From: Eliezer S. Yudkowsky (sentience@pobox.com)
Date: Mon Apr 08 2002 - 09:21:19 MDT
Ben Houston wrote:
>
> Mitochondrial mRNA, unlike standard DNA, uses UGA to encode tryptophan
> rather than the usual terminator code. Most animal mitochondria use AUA
> to code for methionine instead of isoleucine. Vertebrate mitochondria
> use AGA and AGG to code for the terminator code rather than the usual
> arginine. Yeast mitochondria assign all codons that begin with CU to
> threonine instead of leucine. And some unicellular eukaryotes use one
> or two of the codons that usually are the terminator code for amino
> acids.
>
> Also in some organisms UGA, usually a stop code, can code for the rare
> amino acid selenocysteine -- it is not part of the standard 20 amino
> acids.
Obviously the genetic code did evolve originally, and it is obviously not
impossible that mitochondria should have a different mapping, if either (a)
they diverged from modern multicellular animal life before the genetic code
became fixed or (b) they or one of their ancestral stages were simple enough
(had a small enough base of genetic complexity) that the DNA could go on
evolving, i.e., occasionally jump to a new mapping without destroying the
whole organism. I am simply pointing out that relative to a complex
organism such as humans, DNA is absolutely fixed because of the number of
simultaneous dependencies.
-- -- -- -- --
Eliezer S. Yudkowsky http://intelligence.org/
Research Fellow, Singularity Institute for Artificial Intelligence
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