Which initiation step involves detection of the start codon?

Which initiation step involves detection of the start codon?

• A. Detection: of start codon
• B. Binding of initiator tRNA to small subunit
• C. Attachment: of large subunit
• D. Binding: of small subunit w/ tRNA to mRNA

Answer: (A)

Recognizing the exact start site is the key moment in initiation. The initiator tRNA carrying the first amino acid pairs its anticodon with the start codon in the mRNA, positioning in the P site of the small ribosomal subunit. This codon–anticodon interaction confirms AUG as the correct start and sets the reading frame, allowing initiation factors to promote joining of the large subunit and form the full ribosome to begin elongation. The other steps describe assembling the ribosome and positioning tRNA, but they don’t specify the critical recognition of AUG itself.

Recognizing the exact start site is the key moment in initiation. The initiator tRNA carrying the first amino acid pairs its anticodon with the start codon in the mRNA, positioning in the P site of the small ribosomal subunit. This codon–anticodon interaction confirms AUG as the correct start and sets the reading frame, allowing initiation factors to promote joining of the large subunit and form the full ribosome to begin elongation. The other steps describe assembling the ribosome and positioning tRNA, but they don’t specify the critical recognition of AUG itself.