Gzh

Maintained by the ik^c National Science / ■ \ rO. Teachers Association figure 11-7

Mutations in proto-oncogenes or tumor-suppressor genes can destroy normal gene functioning, possibly resulting in cancer. A mutation in a proto-oncogene may cause the gene to become an oncogene, a gene that can trigger cancer.

Proto-oncogenes

Tumor-suppressor genes

Normal

Code for proteins that help regulate cell division

Normal

Code for proteins that prevent uncontrolled cell division co ConnectionJ

Secondhand Tobacco Smoke

In 1992, the Environmental Protection Agency (EPA) declared secondhand tobacco smoke, also called environmental tobacco smoke, to be a human carcinogen. According to the EPA standards, secondhand smoke contains more than 3,000 chemical compounds, including four known human carcinogens and several other probable human carcinogens. In fact, the air in an enclosed room of smokers could contain up to six times the air pollution of a busy highway.

Thousands of nonsmokers die of lung cancer each year as a result of breathing secondhand tobacco smoke. There is no safe level of tobacco smoke. As more is learned about the contents and effects of tobacco smoke, regulations to protect people from secondhand smoke are being enacted across the United States. In fact, smoking indoors is now prohibited in many public places.

Gene Expression in Cancer

The expression of oncogenes and mutated tumor-suppressor genes causes cancer cells to behave differently than normal cells. A normal cell must be attached to other cells, to a membrane, or to fibers between cells in order to divide. Also, normal mammalian cells will stop dividing after about 40 cell divisions—or sooner if they become too crowded. Cancer cells, however, continue to divide indefinitely, even when they are very densely packed, seemingly ignoring the normal cellular message to stop dividing. They also continue dividing even after they are no longer attached to other cells, a trait that facilitates the spread of cancer cells throughout the body. The spread of cancer cells beyond their original site is called metastasis (muh-TAS-tuh-sis). When metastasis occurs, cancer cells can invade healthy tissues and begin forming new tumors.

Causes of Cancer

The mutations that alter the expression of genes coding for cell-regulating proteins can occur spontaneously but are more likely to occur as a result of the organism's exposure to a carcinogen (kahr-SIN-uh-juhn). A carcinogen is any substance that can induce or promote cancer. Most carcinogens are mutagens (MYOOT-uh-JUHNZ), agents that cause mutations to occur within a cell.

Well-known carcinogens include the chemicals in tobacco smoke, asbestos, and ionizing radiation, such as X rays or ultraviolet light from the sun. For example, tobacco smoke, shown in Figure 11-8, has been found to be the cause of more than 85 percent of all lung cancers. Certain viruses can cause cancer. Many viral genes are actually oncogenes. Viruses can also stimulate uncontrolled growth in host cells by causing mutations in proto-oncogenes or tumor-suppressor genes, thus accelerating the rate of cell division in the host cell. Or they may activate the cell's own oncogenes. Viruses have been found to cause some cancers in blood-forming tissues, and the human papilloma virus has been shown to cause cervical cancer.

figure 11-8

Tobacco smoke contains more than 60 known carcinogens, including cyanide, formaldehyde, and lead.

1992 The first DNA chip patent is issued.

1997 Dolly, a successful clone of an adult sheep, is born.

1998 The first genome sequence of a multicellular organism, the roundworm, is completed.

2002 Mouse genome sequencing is completed, one year ahead of schedule.

Timeline

1983 The polymerase chain reaction is invented.

1990 The Human

Genome Project is launched.

1992 The first DNA chip patent is issued.

1997 Dolly, a successful clone of an adult sheep, is born.

1998 The first genome sequence of a multicellular organism, the roundworm, is completed.

2002 Mouse genome sequencing is completed, one year ahead of schedule.

2003 Sequencing of the human genome is completed.

Was this article helpful?

0 0
Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook


Post a comment