Decoding the first codon: Which experimental approach did Nirenberg and Matthaei (1961) use to determine that UUU codes for phenylalanine?
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AIn vitro synthesis of a polypeptide using a poly-U (UUUUU...) RNA template
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BLabeled peptide binding to ribosomes without translation
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CMixed copolymer mRNA synthesis as the first demonstration
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DNone of the above
Answer
Correct Answer: In vitro synthesis of a polypeptide using a poly-U (UUUUU...) RNA template
Explanation
Introduction / Context:Cracking the genetic code began with cell-free translation experiments. Nirenberg and Matthaei famously used synthetic RNA polymers to show which codons specify which amino acids.
Given Data / Assumptions:
- Poly-U RNA directs incorporation of phenylalanine into polypeptides.
- Experiment performed in a cell-free E. coli extract with radiolabeled amino acids.
- Question asks which technique revealed the first codon assignment.
Concept / Approach:A homopolymer (poly-U) mRNA was added to a translation extract. Only phenylalanine was incorporated into the newly synthesized protein, proving that UUU encodes Phe.
Step-by-Step Solution:
Prepare a cell-free translation system.Add synthetic poly-U mRNA.Measure incorporation of radiolabeled phenylalanine; observe strong incorporation specific to UUU.Verification / Alternative check:Subsequent use of poly-A and poly-C corroborated assignments (Lys for AAA, Pro for CCC), and mixed copolymers refined the code later.
Why Other Options Are Wrong:
- Ribosome binding without translation: used later for codon–anticodon binding studies, not the first discovery.
- Mixed copolymers: were later refinements by Nirenberg and Khorana’s group.
- None: the poly-U experiment is the correct historical answer.
Common Pitfalls:Confusing the timeline of genetic code elucidation; separate milestones involved different techniques.
Final Answer:In vitro synthesis of a polypeptide using a poly-U (UUUUU...) RNA template