From: Terry W. Colvin (fortean1@mindspring.com)
Date: Tue Dec 03 2002 - 11:53:34 MST
a non-biologist's question (forwarded):
> I don't think that the artice explains why such harmful
> genes/proteins do not accumulate in gametes, which
> must afterall be very old indeed.
> My understanding is that DNA molecules come equiped with
> a chain of residual non-genetic material on the end of
> each chromosome, that when deviding is successively
> reduced in size until, after a certain number of
> divisions, the cells are unable to successfuly multiply.
> The method of cell production which preserves these chains
> is expensive and not normally done except for the case of
> gametes. Thus in old age there are less cells available to
> perform the growth required for continual repair and
> deterioration is seen.
> The specific genes that might be damaged are simply the
> ones at the ends of DNA chains.
> How's that for a theory? I don't rate the significance of
> the two genes they identified.
> Just a thought.
--------------------
> a non-biologist's question (forwarded):
>
> > I don't think that the artice explains why such harmful
> > genes/proteins do not accumulate in gametes, which
> > must afterall be very old indeed.
Gametes lead relatively protected lives, and thus accumulate damage slower
than the typical somatic (body) cell. Gametes do get genetic damage, but
damaged gametes are selected against during reproduction, and are less
likely to be passed to progeny. Some damage does get passed on, and it is
seen in genetic drift, and is the source of variation which is seen over
time in the evolution of a lineage. A relatively undamaged pair of
gametes is selected and passed into each offspring, but an animal's body
has to live with all its cells, each accumulating damage and each changing
more and more as time goes on from the starting genotype.
Cells can escape non-genetic damage such as goop like lipofuscin by
outgrowing it, growing and dividing faster than the goop accumulates. Or
when a cell divides, the goop can end up mostly in one cell, and the
other becomes heathier. An animal, of course, stops growing, and goop
accumulates. Single-celled organisms like bacteria and gametes can use
these strategies to escape aging.
> > My understanding is that DNA molecules come equiped with
> > a chain of residual non-genetic material on the end of
> > each chromosome, that when deviding is successively
> > reduced in size until, after a certain number of
> > divisions, the cells are unable to successfuly multiply.
These are telomeres. In the worms in the paper, C. elegans, the cells
divide in an exactly choregraphed pattern and then stop dividing when the
worm becomes an adult. These initial adult cells age during the aging of
the worm. Because of this, telomeres aren't thought to play a role in
worm aging.
> > The method of cell production which preserves these chains
> > is expensive and not normally done except for the case of
> > gametes. Thus in old age there are less cells available to
> > perform the growth required for continual repair and
> > deterioration is seen.
In dividing cells with telomerase active, the telomere length is
maintained. Having telomerase on is not 'expensive', but has drawbacks
which is likely why it isn't on in most somatic (body) cells. Aging of
the body's tissues is partially due to declining numbers and turnover of
frequently replaced cells, and partially due to accumulating damage to
the stem cells and permanent cells.
> > The specific genes that might be damaged are simply the
> > ones at the ends of DNA chains.
This doesn't happen, AFAIK.
Jim Lund
--
Terry W. Colvin, Sierra Vista, Arizona (USA) < fortean1@mindspring.com >
Alternate: < fortean1@msn.com >
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