Re: The Limits of Organic Life in Planetary Systems

On Jul 10, 5:13 pm, c...@xxxxxxx wrote:

Well, now we get into chemical evolutionary constraints. For
instance, adenine condenses from five molecules of HCN rather
easily. It's going to be preferred in early synthesis.

Sure. Nobody's saying that the Standard 20 are completely random.
But there's no reason to think evolution was compelled to settle on
20, nor on those particular 20.


As for four letters -- or, I should say, 2 x 2 -- it's also
a local optimum. You'd need six codons to code for an amino
acid with a palette of ~20 if the genetic code were 1 x 2,
with little redundancy in the coding system. For a 3 x 2
code, you'd only need two codons, but you'd also have very
little redundancy (and in this case, it would have to
depend on the chemical properties of the codons themselves).

2x2, 3 codons -> 64, which seems rather a lot of redundancy. But who
knows? We're not even sure which came first.


Yeah, this has been done in the lab for at least a
generation. Also, google selenocysteine for a surprise.

What an interesting mechanism. So, really there are 65 codon
combinations, producing 21 acids. At least.

I wonder that it isn't used more. Could we have an alien biochem that
has more amino acids than codon outcomes, because different codon sets
do different things, depending? We have a proof-of-concept...

Anyway. As you point out, the Twenty are a mixture of "you'd expect
this" and "you'd expect something like this". What I wonder is, are
there amino acids that would seem plausible candidates -- because
they're simple, arise easily from precursors, are favored in plausible
reactions, and/or do something useful -- but that aren't in the
Twenty? IOW, are there Pete Best amino acids?


Doug M.

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