At each round of DNA replication, for some cells telomerase adds onto the existing telomere ends the TTAGGG repeats that would otherwise have been lost, thus maintaining telomere length. Telomerase thus selectively acts as an "immortalizing" enzyme. (It also has several other associated proteins.) Since human fibroblast cells (in virtually all tissues) normally lack active telomerase, they will double only ~50 times in culture from fetal tissue. Then further cell reproduction destructively starts losing needed chromosome ends. The individual is then effectively (usefully) aging future cells to death.
Back in the distant past if some species had escaped this useful valuable chromosome DNA shortening nature, (so necessary to achieve fatal aging to help compel and perpetuate continued evolution), such that their adults weren't dying when aging rather old, then the longer they had thus harmlessly aged, while thus not having to often reproduce, then they would have failed ever more badly to keep up with other better more rapidly reproducing (and dying) and thus quicker evolving and better advancing competing species.
But some cells becoming mutated cancerous may unfortunately harmfully become protected to keep forever reproducing, by activating telomerase within themselves. About 9/10 of all human tumors produce telomerase. The only normal adult somatic cells that produce telomerase are stem cell populations found e.g. in skin, the hematopoietic system, germ cells, and gut epithelia.
Telomerase is the most specific property that distinguishes cancer cells from normal cells. If some method could be found to interfere with telomerase activity in cancer cells, then an effective control against cancer might be developed. 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. In most multicellular eukaryotic organisms, telomerase is active only in germ cells, stem cells, and certain white blood cells which require extensive cell division.
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Evolution and the nature of aging (prophet666.com)