Explain what occurs at the A, P, and E sites during the translation of mRNA into a polypeptide.

The A site is where the mRNA is recognized by the appropriate tRNA, which has an amino acid bound ready to add to the growing peptide chain. The P site is where translation is initiated by the association of the methionine tRNA with the mRNA and the ribosome. The P site is also the site of the formation of peptide bonds that are catalyzed by the polypeptide transferase, which uses the hydrolysis of GTP to drive the reaction. As the chain moves along, the empty tRNA exits the ribosome at the E site.