Ribosome–mRNA interaction: On the ribosome, to which component does the mRNA physically bind during translation?
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ABetween the two subunits without preference
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BPrimarily to the large subunit
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CPrimarily to the small subunit
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DNone of these
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EExclusively to elongation factors, not the ribosome
Answer
Correct Answer: Primarily to the small subunit
Explanation
Introduction:Correct placement of mRNA on the ribosome dictates reading-frame selection and start-codon recognition. While the functional ribosome is a two-subunit assembly, the small subunit is the primary site for mRNA binding and decoding interactions, assisted by initiation factors and rRNA elements.
Given Data / Assumptions:
- Bacterial 30S and eukaryotic 40S subunits engage mRNA early in initiation.
- 16S/18S rRNA participates in mRNA alignment (e.g., Shine–Dalgarno pairing in bacteria).
- Large subunit catalyzes peptide bond formation rather than codon decoding.
Concept / Approach:mRNA binding determinants—such as the 3′ end of 16S rRNA in bacteria or the eukaryotic cap-binding complex associated with the small subunit—position the start codon in the P site of the small subunit. The large subunit's peptidyl transferase center acts after correct decoding, making it less central to initial mRNA binding.
Step-by-Step Solution:
Small subunit engages the mRNA via rRNA–mRNA interactions (or cap-binding proteins in eukaryotes).Initiator tRNA pairs with the start codon in the P site on the small subunit.Large subunit joins after start-site selection to catalyze peptide bond formation.Elongation proceeds with decoding still monitored by the small subunit.Verification / Alternative check:Structural studies show the mRNA channel in the small subunit; mutating 16S/18S rRNA elements alters initiation, confirming primary mRNA binding at the small subunit.
Why Other Options Are Wrong:
- Between subunits/none/exclusive EF binding: contradict structural data.
- Large subunit: main role is catalysis, not initial mRNA binding/decoding.
Common Pitfalls:Equating the entire ribosome with the decoding function; decoding specificity resides in the small subunit.
Final Answer:Primarily to the small subunit