On the origin of the genetic code

It is argued that three chemical criteria determined the evolution of the genetic code: codon-anticodon pairing; codon-amino acid pairing; amino acid pairing. The first criterium determined the set of interactive nucleotides; the second, the set of nucleotides interactive with amino acids; the third, the set of mutually interactive amino acids. The code resulted from the intersection of these sets. This hypothesis explains the specificity and universality of the code as well as the “choice” of the standard amino acids and nucleotides from among those available in nature. The specific mechanism for codon-amino acid pairing assumed here is the “backwards” (Crick, 1967) Pelc-Welton (1966) models. Three types of evidence support “backwards” pairing: parallel genetic coding of amino acid pairs (Root-Bernstein, 1982); results of binding experiments by Saxinger and Ponnamperuma (1974); reinterpretation of Jungck's (1978) correlations between the properties of amino acids and their respective anticodon nucleotides. The inversion of the code to its present state occurred as a result of the evolution of tRNA molecules which supplanted parallel codon-amino acid interactions with antiparallel codon-anticodon ones. The paper concludes with suggestions for testing the hypothesis.