Definition
Telomeres are repetitive TTAGGG sequences that cap the ends of chromosomes. Because DNA polymerases cannot fully copy chromosome ends, telomeres shorten with each cell division, and critically short telomeres trigger replicative senescence. Telomerase counteracts this by adding repeats using its own RNA template (TERC) and a reverse transcriptase subunit (TERT). It is active in stem cells, germ cells and most cancer cells but low in most adult somatic cells. Elizabeth Blackburn, Carol Greider and Jack Szostak shared the 2009 Nobel Prize for this work. Telomerase activity is usually measured by the TRAP assay. Some peptides, such as epitalon, have been reported to affect telomerase in cell culture, with limited independent replication.