The stereochemical consideration on the structure of direct RNA templates (DRTs) for peptide synthesis under the principle of replication parsimony. The dark gray, black and light gray portions of the RNAs correspond to the aa-binding sites for three different amino acids, the square, circle and triangle, respectively. (a) If the aa-binding sites in a DRT were organized one after another, stereochemical limitation at their joints (empty arrows) would prevent the folding to bring the amino acids adjacent. Note: it is for convenience to draw the bound amino acid inside the internal loop, with the understanding that the nucleotide residues in other portions of the aptamer domain might also participate in the binding. (b) The stereochemical limitation could be overcome by the introduction of spacer sequences (dashed lines) at the joints. (c) The stereochemical limitation might also be "compensated" by the spatial stretch of adaptors (S-shape lines, only as an abstract structural model), which recognize the template RNA at one end while charge amino acids at the other end (Left). In the CCH hypothesis , the spatial stretch of the supposed adaptors, each composed of a nucleotide triplet (anticodon) charged with an amino acid, is limited (Right). Though the distance between the amino acids in this case would be only in measure of about three nucleotides, the amino acids could not reach each other yet, especially considering that an amino acid is even much smaller than a single nucleotide.