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 EST2 and the telomerase RNA template component. Deletion causes telomere shortening and senescence. Belongs to the reverse transcriptase family. Telomerase subfamily.