The origin of the genetic code is probably the central problem of the studies on the origin of life. The key question to answer is the molecular mechanism that allows the association of the amino acids with their triplet codons. We proposed that the codon-anticodon duplex located in the acceptor stem of primitive tRNAs would facilitate the chemical reactions required to synthesize cognate amino acids from simple amino acids (glycine, valine, and aspartic acid) linked to the 3' acceptor end. In our view, various nucleotide-A-derived cofactors (with reactive chemical groups) may be attached to the codon-anticodon duplex, which allows group-transferring reactions from cofactors to simple amino acids, thereby producing the final amino acid. The nucleotide-A-derived cofactors could be incorporated into the RNA duplex (helix) by docking Adenosine (cofactor) into the minor groove via an interaction similar to the A-minor motif, forming a base triple between Adenosine and one complementary base pair of the duplex. Furthermore, we propose that this codon-anticodon duplex could initially catalyze a self-aminoacylation reaction with a simple amino acid. Therefore, the sequence of bases in the codon-anticodon duplex would determine the reactions that occurred during the formation of new amino acids for selective binding of nucleotide-A-derived cofactors.