DGI: non-universality of codon->amino acid mapping

From: Ben Houston (ben@exocortex.org)
Date: Mon Apr 08 2002 - 09:14:31 MDT

Evolution never refactors its code. It is far easier for evolution to
stumble over a thousand individual optimizations than for evolution to
stumble over two simultaneous changes which are together beneficial and
separately harmful. The genetic code that specifies the mapping between
codons (a codon is three DNA bases) and the 20 amino acids is
inefficient; it maps 64 possible codons to 20 amino acids plus the stop
code. Why hasn't evolution shifted one of the currently redundant
codons to a new amino acid, thus expanding the range of possible

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

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

Unfortunately, there are very few 'absolute' rules in biology. :-/


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