Telomeres are nucleoprotein structures that comprise repetitive nucleotide sequences bound to specialized proteins at the ends (tails) of chromosomes. Often compared to the plastic tips on shoelaces, telomeres cap the ends of chromosomes to prevent them unravelling or tangling. When a cell divides, it loses base pairs from the ends of its telomeres so that, with ageing, the telomeres become shorter; cells with critically short telomeres undergo senescence (also known as the Hayflick limit). As well as normal ageing, several different premature ageing diseases are associated with telomere shortening, a process that affects the cellular and metabolic machinery.