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 entirely of adenine (poly-A)?

A. the peptide Met-Lys-Lys...
B. a polymer of lysine: Lys-Lys-Lys...
C. a polymer of phenylalanine: Phe-Phe-Phe...
D. a polymer of methionine: Met-Met-Met...
E. a polymer of alanine: Ala-Ala-Ala...

Answer 1

A synthetic mRNA made entirely of adenine (poly-A) would lead to the translation of a polypeptide composed of a repeating sequence of the amino acid lysine (Lys), making option B the correct answer.

Step-by-step explanation:

The question asks which peptide would be synthesized by a synthetic mRNA composed entirely of adenine (poly-A) in a cell-free extract without the need for a start codon. From the experiments by scientists like Nirenberg, Matthaei, and Khorana, we learned that the genetic code is universal and degenerate, with 64 triplet codons specifying only 20 amino acids. Specifically, the mRNA triplet AAA codes for the amino acid lysine (Lys). Therefore, a synthetic mRNA polymer composed of only adenine would lead to the synthesis of a polypeptide consisting of repeating lysine amino acids. So, the correct answer to the student's question would be B. a polymer of lysine: Lys-Lys-Lys...