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When Control Is Lost Cancer When U.S. President Richard Nixon recruited biologists to join a “War on Cancer” in 1972, we did not know very much about the causes of cancer, although many Americans were dying of it. In the 30 years since then, biologists have learned a lot. Cancer is a growth defect in cells, a breakdown of the mechanism that controls cell division. Cell Cycle Regulation Certain genes contain the information necessary to make the proteins that regulate cell growth and division. If one of these genes is mutated, the protein may not function, and regulation of cell growth and division can be disrupted. , the uncontrolled growth of cells, may result. Cancer is essentially a disorder of cell division. Cancer cells do not respond normally to the body’s control mechanisms. Some mutations cause cancer by overproducing growth-promoting molecules, thus speeding up the cell cycle. Others cause cancer by inactivating the control proteins that normally act to slow or stop the cell cycle. pg. 3 1 2 3 Although all cancers are not curable, great progress has been made in cancer research over the last 30 years. Cell Quality Control The cell cycle has built-in quality control checkpoints that monitor the cell cycle and can stop it if something goes wrong. For instance, near the end of the G1 stage, the cell monitors its DNA for damage and can stop the cell cycle before entering the S stage of interphase if something is wrong. There are other quality control checkpoints during the S Cancer is caused by mutations, or changes, in segments of DNA that code for production of proteins, including those that regulate the cell cycle. Often, cells can fix mutations in DNA. If the repair system fails, cancer can result. Environmental factors can increase the risk of cancer. stage and after DNA duplication in the G2 stage. During mitosis, the cell checks the spindle fibers before it undergoes cytokinesis. If the cell detects a failure, the cell cycle stops. Sometimes control of the cell cycle fails. When cells do not respond to control mechanisms, cancer results. Cancer is the uncontrolled growth and division of cells. Cancer cells grow and divide as long as they receive nutrients. They crowd normal cells causing tissues and organs to stop working. Cancer can kill an organism. Substances that are known to cause cancer are caled carcinogens (kar SIH nuh junz). Tobacco, tobacco smoke, alcohol, some viruses, and radiation are examples of carcinogens. Avoiding carcinogens can help reduce the risk of cancer. Federal laws protect people from exposure to carcinogens in the workplace and in the food supply. People can reduce their risk of cancer by avoiding all tobacco (including secondhand smoke and smokeless tobacco) and by using sunscreen to protect their skin from ultraviolet radiation from the Sun. 1 2 3 years indicates that mutation of only a few genes can transform normal cells into cancerous ones. All of these cancer-causing genes are involved with regulating how fast cells grow and divide. How is cell division regulated? As a crude analogy, imagine a car parked on the side of a road. To get it going, you Mutations and Cancer must step on the accelerator and release the brake. How many mutations are required to produce cancer? Research in the last several years indicates that mutation of only a few genes can transform normal cells into cancerous ones. All of these cancer-causing genes are involved with regulating how fast cells grow and divide. How is cell division regulated? As a crude analogy, imagine a car parked on the side of a road. To get it going, you must step on the accelerator and release the brake. Stepping on the Accelerator A cell divides when it receives a signal to do so. A “divide” signal is usually in the form of a chemical substance released by another cell. A protein on the surface of the receiving cell binds the substance. This binding activates a second protein inside the cell—relaying the signal from the outside of the cell to the inside. Here, a family of proteins then relays the signal inward to the nucleus. One protein molecule passes the signal to the next like a baton in a relay race. The genes for these signalcarrying proteins are called oncogenes (onco is Greek for “mass” or “tumor.”). If oncogenes are changed by mutation to become more active, cancer can result. Like stepping on the accelerator of a car, an increase in the activity of these proteins amplifies the “divide” signal. This causes the cell to divide more often. Releasing the Brakes At the nucleus, the divide signal overrides a set of genes that act as “brakes.” These braking genes—called tumor suppressor genes— prevent cell division from occurring too often. In cancer, these tumor suppressor genes are damaged. Like removing pressure from the brakes of a car increases a car’s speed, decreasing the activity of tumor suppressors speeds up cell division. Cells have three kinds of tumor suppressors, all of which must be disabled before cancer can occur. The genes for these signal-carrying proteins are called oncogenes. 1 2 Why does cancer run in families? Cancer runs in some families. People might inherit one or more DNA mutations from their parents, increasing their risk of developing cancer. Cancer can occur in people of all ages, but older people have a higher risk. This might be because it takes more than one DNA mutation to change an abnormal cell into a cancer cell. Older cells have had more time to accumulate the mutations that lead to cancer. We now know that many cancers can be largely avoided. To sharply reduce your risk of lung cancer, for example, don’t smoke. Many other cancers can be treated successfully when detected early. Colon cancer, for example, develops slowly from intestinal tissue growths called polyps. A simple medical examination enables the detection and removal of the polyps. Great progress is being made in curing many cancers. Anticancer drugs that stick to these extra cell proteins, gumming them up so they cannot promote excessive cell division, appear to offer great promise.