Abstract
The birth of primitive genetic code is discussed based upon the probable synthetic pathways of nucleotide bases and protein amino acids in the course of chemical evolution (Table I).
Protein amino acids can be classified into two groups: purine group amino acids and pyrimidine group amino acids. Purine bases and pyrimidine bases are predominant in primitive two-letter codons of amino acids belonging to the former and the latter group respectively.
Guanine, adenine, and amino acids of purine group may be regarded as synthesized from C1 compounds and N1 compounds, mostly through glycine, in the early stage of chemical evolution.
Uracil, cytosine, and amino acids of pyrimidine group may be regarded as synthesized directly or indirectly from three-carbon chain compounds. It was made possible after the accumulation of three-carbon chain compounds and their derivatives in the primeval sea.
Purine group can be further classified into guanine or (Gly+nC1) subgroup and adenine or (Gly+nC2) subgroup. Pyrimidine group can be further classified into uracil or C3C6C9 subgroup and cytosine or C5-chain subgroup (Table I).
