Telomeres are actually composed of a short sequence of nucleotides repeated over and over. During DNA replication, DNA polymerase is unable to copy chromosomes all the way to the tip. In human cells, telomeric segments are lost as a cell ages until they decline to a threshold length that signals the cell to simply stop reproducing. This loss of the capability to divide may be the cause for some of the effects of human aging, such as a reduced ability to repair local wounds and damage to blood vessel walls.
Some human cells, such as embryonic cells and cells in the gonads, undergo repeated cell division without aging. In these cells, an enzyme called telomerase takes over to replicate the telomeres and to prevent the shortening of chromosomes and eventual death of the cells.
However, researchers recently found telomerase in human cancer-cell lines in the laboratory and in ovarian tumors in the human body. Scientists believe that normal cells are able to respond to the signal given by the shortened telomeres to stop dividing. However, cells with genetic mutations that cause them to ignore this signal will continue to divide. Some cells may die when they reach a point when the telomeres are almost completely gone. Other cells may continue to obtain genetic derangements and may start to synthesize telomerase. Now the shortened telomeres will be maintained by the telomerase, and the cell will continue to live and gain the immortality of cancer cells.
Exciting possibilities exist in cancer therapy as the presence of telomerase in human cancers may serve as a target for anti-cancer drugs. Treatments that disrupt the functioning of telomerase may kill tumor cells without harming normal cells. However, there are some normal cells, such as cells in the bone marrow, which may manufacture and use telomerase in their normal activities. Researchers do note though that the telomeric lengths of cancer cells are so short that the cancer cells may die before normal cells with longer telomeres are harmed.
Source: "When 'immortal' cancer cells lose their edge" by Boyce Rensenberger, Washington Post National Weekly Edition September 11-17, 1995