Fig. 6.

The nucleopeptide Initial Darwinian Ancestor. In this cartoon model, a short strand of XNA has the functionality of both a primordial p-mRNA and a p-Rib. Primitive XNA molecules loaded with amino acids (p-tRNA) bind to the p-Rib via codon–anticodon pairing. This allows adjacent amino acids to undergo peptide bond formation and a short peptide chain is produced. A certain peptide sequence is able to act as a primordial XNA-dependent XNA polymerase (p-Pol) able to copy both + and – p-Rib strands to eventually produce a copy of the p-Rib(+) strand.