ABSTRACT
Aberrant codon usages typically seen in unicellular eukaryotes with AT-rich genomes, as well as in mitochondria with extremely small genomes, appear to indicate that there has never been a priori coupling between two parts of transfer ribonucleic acid molecule. These are: the upper domain, recognized by an amino acid acylating enzyme and a lower domain containing an anticodon. CG deficiency is generally attributed to CG methylation; methylated CG being converted to TG or CA. As CG methylation is extremely widespread, affecting prokaryotes and eukaryotes alike, the universality of CG deficiency, indeed, appears explainable by CG methylation. The universal codon assignment must be viewed as a fortunate frozen accident. The fraction of base trimers to be used as codons is determined by a reading frame choice in translation of a set of repeating base tetramers in which a given trimer resides.
