Telomerase is the enzyme that adds repetitive DNA sequences to the ends of chromosomes in most eukaryotic cells.
Chromosome ends are called telomeres. In humans, telomeres consist mainly of repeated TTAGGG DNA sequences, with a protective protein complex attached. They prevent chromosome ends from being mistaken for broken DNA and help stop neighboring chromosomes from joining together.
Ordinary DNA polymerases cannot fully copy the very ends of linear chromosomes. Telomerase solves this problem by carrying its own RNA template and extending the telomere, allowing the chromosome-completion machinery to copy further. Telomerase is especially active in germ cells, many stem cells, and most cancer cells.
Telomeres generally become shorter as many ordinary human somatic cells divide, although this is not a simple clock for an individual’s age. Telomerase does not make chromosomes circular, and it does not add unique genes to chromosome ends; it maintains repetitive protective DNA.