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). Expressed at highest levels in gonads and brain, and at lower levels in heart, spleen, kidney, gill, muscle and skin (PubMed:18312429, PubMed:21547060). Detected in embryonic stem cell lines before and after differentiation (PubMed:21547060). Isoform F is expressed in gonads, with higher levels in testis relative to ovary, but is not detected in other tissues (PubMed:18312429). Isoform B is expressed predominantly in testis (PubMed:21547060). Isoform C is up-regulated in embryonic stem cell lines after differentiation (PubMed:21547060). Expressed at high levels from the embryonic 2-cell stage through to the blastula stage, possibly due to inheritance of maternal transcripts. During later stages levels gradually decline. Belongs to the reverse transcriptase family. Telomerase subfamily.