Telomeres are the repeated DNA sequences found at the ends of chromosomes. A little of that length is lost with each cell division, and this serves as a part of the mechanism that limits the number of times a somatic cell can divide. Stem cells employ telomerase to maintain long telomeres through the asymmetric divisions needed to supply tissues with new daughter somatic cells equipped with long telomeres. This split of responsibilities between many restricted cells and a few privileged cells is the primary strategy by which multicellular organisms keep the risk of cancer low enough for evolutionary success.
Given this arrangement, average telomere length in any given tissue is a blurred measure of how fast cells divide and how frequently new cells are delivered by the supporting stem cell population. Over large populations of people, shorter telomere length tends to correlate with greater age, most likely because stem cell activity declines with age. Unfortunately, it is the case that telomere length as presently measured – in leukocytes from a blood sample – is quite dynamic in response to day to day environmental circumstance, and is thus only poorly correlated to aging
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