What are the three sites of the ribosome and what occurs at each site?

• A. A site accepts aminoacyl-tRNA; P site holds the growing polypeptide; E site is where empty tRNAs exit
• B. A site binds the ribosome; P site degrades misfolded proteins; E site initiates translation
• C. A site catalyzes peptide bonds; P site opens the ribosomal subunits; E site terminates translation
• D. A site holds the growing polypeptide; P site accepts aminoacyl-tRNA; E site binds mRNA

Answer: (C)

In translation, the ribosome has three tRNA-binding sites that work in order: A, P, and E. The A site is where an aminoacyl-tRNA enters and pairs with the mRNA codon to bring in the next amino acid. The P site holds the tRNA carrying the growing polypeptide chain, so the chain is extended there as the peptide bond forms. The E site serves as the exit for tRNAs that have already donated their amino acid and are leaving the ribosome.

The actual peptide bond is formed by the ribosome’s peptidyl transferase activity in the large subunit, not by the A site itself. After bonding, the tRNAs shift positions: the one at A moves to P, and the one at P moves to E and exits.