Ribosome tRNA sites and active codon-anticodon pairing

Steps:

• Ribosomes contain three tRNA binding sites: A (aminoacyl), P (peptidyl), and E (exit).
• During elongation, codon-anticodon interactions form in the A and P sites to position tRNAs for peptide bond formation.
• The E site binds deacylated tRNA post-translocation, but this interaction is transient and not essential for ongoing synthesis.
• Thus, the maximum simultaneous active interactions occur with tRNAs in A and P sites only.

Why A is correct:

• Standard translation models define active codon-anticodon pairing as limited to the A and P sites per ribosomal cycle, per the central dogma of protein synthesis.

Why the others are wrong:

• B. 3: Overcounts by including the E site's transient binding, which does not sustain active decoding.
• C. 4: Ribosomes lack a fourth tRNA site for codon interaction.
• D. 6: Far exceeds the three-site structure, with no basis in ribosomal anatomy.

Final answer: A