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) Component of the telomerase holoenzyme complex composed minimally of the catalytic subunit p123 and the telomerase RNA template component. Belongs to the reverse transcriptase family. Telomerase subfamily.