Re: Why are we allowed to age?
Carl Feynman (carlf@atg.com)
Thu, 12 Jun 1997 14:14:56 -0400
At 08:37 AM 6/12/97 -0700, Hal Finney wrote:
>Is it just that the genes can't do any better, that despite their best
>efforts, errors and waste accumulate and the cells just don't work
>as well? Why then are egg cells kept so fresh and clean generation
>after generation? Think about it: every cell in your body is the direct
>offspring via fission of an earlier cell, and that line *literally* goes
>back billions of years. Somehow it was able to keep itself clean and
>avoid accumulated junk, until the last couple of dozen cell generations,
>since it became part of your kidney or whatever. All of a sudden it's
>just too difficult to prevent damage? After millions and millions
>of generations? That's hard to believe.
The egg and sperm cells ('germ line') avoid accumulated junk by killing
mutants. The number of DNA changes per human reproduction is on the order
of five, i.e. an error rate per base of 10^-9. Most of the changes either
don't make any difference, or are so bad that the embryo is spontaneously
aborted. The remainder are fodder for natural selection.
I don't think the mutation rate is different for germ line than for other
('somatic') cells. Remember that a single wrong mutation in a somatic cell
can cause cancer, and there are trillions of somatic cells, some of which
reproduce every few weeks.
--CarlF