Decoding the first codon: Which experimental approach did Nirenberg and Matthaei (1961) use to determine that UUU codes for phenylalanine?

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:

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