I think you can invoke an argument of parsimony in this explanation; selection
should favor the simplest system. Our present repertoire of enzymes and
structural proteins do very well within the limits of 20-ish amino acid
forms. Our systems accomplish what they need to accomplish under these
circumstances. It would take a lot more components to deal with synthesizing
a lot of D aa's as well as L aa's, plus you would have to activate the tRNAs
and modify the meanings of codons too. It's a lot of work for what appears
to be superfluous variation.
--
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* Lou Hom * "All folks are family." *
* lhom at ocf.berkeley.edu * -- John Saponara *
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