FACTORS THAT ACTIVATE SENESCENCE
Many different factors activate senescence in cells and have been implicated in the aging process. In normal cells, the length of the telomeres controls whether or not cells senesce. During the replication of chromosomes, DNA polymerase cannot replicate the DNA at the distal tips of the chromosomes. Consequently, each chromosome shortens each time the cell divides ( Fig. 20.1 ). When the telomeres reach a critically short length, the cell enters senescence.
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