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. Required to prevent genome instability induced by breakage-fusion-bridge (BFB) cycles. Can extend completely non-telomeric sequences using RNA template in vitro. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Component of the telomerase ribonucleoprotein complex (Probable). Interacts with POT1A. Low rates of telomere shortening at each generation. Null mutant plants can survive up to 10 generations before dying. Accelerated proliferation and cell death in dedifferentiated cells. In the absence of telomerase, telomeres decline by approximately 500 bp per generation, a rate 10 times slower than seen in telomerase-deficient mice. This may be due to alternative telomere lengthening, a process which is activated in early embryonic development. Belongs to the reverse transcriptase family. Telomerase subfamily.