What happens to telomeres as a cell divides repeatedly?

As a cell divides repeatedly, the telomeres at the ends of its chromosomes gradually shorten. This occurs because the DNA replication machinery cannot fully replicate the very ends of chromosomes, leading to the progressive loss of telomeric DNA with each cell division. After many divisions, the telomeres become critically short, which triggers a cell to enter senescence (permanent growth arrest) or undergo apoptosis (programmed cell death). This process serves as a mechanism to limit the potential for uncontrolled cell growth and the accumulation of genetic mutations, but it also contributes to aging.