The six-base DNA sequence TTAGGG is repeated at the ends of human chromosomes to form telomeres.
Telomeres protect chromosome ends from being mistaken for broken DNA. They also help prevent chromosome ends from sticking to one another. Because ordinary DNA replication cannot fully copy the very end of a linear chromosome, telomeres generally become shorter in many dividing somatic cells.
The enzyme telomerase can restore telomeric repeats by using an internal RNA template. Telomerase is active in many stem cells, germ cells, and cancer cells, but is usually limited in most mature human somatic tissues.
Telomere shortening is not a simple biological clock that predicts an individual’s exact lifespan. Telomere length varies among people and tissues, and it is affected by genetics, cell division, disease, and environmental factors.