Discussion
Two RNAs shown below were used separately as templates in an in vitro translation system, which can generate proteins in all possible reading frames.
RNA1: 5′ − (AG)n − 3′
RNA2: 5′ − (AAG)n − 3′
The RNA1 translated product contained Arg and Glu.
The RNA2 translated product contained Arg, Glu, and Lys.
Which one of the following codons directs the incorporation of Arg?
Answer: B
Arg is encoded by AGA.
RNA1, (AG)n, reads AGA GAG AGA ... in every frame, so its only codons are AGA and GAG, giving an alternating Arg-Glu product. RNA2, (AAG)n, gives a different codon in each frame: AAG, AGA and GAA, one homopolymer each of Lys, Arg and Glu.
Arg appears in both products, so its codon must be one both templates contain. The only codon common to the two sets is AGA, so AGA codes for Arg. It follows that GAG, the other codon of RNA1, is Glu; then GAA, the remaining RNA2 codon besides AGA, also gives Glu, leaving AAG for Lys.
AAG and GAA are absent from RNA1, and GAG is absent from RNA2, so none of them could account for Arg in both products.
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