In the terminology of tRNA binding sites, which site is designated as the start codon?

• A. P - amino acid is transferred to
• B. E - first tRNA exits and 2nd and 3rd tRNA molecules shift opening A site for the next tRNA, continues transfers from P to A site until translation is complete
• C. B - exit site
• D. A - start codon

Answer: (D)

The main idea is how the ribosome uses its three tRNA binding sites during translation. The start signal on the mRNA—AUG—is recognized by the initiator tRNA carrying methionine, and this initiator tRNA binds in the P site. This sets the reading frame and marks the beginning of polypeptide synthesis. After initiation, the next aminoacyl-tRNA enters the A site, a peptide bond forms transferring the growing chain from the P-site tRNA to the A-site tRNA, and the ribosome translocates so the tRNA carrying the now‑polypeptide moves to the E site for exit while the A site becomes ready for the next tRNA. So, the start codon is associated with the P site, not the A site. The A site is for new tRNA entry, and the E site is the exit path for tRNA after it has contributed its amino acid.