Figure 1.

Proposed hairpin duplication origin of tRNA. Hairpin monomer (top left) is in equilibrium with partial duplex (top middle), both of which are able to be specifically aminoacylated with glycine by the RNA predecessor of contemporary glycyl-tRNA synthetase (bottom left and middle). The defining moment in the origin of tRNA was the ligation of the partial duplex, which created a covalently joined molecule (top right), anticodon loop, and anticodon from the 3'-terminal CCA sequence of the upstream hairpin [1,2,32]. Mutations (principally in the central loops) produced the precursor to contemporary glycine tRNA (bottom right). Subsequent duplication and mutation to re-evolve the amino acid-specific RNA operational code sequences of the other amino acid-accepting hairpins (with accompanying mutation of the anticodon) led to a proliferation of tRNA sequences and, eventually, coded protein synthesis.