Telomere, << TEHL uh muhr or TEE luh muhr, >> is a protective structure that forms the very ends of chromosomes in most organisms. Chromosomes are threadlike parts in cells that carry genetic (hereditary) information. This information is encoded in the structure of a chainlike molecule called DNA (deoxyribonucleic acid). A telomere consists of a particular, highly repetitive sequence of DNA at the chromosome’s end, along with specialized proteins that form a protective “cap.“ The cap prevents the end of the chromosome from fraying, much as the plastic tips of shoelaces prevent the strands from unraveling. Telomeres are necessary to ensure that the entire chromosome is duplicated during the cell division that occurs in growth. They also enable a cell to distinguish a break in the chromosome—which must be repaired—from the chromosome’s natural end. Many scientists believe that telomeres play an important role in cancer and in other diseases associated with aging.
Function.
Each time a cell divides, it must copy the DNA in its chromosomes. The copying is done by a particular enzyme. Enzymes are molecules that control and speed up certain chemical reactions in the body. The enzyme that copies DNA, however, cannot copy all the way to the end of a chromosome. As a result, some DNA is lost from the chromosome’s end. If the chromosome did not have a telomere, such a loss might damage an important gene, a sequence of DNA that carries the instructions for making a particular protein. The DNA lost from the telomere, on the other hand, is highly repetitive and does not contain genes. Thus, although the telomere grows shorter in copying, the chromosome’s genes are preserved.
In most organisms, an enzyme called telomerase << tehl uh MEER ays >> replenishes telomeres. Telomerase extends the telomere by adding new units of telomeric DNA to its end. This lengthening compensates for the loss of DNA that occurs during cell division. However, many cells—including most cell types in the human body—do not have sufficient amounts of telomerase to completely counteract the shortening of telomeres. Scientists have thus observed that telomeres tend to shorten with age.
Role in illness and aging.
If telomeres become too short, the ends of the chromosomes can become “sticky.” Such sticky ends may fuse to other portions of the chromosome or to other telomeres. These fused chromosomes can break and reform in harmful rearrangements. Scientists believe the shortening of telomeres may thus be a major cause of genetic illnesses associated with aging. For example, one of the earliest changes known to occur in the development of cancers is that the chromosomes are disturbed or rearranged in some cells.
Telomerase may also play a role in cancer. Cancer cells undergo rapid, uncontrolled division. Such growth might be expected to quickly deplete the cancer cells’ telomeres, limiting the spread of the disease. However, cancer cells often contain abnormally high levels of telomerase. Scientists believe that certain changes occur in cancer cells to enable these cells to produce telomerase. Researchers have developed sensitive techniques to detect telomerase in the body. They hope to use these techniques to diagnose cancer at an early stage, when it can be treated more effectively. Scientists are also trying to develop anticancer drugs that work by blocking the action of telomerase. The drugs, called telomerase inhibitors, would prevent cancer cells from replenishing telomeres and thus stop the cells from dividing uncontrollably.
Telomeres may play a broader role in aging. Many scientists think that the number of times a cell can divide is limited in part by the length of its telomeres. They have observed that when human cells are grown in a laboratory, the cells’ telomeres shorten until the cells can no longer divide. Such cells often do not function as well as they did when they were younger. However, scientists can artificially rejuvenate the cells using telomerase. This fact makes scientists wonder whether many of the effects of aging result from the gradual shortening of telomeres in the cells of the body.
