Fig. 3.

The organization of the genetic code and AARSs. The code is divided into two halves, pro-diversity (unshaded area) and pro-robustness (shaded area), according to sensitivity of the codons to purine (AG) content changes. AARSs are also divided into two types and the Type II enzymes are underlined. There are both types of AARSs for lysine although it is underlined in this figure. After C was recruited as an essential building block, the code was extended to include more amino acids with its C-containing codons. The rule of extension for AARSs followed a G–C conversion trend except the six-fold codons (L, R, and S). For instance, the pairs, such as CAR and GAR, CAY and GAY, GCN and GGN, share the same class of AARSs.