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The question "why do we age" has been asked numerous times. But why are we young? The cells of the adult human being are an age (time>0), but how can old cells create new cells that are younger than themselves?

I.e, how are young people made from the cells of old people?

I realize that senescence, the act of a cell aging, has to do with the shortening of telomerase over time which puts a limit (the Hayflick limit) on the life of an autosomal cell. So in theory, because cells only make direct copies of themselves but the telomerase shortens with every division, each new daughter pair would be as old as the parent and would continue aging. But how come new people start senescence at time t=0?

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Telomeres shorten with every cell division. Telomerase is an enzyme that adds extra copies to the ends of telomeres, extending a cell's lifespan. The assumption that telomeres only ever get shorter is incorrect (though that doesn't invalidate the rest of your question). –  octern Apr 21 '13 at 3:53
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Gametes (sperm and ovum), which fuse to form a zygote, arise from germ cells (spermatogonia and oogonia). Germ cells, like stem cells, are maintained carefully i.e the genome is preserved and transposition/recombination events are tightly controlled via different mechanisms.

So these germ cells don't have shortened telomeres. Also, during early embryonic development the future germ cells quickly migrate to an end of embryo and remain there while the rest of the body takes shape.

I'm not sure if it has been demonstrated in an article or not but once I happened to talk to Dr Wakayama (who cloned a mouse frozen for 16 years) and got to know that animals cloned by the process of Somatic Cell Nuclear Transfer have reduced lifespan. (SCNT: the nucleus of a somatic cell is transferred to an ovum and triggered to form zygote).

So, to conclude, if germ cells were not protected from mutations and deleterious recombinations, lifespan would have been reduced with every generation.

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I guess it would be good if someone could give a word about epigenetics too! Gametes do not carry the same epigenetics modifications than cells of old people. –  Remi.b Jul 3 at 22:04
    
That is a good point. It is observed (to an extent) that transfer of somatic nucleus to egg cytoplasm causes epigenetic reprogramming. However, I am sure that the old epigenetic marks are not completely erased. –  WYSIWYG Jul 4 at 4:59
    
And I guess it is intuitive that no two cell types have same epigenetic marks. –  WYSIWYG Jul 4 at 5:02
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