n. a structure found at the ends of chromosomes that protects fragments from rejoining after a chromosome has divided. Telomeres become progressively shorter with each cell division, and after a certain number of cell divisions, they become so short that they trigger the cell to stop dividing or to undergo programmed cell death. Telomeres are said to act as a genetic clock that ticks down the number of times the cell can divide safely. See also Hayflick limit.