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The C Word

depends on the type of cell); eventually they die and are replaced with new cells. Cell division is controlled by the DNA, or genes , that exist in each cell. In a healthy person, cell division is a controlled process that only happens when needed. For instance, if you cut your finger, the normal level of cell division in your skin will increase for a time, creating more skin cells to replace the damaged ones. Each new cell

Educational Video Scan this code for a video about the basics of cell division.

will be identical to the previous one, and the process will continue until the injury is healed. At that point, skin-cell division will return to its usual pace. But sometimes cell division goes wrong, and the new cell is not identical to the original. This is called a mutation . Mutations can happen randomly during cell division, when the DNA doesn’t copy correctly, or they can be caused by external factors (more on this in the next section). Cell mutations are actually quite common, and they don’t necessarily cause problems. Much of the time, mutated cells simply don’t survive—the immune systemwipes them out, or they die on their own. But sometimes that doesn’t occur, and the mutated cells are able to reproduce themselves. These are called precancerous cells. Although they continue to fulfill their usual role, they can become a problem if they continue to mutate. Out-of-control cell growth is a bit like the proverbial snowball rolling downhill: the more mutated cells there are, the more mutations will occur, and the harder it becomes for the body to stop the reproduction from happening. This is how cancer begins. Mutated cancer cells reproduce unchecked, and they often destroy nearby healthy cells, too. In time, cancer cells may travel throughout the body, usually through the lymphatic system , and invade other tissues. This

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