Codons and hypercycles.

Several hypotheses on the origin of codon assignments imply that the present protein synthesizing machinery was already in place when the assignments were made. These are examined by computer modeling. The results do not suggest that assignments were optimized for resistance to reading and mutation errors, nor that the assignments are random. It is improbable that the number of species of amino acids increased in the course of evolution. An originally ambiguous dictionary is likely to have been subject to error catastrophe and is improbable. A relation between amino acid properties and their codons exists, and suggests that the codon assignments were established at the time of origin of the hypercycle, i.e. a system of aminoacyl synthetases which attaches amino acids to tRNA, and before the present protein synthesizing machinery was in place. The origin of a hypercycle is only possible if the system began with components which were catalytically active even when they did not form a self-replicating system. A model of such a system is proposed.