Figure 3.

The evolutionary scenarios of the genetic code A. The assumed early code that encodes 7 amino acids with triple duplexes, one stop codon, and one start codon. Individual nucleotide may be not necessary to be distinguished other than purines (R) and pyrimidines (Y). B. As the emergence of protein synthesis machinery, the translational machinery may become more precise so that individual nucleotides are recognized. The first-phase code (shaded in green) may have expanded into the second phase where G is involved in protein-coding. Since GU and AG are involved in splice sites, AGN and GUN (shaded in pink) may not be part of the second-phase genetic code.