The number of times our cells can divide is dictated by telomeres, stretches of DNA at the tips of our chromosomes.
The act of preserving telomeres through telomerase is a hallmark of only certain cells, particularly those in developing embryos. In adults, telomerase is active in stem cells, certain immune system cells and, most notably, cancer cells. Understanding how telomeres keep our chromosomes – and by extension, our genomes – intact is an area of intense scientific focus in the fields of both aging and cancer.
"We know that disabling this protein in humans will most likely lead to senescence, which is of particular interest in cancer, because telomere lengthening is one of the ways cancer cells obtain their immortality."
Protein clamps tight to telomeres to help prevent aging ... and support cancer
http://www.eurekalert.org/pub_releases/2010-09/twi-pct091510.php
Telomeres are important to cell division because they serve as sort of a timing mechanism that can, in effect, limit the number of times a normal cell can divide. As each cell divides, it must first replicate – or copy – the DNA of its chromosomes in exacting detail. However, the proteins in cells that make this replication possible physically cannot copy the last few base units of DNA at the tips of the chromosomes, which effectively shortens the telomere each time a chromosome is copied.