Telomerase can extend chromosome ends by adding repeated DNA sequences.
Chromosome ends are protected by structures called telomeres. In humans, telomeres consist mainly of repeated TTAGGG DNA sequences bound by specialized proteins. Because conventional DNA replication cannot fully copy the very end of a linear chromosome, telomeres tend to shorten in many dividing cells.
Telomerase solves this end-replication problem by carrying an internal RNA template and using it to add telomeric repeats. The enzyme is especially active in germ cells, many stem cells, and most cancer cells, while it is low or absent in many ordinary somatic cells.
Telomere shortening is associated with cellular senescence, but it is not a simple standalone measure of ageing. Telomerase activity can help cells keep dividing, yet persistent activation may support cancer development. Telomeres also help distinguish chromosome ends from broken DNA, preventing inappropriate repair reactions.