To crack the genetic code, researchers introduced synthetic messenger RNAs into in vitrotranslation systems and determined which proteins were produced from these synthetic mRNAs. mRNAs consisting of poly-UUC led to production of three different proteins: poly-Phe, poly-Ser, and poly-Leu. What best explains this result?

A. Each amino acid is specified by several codons.
B. Transcription started at three different places.
C. The synthetic mRNA was read in all three reading frames.
D. Each codon codes for several different amino acids.

C. The synthetic mRNA was read in all three reading frames.

Explanation: During translation, codons are read in a successive, nonoverlapping manner in which a new codon begins every three nucleotide residues. This is called a reading frame. A specific first codon in the sequence begins the reading frame. In the experiment involving a poly-UUC mRNA strand, the strand is read in all the reading fraMrs. Consider the poly-UUC = UUCUUCUUCUUCUUCUUC strand, the strand may be read from the first amino acid in the sequence as UUC. UUC is a codon that codes for phenylalanine, this reading frame will result in a protein with a poly-phe sequence.

If the reading frame starts from the second amino acid U, the codons will be UCU, which will give rise to a protein with a poly-ser amino acid sequence because UCU codes amino acid serine.

Lastly, if the reading frame starts from the third amino acid in the sequence, the codons will be CUU. This will produce a protein with a poly-leu amino acid sequence because CUU codes for amino acid Leucine.

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