Telomerase is a ribonucleoprotein enzyme essential for the replication of chromosome termini in most eukaryotes. It elongates telomeres. It is a reverse transcriptase that adds simple sequence repeats to chromosome ends by copying a template sequence within the RNA component of the enzyme. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Catalytic subunit of the telomerase holoenzyme complex composed minimally of TERT and the telomerase RNA template component (TERC). Detected at highest levels in gill, ovary and testis, and at lower levels in brain, eye, heart, skin, spleen and stomach. Expressed at higher levels in actively dividing cells. Belongs to the reverse transcriptase family. Telomerase subfamily.