After peptide bond formation during elongation, which site contains the unbound tRNA and which site contains the peptidyl-tRNA?

After peptide bond formation during elongation, which site contains the unbound tRNA and which site contains the peptidyl-tRNA?

• A. A site; P site
• B. E site; P site
• C. P site; A site
• D. P site; E site

Answer: (C)

During elongation the ribosome holds tRNAs in three sites: A for incoming aminoacyl-tRNA, P for the tRNA carrying the growing chain, and E for the exit of the deacylated tRNA. When a peptide bond is formed, the growing polypeptide is transferred from the tRNA in the P site to the tRNA in the A site, so the A-site tRNA becomes the peptidyl-tRNA carrying the nascent chain. The tRNA in the P site, now uncharged, remains there briefly and will be moved to the E site and exit after translocation. Therefore, immediately after peptide bond formation, the uncharged tRNA sits in the P site and the peptidyl-tRNA sits in the A site.

During elongation the ribosome holds tRNAs in three sites: A for incoming aminoacyl-tRNA, P for the tRNA carrying the growing chain, and E for the exit of the deacylated tRNA. When a peptide bond is formed, the growing polypeptide is transferred from the tRNA in the P site to the tRNA in the A site, so the A-site tRNA becomes the peptidyl-tRNA carrying the nascent chain. The tRNA in the P site, now uncharged, remains there briefly and will be moved to the E site and exit after translocation. Therefore, immediately after peptide bond formation, the uncharged tRNA sits in the P site and the peptidyl-tRNA sits in the A site.