During translation initiation, the first amino acid-tRNA enters the ribosome at which site?

During translation initiation, the first amino acid-tRNA enters the ribosome at which site?

• A. True
• B. False
• C. Only in prokaryotes
• D. Partly true

Answer: (B)

The key idea is where the initiator tRNA binds to start translation. The initiator tRNA carrying the first amino acid sits in the P site of the ribosome when the start codon is recognized. This positioning is essential because the P site holds the growing peptide chain, and only after initiation does the ribosome shift and make room in the A site for subsequent aminoacyl-tRNAs during elongation. The A site is where new aminoacyl-tRNAs enter during elongation, not at initiation. So it’s false to say the first amino acid-tRNA enters at the A site—the correct site is the P site. In prokaryotes, that initiator tRNA is formylmethionine-tRNA, while in eukaryotes it’s methionine-tRNA.

The key idea is where the initiator tRNA binds to start translation. The initiator tRNA carrying the first amino acid sits in the P site of the ribosome when the start codon is recognized. This positioning is essential because the P site holds the growing peptide chain, and only after initiation does the ribosome shift and make room in the A site for subsequent aminoacyl-tRNAs during elongation. The A site is where new aminoacyl-tRNAs enter during elongation, not at initiation. So it’s false to say the first amino acid-tRNA enters at the A site—the correct site is the P site. In prokaryotes, that initiator tRNA is formylmethionine-tRNA, while in eukaryotes it’s methionine-tRNA.