a cell, or the cell cycle. All living tissue is made up of cells. The cell cycle is a series of steps that both normal cells and cancer cells go through in order to form new cells.

The cell cycle has 5 phases which are labeled below using letters and numbers. Since cell reproduction happens over and over, the cell cycle is shown as a circle. All the steps lead back to the resting phase (G0), which is the starting point.

After a cell reproduces, the 2 new cells are identical. Each of the 2 cells made from the first cell can go through this cell cycle again when new cells are needed.

The Cell Cycle G0 phase (resting stage): The cell has not yet started to divide. Cells spend much of their lives in this phase. Depending on the type of cell, G0 can last from a few hours to a few years. When the cell gets a signal to reproduce, it moves into the G1 phase. G1 phase: During this phase, the cell starts making more proteins and growing larger, so the new cells will be of normal size. This phase lasts about 18 to 30 hours. S phase: In the S phase, the chromosomes containing the genetic code (DNA) are copied so that both of the new cells formed will have matching strands of DNA. The S phase lasts about 18 to 20 hours. G2 phase: In the G2 phase, the cell checks the DNA and gets ready to start splitting into 2 cells. This phase lasts from 2 to 10 hours. M phase (mitosis): In this phase, which lasts only 30 to 60 minutes, the cell actually splits into 2 new cells.

Normal cells spend most of their life cycle in the G0 (resting) phase. They only get the signal to enter G1 and start growing and reproducing to replace cells that have been injured or have worn-out. However, cancer cells divide much more often than normal cells. So, chemotherapy drugs target and kill cells that are in the process of dividing. Because cancer cells divide much more often than normal cells, they are more likely to be killed by chemotherapy.

Different chemotherapy drugs focus on different stages of the cell cycle. For example, some drugs interfere with a cell’s ability to replicate its DNA during the S phase, while others interfere with the cell’s ability to divide its DNA during mitosis. Often, multiple drugs are used together to be more effective. Doctors also use their understanding of the timing of the cell cycle to plan how often each dose should be given.

While chemotherapy is meant to target and kill the rapidly dividing cancer cells, it also affects healthy body tissues where the cells are constantly growing and dividing. (For example, the cells in hair follicles are constantly growing, as are the cells that line the digestive system.) This is what causes side effects like hair loss and nausea.

Even though these side effects are unpleasant for cancer patients, the damage to the healthy cells usually does not last. Once the chemotherapy treatment is over, the normal cells can replace or repair the healthy cells that were lost.

Check for Understanding: Please answer the following on a separate piece of paper.

1) How does the cell cycle of cancerous cells differ from the cell cycle of healthy cells?

2) How do chemotherapy drugs take advantage of these differences?

3) How do doctors use their understanding of the cell cycle to more effectively plan & administer chemotherapy?