The protective caps on your chromosomes wear down every time a cell divides
Telomeres are repetitive DNA sequences capping the ends of chromosomes, and they shorten a little with each cell division. Elizabeth Blackburn, Carol Greider and Jack Szostak discovered how telomeres and the enzyme telomerase protect chromosomes from degrading — work that won them the 2009 Nobel Prize in Physiology or Medicine. Telomere length has since become one of the most studied biological markers of cellular ageing.