From: John Clark (jonkc@worldnet.att.net)
Date: Thu Aug 27 1998 - 22:22:56 MDT
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Damien Broderick <damien@ariel.ucs.unimelb.edu.au> Wrote:
>My point is that this mutation calls for a purine
>double ring to be changed deftly into a pyramidine single ring,
>with all manner of small but precise replacements of Ns and
>Os and Hs. [...] But how the hell does the equivalent of a
> random impact do that?
A mutation doesn't have to slowly create a purine or pyramidine
anymore than a rock randomly thrown on a keyboard must create
all the intricate loops and squiggles of an ASCII character.
Bases
of all sorts are ubiquitous in a cell, all a mutation needs to
do is
insert a wrong base into a sequence when the double helix
unzips itself into two single helixes, and this must happen
whenever DNA duplicates itself. The 2 single strands then
rebuild
their missing stands using bases that already exist until you
have
2 double helix strands where you only had one before.
Adenine normally pairs with thymine but about one base in 10^5
is in a different tautomeric form than normal, that is, a
hydrogen
atom is in a slightly different place. With such abnormal bases
adenine will bind with cytosine and fit into the DNA helix
perfectly.
Also, guanine normally binds with cytosine but even with the
normal tautomeric form it can be made to bind with thymine,
not as well as with cytosine and it puts strain on the DNA helix
but if something like radiation or a very reactive chemical
gives
it an extra push it can snap into place. If you have a big
enough
hammer you can make almost anything fit. Proofreading
enzymes correct most of these copying errors but about
one in 10^9 slips through and as a result changes the blueprint
for a protein.
John K Clark jonkc@well.com
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