Origin of the translation system and the genetic code in the RNA World. Step 6: Amino-acid-specific variants of RNA T evolve by duplication and subfunctionalization. The specifics of T RNA-amino acid interaction depend on the mode amino acid recognition, CRM, ARM, or FAM. A. ARM: recognition of the amino acid residue by the anticodon loop and of the amino acid backbone by the stem of the RNA T. B. ARM: formation of dimers facilitates the stereochemically unhindered binding of amino acids. C. CRM: RNA T exists in two alternative folding conformations. In one conformation, the codon is complementary paired with the anticodon; in the other confirmation, the codon binds a cognate amino acid, and the anticodon is exposed. D. FAM: the amino acid is recognized by an "ad hoc" site unrelated to the codon or the anticodon.