  Information theoretic analysis of genetic languages indicates that the
naturally occurring 20 amino acids and the triplet genetic code arose by
duplication of 10 amino acids of class-II and a doublet genetic code having
codons NNY and anticodons $\overleftarrow{\rm GNN}$. Evidence for this scenario
is presented based on the properties of aminoacyl-tRNA synthetases, amino acids
and nucleotide bases.
