Proposed hairpin duplication origin of tRNA, based on Di Giulio [31–33]. RNA hairpin (left) was specifically aminoacylated with glycine, enabling it to participate in noncoded peptide synthesis. The hairpin monomer was in equilibrium with the partial duplex (middle), which underwent ligation to form a covalently joined molecule possessing an anticodon loop with the anticodon derived from the 3'-terminal CCA sequence of the upstream hairpin. Mutations produced the first tRNAGly (far right), also a substrate for noncoded protein synthesis. Subsequent gene duplication and mutation led to a proliferation of tRNA molecules with different amino acid specificities.