February 19, 2004 | David F. Coppedge

How to Get a Genetic Code by Chance

The Feb. 17 issue of Current Biology1 has a Q&A magazine feature on the genetic code.  After dismissing some myths about it being universal, consisting of only 20 amino acids and obligated to only three codons (there are some minor exceptions to these mostly-true principles: see 04/30/2003), the authors tackle the big question: where did it come from?

I heard about a ‘frozen accident’�
One of the first proposals, in 1968, for the origin of the code, was Francis Crick’s ‘frozen accident’ model.  But the discovery of alternative codes showed that the code is not frozen.  And similar codons are assigned to similar amino acids, indicating that the code is not an accident.

So, how did the code evolve?
There are several theories that try to explain the origin of the code.  Most can be classified in one of three major groups.

Chemical: posits that direct chemical interactions between amino acids and their cognate codons/anticodons influenced codon assignment.  Studies of binding of RNA aptamers to amino acids showed that, for at least some amino acids – arginine, tyrosine and isoleucine – such chemical interactions do exist.  These theories fail to explain the assignment of codons that do not show direct interactions to their cognate amino acids.
Historical: proposes that an initially smaller code grew by incorporation of new amino acids. For example, new amino acids may have captured codons from their metabolic precursors, contributing to the assignment of similar amino acids to similar codons.
Selection: suggests that the code was selected to minimize the phenotypic effects of point mutations.  The code’s organization supports this: nonsynonymous substitutions often lead to replacement of an amino acid by one chemically similar, causing little disruption in the protein.
Accumulating evidence for these models suggests that they are not mutually exclusive.  Rather, the code probably evolved by an interplay among some or all of them.  Direct interactions of short RNA molecules and amino acids may have fixed the assignment of certain codons, while subsequent assignments may have been driven by history and selection.

(Emphasis in original.)

1Andre R.O. Cavalcanti and Laura F. Landweber, “Magazine: Genetic Code,” Current Biology Vol 14, R147, 17 February 2004.

They just violated Occam’s razor.  They also violated the rule that three wrongs don’t make a right. 

  • The “Chemical” theory is the old biological predestination idea that Dean Kenyon abandoned.  If RNA happens to bind to three amino acids better than the 17 others, that does not explain how they subsequently linked via peptide bonds to form a polypeptide with any catalytic activity.  Amino acids do not have the ability to link up by themselves.  Getting just one element of the complex protein machinery that can translate DNA and construct a protein is astronomically improbable, to put it mildly (see our online book).
  • The “Historical” theory is hysterical, because it personifies amino acids.  One cannot ascribe purposeful processes to chemicals.  No cheating with natural selection, either; it cannot even begin to a player unless an accurate system of self-replication is already working.
  • The “Selection” Theory also personifies the chemicals: the code was selected to minimize … point mutations”  Enough of this passive-voice nonsense.  Who selected it, and why would he/she/it want to, if not to optimize the system?  The sentence makes perfect sense in intelligent design theory, but is bizarre otherwise.  No cheating with natural selection here, either.
        The authors committed one more foul: card stacking.  All their theories assume naturalistic evolution.  They left out the only theory that explains the observations without violating Occam’s razor: intelligent design.
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