Figure 2

Reciprocity of peptides and nucleic acids. Schematic depicting the mutual catalytic dependence between nucleic acids and peptides in living systems, where a dashed arrow from X to Y indicates that there exist reactions involving molecules in Y which are catalysed by molecules in X. While all possible such arrows are present in the diagram, both groups of molecules are “closed” in the sense that there are no reactions combining nucleotide and amino acid monomers in the same polymer.