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On the one hand, somatic cells's DNA age as life goes on, mitosis after mitosis, and that is reflected into reduced telomeres.

On the other hand, during life, reproductive stem cells must age too. However one half of the DNA of some random old reproductive stem cell is eventually transferred through reproduction, and the ageing cycle begins again, with new somatic cells with long telomeres.

What mechanisms can explain reproductive stem cells DNA do not age as somatic cells' DNA ? Or do we know how telomeres are growing back from old reproductive stem cells to gamete during meiosis ? Put another way, how is it possible that we are born with long telomeres ?

I am sorry for the formulation of my question. I think it is understandable but not really well written. However I don't know how to formulate it another way yet. Do not hesitate to reformulate it if you feel like it.

Thanks.

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    $\begingroup$ Some cell types have an active telomerase, stem cells are among these cells. Additionally sperm cells/eggs do not divide any further once they are haploid. $\endgroup$ – Chris Apr 14 '15 at 14:16
  • $\begingroup$ I was not aware of the "active telomerase". It seems to me like some great advantage for a cell. Why would some have it and others not ? $\endgroup$ – Rodolphe Apr 14 '15 at 14:36
  • $\begingroup$ One advantage of somatic cells not having active telomerase enzymes is that they die when become too old / damaged / used etc. and new somatic cells can take their place. These new somatic cells from progenitor cells aka "adult stem cells". This way we can renew our skin, relplace old red blood cells (although they don't even have nucleus) etc. $\endgroup$ – Nandor Poka Apr 14 '15 at 14:56
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    $\begingroup$ We have to be careful about how much we mix telomere length and aging. Yes diseases that cause artificial telomere shortening accelerate aging, but exercise causes widespread telomere shortening but not accelerated aging. And constitutive telomerase expression does not mean that cells are immortal, for example mice have constitutive telomerase expression in every cell. I believe the more plausible advantage to somatic cells not expressing telomerase is reduced transformative capacity because pre-cancers must first find a way to express telomerase. $\endgroup$ – The Nightman Apr 14 '15 at 17:46
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    $\begingroup$ Possible duplicate of How are new people created from the DNA of an aged person. i.e. Why are we young? $\endgroup$ – March Ho Nov 26 '15 at 5:59

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