The chemical mechanisms used to avoid errors in protein synthesis are different from those used during DNA replication. DNA polymerases use a 3 ′ to 5 ′ exonuclease proofreading activity to remove mispaired nucleotides incorrectly inserted into a growing DNA strand. There is no analogous proofreading function on ribosomes. Instead, proofreading for protein synthesis is carried out by the aminoacyl‑tRNA synthetases. ldentify the mechanisms aminoacyl-tRNA synthetases use to ensure that the correct amino acid is attached to the appropriate tRNA.(A) To distinguish between two amino acids with similar structures, some aminoacyl-tRNA synthetases use a proofreading site that recognizes and hydrolyzes the incorrect amino acid.(B) The active site in each aminoacyl-tRNA synthetase has a high specificity for the correct amino acid substrate. After binding the correct amino acid, an aminoacyl-tRNA synthetase recruits the appropriate tRNA to the active site by recognizing a number of sequences and structural features in a specific tRNA.(C) Aminoacyl-tRNA synthetases have an additional active site that binds to non-cognate tRNAs. The tRNAs that bind to this second active are hydrolyzed and released from the enzyme.(D) First, the aminoacyl-tRNA synthetase binds to its cognate tRNA by recognizing a number of specific sequences and structural features in the tRNA. Next, the synthetase-tRNA complex recruits the correct amino acid to the enzyme active site.(E) Aminoacyl-tRNA synthetases can rapidly hydrolyze the ester linkage between incorrectly paired RNAs and amino acids