On the origin of the genetic code.

Mechanisms underlying how the genetic code was generated by Darwinian selection have remained elusive since the code was cracked in 1965. Here, I propose a hypothesis on the emergence of the genetic code and predict that its emergence was driven by sequential distinct selective pressures. According to the hypothesis, aminoacyl-RNAs for Glu, Asp, Lys, Tyr, His, Arg, Cys and Ser were first selected as cartridge-type subunits of three-subunit ribozymes. Aminoacyl-RNA subunits acting as cofactors were accommodated by the proto P-site of the large subunit of ribozymes. Importantly, I predict that there was no direct relationship between amino acids and codon and anticodon pairs. Duplication of the proto P-site could have created the proto A-site, enabling multi-subunit ribozymes to simultaneously interact with two-cartridge-type aminoacyl-RNA subunits. Random insertion of two cartridges would have instantly abolished enzymatic activity of multi-subunit ribozymes. On the other hand, if two tandemly aligned pairs of codons and anticodons specify two cartridges, dozens of different active pockets in multi-subunit ribozymes would have rapidly emerged, leading to the rise of extant organisms' metabolic pathways. The strong driving force of Darwinian selection described here could have created the primary genetic code for catalytic amino acids. Evolution of the protein translation system and events leading to the expansion of the genetic code until the time it was "frozen" are presented in detail.