How many tRNA binding sites are on a ribosome?

• A. Two
• B. Three
• C. Four
• D. Five

Answer: (B)

Three tRNA binding sites are used by the ribosome to coordinate translation: A, P, and E. The A site accepts the next aminoacyl-tRNA that matches the codon on the mRNA. The growing polypeptide is held at the P site on the peptidyl-tRNA, where peptide bond formation occurs between the new amino acid and the growing chain. The deacylated tRNA exits through the E site after its amino acid has been transferred. As the ribosome moves along the mRNA, tRNAs sequentially shift from A to P to E, enabling continuous elongation. Options with two, four, or five sites don’t fit this well-established mechanism, since three sites are needed to accommodate the incoming tRNA, the growing chain, and the exiting tRNA.