Previous evidence has suggested that telomere length is crucial to determining the life span of cells. But in cells in which we activated the telomerase enzyme, we observed cells thriving despite the fact that their telomeres were very short. Telomerase synthesizes telomeres, snippets of DNA on the ends of chromosomes that function in part like the plastic tips on the ends of shoelaces, preventing the gene-bearing, threadlike spindles from unraveling. With each cycle of cell division, the tips of telomeres drop off, until the chemical bookends become so eroded that the chromosomes become unstable, a condition that signals cells to stop dividing. This natural control on cell division is known as the Hayflick limit.
Cancers develop when cells increasingly ignore or override such signals of chromosome impairment.
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