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During the process of DNA replication, small segments of DNA at each end of the DNA strand(telomeres) are unable to be copied and are lost after each time DNA is duplicated .The telomeres are a region of DNA which code for no proteins; they are simply a repeated code on the end region of DNA that is lost. Eventually, after many divisions, the telomeres become depleted and the cell commences apoptosis.This is a defense mechanism of a cell to prevent replicating error that would cause mutations in DNA
The enzyme telomerase allows for replacement of short bits of DNA known as telomeres, which are otherwise lost when a cell divides via mitosis.In normal circumstances, without the presence of telomerase, if a cell divides recursively, at some point all the progeny will reach their Hayflick limit. With the presence of telomerase, each dividing cell can replace the lost bit of DNA, and any single cell can then divide unbounded. While this unbounded growth property has excited many researchers, caution is warranted in exploiting this property, as exactly this same unbounded growth is a crucial step in enabling cancerous growth.
As a spin-off of the Question-of-the-Week about immortal life forms, we know that cancer cells divide more-or-less continuously, but do "old" cancer cells ever die from "old age", or do they continue to live indefinitely (repairing themselves etc)?