On the origin of the genetic code

A series of stages in the evolution of the genetic code is postulated, representing a chain of logical steps that leads to the present-day code. The stages described are based on translation machinery between the RNA world and that of amino acids, a model that consists of an RNA assembler strand along which RNA hairpin molecules are lined up, forming a picket-fence-like aggregate. Each hairpin carries an amino acid at the bottom of one of its legs, and the mutual proximity of amino acids achieved in this way facilitates their linkage into oligopeptides, in a sequence governed by the nucleotide sequence along the assembler strand, the code. The order in which amino acids are introduced into the code is in the approximate order of their availability, tempered by polarity and structural considerations.