Biology as Poetry: Genetics

Bacteriophage Ecology Group


Ability of tRNAs to recognize more than one codon.

The idea of wobble has nothing to do with the lack of ambiguity of the genetic code (each codon specific only one amino acid) but is very much a component of the redundancy of the genetic code (amino acids in many cases can be specified by more than one codon). That is, given wobble, it is possible for more than one codon to specify a given amino acid but without having tRNAs that correspond specifically to each of those codons.

The variation among different codons that can be recognized by a single tRNA, as a consequence of wobble, all occurs in the third base. Indeed, when one considers the redundancy of the genetic code, the variation among synonymous codons occurs, at a minimum, at the third base.

Five amino acids in particular are encoded by four codons that differ solely in terms of the third base. These are alanine (GCX), glycine (GGX), proline (CCX), threonine (ACX), and valine (GUX). Three additional amino acids are encoded by codons that, in part, span one such combination: arginine (CGX), and leucine (CUX) and serine (UCX). Isoleucine nearly spans a complete set (AUX) with the exception of AUG, the start codon, which specifies methionine instead.

With the exception of methionine and tryptophan, all other amino acids – (asparagine, aspartate (a.k.a., aspartic acid), cysteine, glutamate (a.k.a., glutamic acid), glutamine, histidine, lysine, phenylalanine, tyrosine – are encoded by two codons which differ only at the third base. Redundancy in the genetic code, in other words, more often than not is specified by the third base of codons and wobble allows tRNAs to recognize codons that differ only at their third base.

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