The enzyme that synthesizes repetitive telomere sequences at chromosome ends is telomerase. It extends the chromosome’s 3′ end by using its own RNA component as a template, solving the end-replication problem that ordinary DNA replication machinery cannot fully overcome.
Telomerase is a ribonucleoprotein whose catalytic protein, telomerase reverse transcriptase, builds DNA from an RNA template. In vertebrates, it adds repeated TTAGGG sequences; these repeats, together with associated proteins, form telomeres. Telomeres help protect chromosome ends from being mistaken for broken DNA or being fused to neighboring chromosomes.
Helicase unwinds DNA, primase makes RNA primers, and DNA polymerases copy DNA, but none is the enzyme specifically responsible for replenishing telomeric repeats. Telomerase activity is high in germ cells, embryonic cells, many stem cells, and most cancers, while most ordinary somatic cells have little or no telomerase activity.
Alexey Olovnikov proposed the compensatory mechanism in 1973. Carol Greider and Elizabeth Blackburn discovered telomerase in the ciliate Tetrahymena in 1984, work that helped earn them and Jack Szostak the 2009 Nobel Prize in Physiology or Medicine.