Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3′ end of telomeres. A telomere is a region of repetitive sequences at each end of a chromosome. Telomeres protect the end of the chromosome from DNA damage or from fusion with neighboring chromosomes.
Telomerase is a reverse transcriptase enzyme that carries its own RNA molecule (e.g., with the sequence 3′-CCCAAUCCC-5′ in Trypanosoma brucei) which is used as a template when it elongates telomeres. Telomerase is active in gametes and most cancer cells, but is normally absent from, or at very low levels in, most somatic cells.
Telomerase restores short bits of DNA known as telomeres, which are otherwise shortened when a cell divides via mitosis.
In normal circumstances, where telomerase is absent, if a cell divides recursively, at some point the progeny reach their Hayflick limit, which is believed to be between 50–70 cell divisions. At the limit the cells become senescent and cell division stops. Telomerase allows each offspring to replace the lost bit of DNA, allowing the cell line to divide without ever reaching the limit. This same unbounded growth is a feature of cancerous growth.
Some experiments have raised questions on whether telomerase can be used as an anti-aging therapy, namely, the fact that mice with elevated levels of telomerase have higher cancer incidence and hence do not live longer. On the other hand, one study showed that activating telomerase in cancer-resistant mice by over-expressing its catalytic subunit extended lifespan.
A study that focused on Ashkenazi Jews found that long-lived subjects inherited a hyperactive version of telomerase.