the foutain of youth.

Allen Smith allens at yang.earlham.edu
Fri Apr 24 13:57:29 EST 1992


In article <9204201824.AA19658 at rust.zso.dec.com>, french at RUST.ZSO.DEC.COM writes
   :
> In article <9204180147.AA16796 at genbank.bio.net> allens at yang.earlham.edu
> (Allen Smith) writes:
>
>> Children do tend to have mutations. However, most mutations appear
>> to be neutral, or at least insufficiently harmful to have a significant
>> effect. Those which aren't are eliminated by natural selection ...
>>
>> There is the protective mechanism (to the mother) of spontaneous
>> abortion; through this mechanism, the mother is kept from having to put
>> energy, etc. into maintaining a defective fetus.
>
> If DNA damage actually does play a major role in aging, then
> the parent's DNA will contain more errors than their offspring.
> This can only be explained by one of the following mechanisms or
> possibly of combination of them working together:
>
>     1. The DNA contained in germ cells is much more immune to the damage
>        mechanisms that introduce errors into the DNA of somatic cells.
>
>     2. A near-perfect error detection mechanism causes germ cells
>        to self destruct if they contain errors.
>
>     3. A near-perfect error correction mechanism removes errors
>        from the parent's DNA when fertilization occurs.
>
> Furthermore, any such mechanism must reduce the overall DNA error rate
> down to the rate of heritable mutations.
>
> If somatic cell DNA is subject to massive error rates then I don't
> believe that propositions 1 and 2 are sufficient to explain the
> relatively low error rate in the DNA of children.  Germ cells are
> subject to the same sources of DNA damage as somatic cells, eggs
> spend a prolonged time waiting in line in the ovaries, and yet most
> eggs remain fertile.
>
> I would expect eggs to self-destruct (and spontaneous abortions) to
> occur at a much higher rate than they actually do unless an error
> correction mechanism was also at work that corrected DNA errors
> when fertilization occurs.  Such an error correction mechanism
> could easily explain the relatively low rate of DNA errors in
> children - all that's required is two independant information
> sources (which are supplied through parent's chromosomes).
>
>
        I suspect that DNA damage is indeed involved in aging- but the
mechanisms that prevent it are one of the things that turn off in old age.
If someone has some data on DNA repair efficiency as related to aging,
that would be nice.
        Spontaneous abortions are at a much higher rate than most people
realize. Generally, they happen before the mother is even aware of the
pregnancy; she will appear to simply have a heavier, possibly off-timed,
period.
        Given that much DNA damage takes place at the time of replication,
it's unsurprising that egg cells wouldn't have that much damage. They
don't replicate after some point (I believe either infancy or in the fetal
state). I'm not sure off-hand how many divisions they are from the initial
fertilized egg, but I'd suspect them not to be very many away.
        -Allen




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