Primordial reading of genetic information

From the consideration of general features of the anticodon loop and stem in tRNA and the properties of present-day translation, we put forward a plausible scenario to explain the evolution of the genetic code from a highly ambiguous triplet code to the present refined decoding system. Our model based on the reading of the code suggests that the anticodon of primordial tRNA could adopt either the 3' or the 5' stacked conformation permitting the formation of the "best two out of three" base pairs, either the first and second codon position or the second and third. Progressive acquisition of precise structural constraint and the modification of bases in the anticodon loop would give way eventually to the less ambiguous "two out of three" reading mechanism having only the 3' stacked conformation. Further adjustments of base composition and modification leads inevitably to the present generalized code. In this way the primordial code encoding 4-8 amino acids or related derivates evolves smoothly to the present code having 20 amino acids.