Figure 5-19 (a) Translating nucleic acid sequence into amino acid sequence requires two steps. Step 1: An aminoacyl-tRNA synthetase couples a specific amino acid to its corresponding tRNA. Step 2: The anticodon base-pairs with a codon in the mRNA specifying that amino acid. (b) Molecular model of the human mitochondrial aminoacyl-tRNA synthetase for Phe in complex with tRNA^Phe.