Telomerase is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres.
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. (Beyond unicellular organisms) Telomerase is active in normal stem cells and most cancer cells but is normally absent from, or at very low levels in, most somatic cells.