Telomere

Telomeres are repeating DNA sequences capping each chromosome, protecting the coding sequence from degradation and from being mistaken for a broken strand. They shorten with each cell division, and when they become critically short the cell stops dividing — cellular senescence.

It matters because telomere length is one of the most-studied biological markers of ageing, and shorter leukocyte telomeres associate with smoking, obesity, chronic stress and cardiovascular disease. The enzyme telomerase can extend them, and is notably active in most cancers.

In practice the direction of causation is far from settled — telomere shortening may be as much a consequence of biological stress as a cause of ageing. Consumer telomere-length tests have limited reliability and no established clinical use, so they belong in the curiosity category rather than the health-decision one.