During translocation, which site serves as the exit for the uncharged tRNA?

During translocation, which site serves as the exit for the uncharged tRNA?

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

Answer: (A)

The key idea is how tRNA moves through the ribosome during elongation. The ribosome has three tRNA binding sites: A for incoming aminoacyl-tRNA, P for the tRNA holding the growing polypeptide, and E as the exit site. After the peptide bond forms, the ribosome shifts one codon. The tRNA that was in P, now deacylated after transferring its peptide, slides into the E site and exits the ribosome. Meanwhile, the tRNA that came in at the A site moves into the P site, and a new aminoacyl-tRNA will enter the now-free A site. So the exit for the uncharged tRNA is the E site. The A site is for entering the next charged tRNA, and the P site holds the peptidyl-tRNA before it moves to the E site; the F site is not a standard site used in this process.

The key idea is how tRNA moves through the ribosome during elongation. The ribosome has three tRNA binding sites: A for incoming aminoacyl-tRNA, P for the tRNA holding the growing polypeptide, and E as the exit site. After the peptide bond forms, the ribosome shifts one codon. The tRNA that was in P, now deacylated after transferring its peptide, slides into the E site and exits the ribosome. Meanwhile, the tRNA that came in at the A site moves into the P site, and a new aminoacyl-tRNA will enter the now-free A site. So the exit for the uncharged tRNA is the E site. The A site is for entering the next charged tRNA, and the P site holds the peptidyl-tRNA before it moves to the E site; the F site is not a standard site used in this process.