  To synthesize peptides alongside the RNAs making the so-called RNA world,
some genetic coding involving RNA had to develop. Herein, it is proposed that
the first real-coding setup was a direct one, made up of continuous
poly-tRNA-like molecules, with each tRNA-like moiety carrying, beyond and near
its 5 prime or 3 prime end, a trinucleotide site for specific amino acid
binding: the sequence and continuity of the tRNA moieties of a particular
poly-tRNA would ensure the sequence and continuity of the amino acids of the
corresponding peptide or small protein. In parallel with these particular
entities, and enhancing their peptide-forming function, a proto-ribosome and
primitive amino acid-activation system would develop. At some stage, one
critical innovation would be the appearance of RNA fragments that could tighten
several adjacent tRNA moieties together on a particular poly-tRNA molecule, by
pairing with the second trinucleotide sequence (identical to the first one
carrying the specific amino acid-binding site) situated at, or close to, the
middle of each tRNA moiety (i.e., the present anticodon site). These RNA
fragments, acting as authentic co-ribozymes in the peptide-synthesizing
apparatus, would constitute the ancestors of the present mRNAs. Later, on these
mRNA-like guiding fragments, free tRNA forms would be additionally used, first
keeping their amino acid-binding sites, then losing them in favor of a specific
amino acid attachment at a CCA arm at their 3 prime end. Finally, these latter
mechanisms would progressively prevail, leading to the modern and universal
indirect genetic coding system. Experimental and theoretical arguments are
presented and discussed in favor of such a scenario for the origin and
evolution of genetic coding.
