Question

The genetic code was originally deciphered, in part, by experiments in which synthetic polynucleotides with repeating sequences were used as mRNAs to direct protein synthesis in cell-free extracts. Under these conditions, ribosomes could be made to start translation anywhere within the RNA molecules, with no start codon necessary. What peptide would be made by translation from a synthetic mRNA made of the repeating dinucleotide CGCG…?
A. a polymer of arginine plus a polymer of alanine
B. the peptide Met–Arg–Ala–Arg–Ala…
C. a polymer of alanine: Ala–Ala–Ala…
D. a peptide containing alternating arginines and alanines
E. a polymer of arginine: Arg–Arg–Arg…

Answer 1

Answer: a peptide containing alternating arginines and alanines.

Explanation:

As the mRNA has a repeating unit of CGCG.

Now if we extend the units with more CG (CGCGCGCGCGCG) the pairing will be as CGC-GCG-CGC-GCG. and if we see the amino acids coded by these codons are arginine for CGC and alanine for GCG.

As this synthetic mRNA doesn't require any initiation codon (AUG) thus the polypeptide will not have methionine, which is the first amino acid in every polypeptide coded by the initiation codon.

Thus, the synthetic mRNA will code for peptide having alternating arginines and alanines amino acids.