WHY 4(3) CODONS

An attempt is made to answer the question posed by the title of this p aper. First we show that in primitive self-replicating oligoribotide s ystems, selection depended from the very start on the existence of 4 k inds of ribotides, forming 2 complementary pairs. Further selection re quired that the condensation reactions involving the two last position s of the 3'-end of a growing oligoribotide fragment and the first posi tion of the 5'-end of another fragment were catalyzed by randomly synt hesized peptides. This established a codon - amino acid concentration correlation and clinched triplet segments as the basis of the translat ion process. Finally, physical arguments are given to show that the mo nochirality of the ribotides arose from stereochemical reasons, as fir stly described by Wald, but that of the amino acids is the result of n atural selection acting during the peptide-assisted stage of oligoribo tide growth.