Point Mutations

John Clark (jonkc@worldnet.att.net)
Fri, 28 Aug 1998 00:22:56 -0400

Hash: SHA1

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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