From: Ben Houston (ben@exocortex.org)
Date: Wed Apr 10 2002 - 17:06:09 MDT
> 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.
I wasn't saying that mitochondria have a different mapping from current
human nuclear DNA but that the mapping that mitochondrion of different
species seems to differ. I'm not sure what you mean by point (a) -- the
mitochondria I am referring to are an important component of prokaryote
cells (which compose all "advanced" multicellular organisms). And in
reference to point (b) -- the change in mitochondrial coding happened
after it was a building block in multicellular organisms.
But this is all details -- I guess I was just sort of uncomfortable with
the original statement that this point consisting of the supposed
"non-mutability of the codon-amino acid mapping" was intended to
support. I don't think that are many significant low-level/archaic
structures that are non-mutable by later evolutionary pressures.
Cheers,
-ben
http://www.exocortex.org
> -----Original Message-----
> From: owner-sl4@sysopmind.com [mailto:owner-sl4@sysopmind.com] On
Behalf
> Of Eliezer S. Yudkowsky
> Sent: Monday, April 08, 2002 11:21 AM
> To: sl4@sysopmind.com
> Subject: Re: DGI: non-universality of codon->amino acid mapping
>
> 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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