Using the book as a model (section 39.2, "Aminoacyl-tRNA Synthetases Have Highly Discriminating Amino Acid Activation Sites" and "Proofreading by Aminoacyl-tRNA Synthetases Increases the Fidelity of Protein Synthesis", page 714 in third edition), explain how an alanine tRNA synthetase might prevent the incorrect addition of the similar amino acids glycine and valine to its target tRNA.

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