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somatic nonreproduc-tive; not an egg or sperm stem cell cell capable of differentiating into multiple other cell types germ cells cells creating eggs or sperm fibroblasts undifferenti-ated cells that normally give rise to connective tissue cells

Using what is called the TRAP assay (telomeric repeat amplification protocol), it has been found that about 90 percent of all human tumors produce telomerase, whereas the only normal adult somatic cells that produce telomerase are stem cell populations found, for example, in skin, the hematopoietic system, germ cells, and gut epithelia. In fact, the presence or absence of telomerase is the most specific property that distinguishes cancer cells from normal cells. This difference is currently under investigation as a diagnostic tool. If a chemical could be found to interfere with telom-erase activity in cancer cells, an effective control of this disease might be found. Several candidate substances have been identified and are undergoing extensive studies in animals.

Telomerase is switched on in virtually all human cells at the moment of conception, but as the embryo matures the telomerase becomes repressed in all but the germ cells and stem cell populations. Further, the level of telomerase expressed in stem cells is much less than that expressed in cancer cells. Interestingly, telomerase expression has been found to occur in all the cells of animals that age slowly or not at all. These are animals, such as the American lobster and the rainbow trout, that do not stabilize at a fixed size in adulthood.

On the human genome, an enzyme known as human telomerase reverse transcriptase (hTERT) is found on the most distal gene on chromosome 5p. The transfection (introduction) of hTERT into cultured normal human fibroblasts has resulted in telomere elongation, telomerase expression, and the immortalization of these otherwise mortal cells. After several hundred population doublings, the transfected cells exhibit some drift from the diploid number of chromosomes but cancer cell properties do not occur. This experiment proves that telomerase is not a cancer enzyme but an immortalization enzyme. The ability to immortalize normal human cells via hTERT has important potential applications. Some immortalized cells could be cultured in the lab to produce therapeutically useful molecules. Others might be used directly within the body to repair tissue or replace lost or damaged cells. see also Chromosome, Eukaryotic; DNA Polymerases; Replication; Reverse Transcriptase.

Leonard Hayflick

Bibliography

Bodnar, A. G., et al. "Extension of Life Span by Introduction of Telomerase into Normal Human Cells." Science 279 (1998): 349-352.

Greider, Carol W. "Telomeres and Senescence: The History, the Experiment, the Future." Current Biology 8 (1998): 178-181.

Hayflick, Leonard. How and Why We Age. New York: Ballantine Books, 1996.

-. "The Illusion of Cell Immortality." British Journal of Cancer 83 (2000):

841-846.

Transcription

Transcription is the process in which genetic information stored in a strand of DNA is copied into a strand of RNA. The sequence of the four bases in DNA, which are adenine (A), cytosine (C), guanine (G), and thymine (T), is preserved in the sequence of the four bases in RNA, which are A, C, G, and uracil (U).

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