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. Identify 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