Chapter 2, Section 4
What Is Cancer?
Cancer is a disease in which cells grow and divide uncontrollably, damaging the parts of the body around them. Cancer is something like weeds in a garden. Weeds can overrun the garden plants, robbing them of the space, sunlight, and water they need. Similarly, cancer cells can overrun normal cells.
Cancer is actually not just one disease. In fact, there are more than 100 types of cancer. Cancer can occur in almost any part of the body. Cancers are often named by the place in the body where they begin. For example, lung cancer begins in the tissues of the lungs. In the United States today, lung cancer is the leading cause of cancer deaths among both men and women.
Figure 16A Breast Cancer Cell A cancer tumor begins as a single cell. A mutation in the cell’s DNA disrupts the normal cell cycle. Relating Cause And Effect How does the cell behave as a result of the mutation?
How Cancer Begins
Scientists think that cancer begins when something damages a portion of the DNA in a chromosome. The damage causes a change in the DNA called a mutation. DNA contains all the instructions necessary for life. Damage to the DNA can cause cells to function abnormally.
Normally, the cells in one part of the body live in harmony with the cells around them. Cells that go through the cell cycle divide in a controlled way. Cancer begins when mutations disrupt the normal cell cycle, causing cells to divide in an uncontrolled way. Without the normal controls on the cell cycle, the cells grow too large and divide too often.
At first, one cell develops in an abnormal way. As the cell divides over and over, the repeated divisions produce more and more abnormal cells. In time, these cells form a tumor. A tumor is a mass of abnormal cells that develops when cancerous cells divide and grow uncontrollably.
How Cancer Spreads
Figure 17 shows how a tumor forms. Tumors often take years to grow to a noticeable size. During that time, the cells become more and more abnormal as they continue to divide. Some of the cancerous cells may break off the tumor and enter the bloodstream. In this way, the cancer can spread to other areas of the body.
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Figure 17How Cancer Spreads A cancerous tumor is a mass of cells that divide uncontrollably. Interpreting Diagrams How can cancer spread from one part of the body to another?
