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. 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. 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. 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. 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. aminoacyl-trna synthetases can rapidly hydrolyze the ester linkage between incorrectly paired rnas and amino acids