Question

Dr. Khorana and his colleagues synthesized an RNA molecule with a repeating sequence of $U{{G}_{n}}$ bases (UG UG UG UG UG UG). It produced a tetrapeptide with an alternating sequence of cysteine and valine is
A. UGU and GUU
B. UGU and GUG
C. UUG and GGU
D. GUG and UGU
E. GUU and UGU

Answer 1

Hint: Dr. Har Gobind Khorana synthesized the polyribonucleotide chemically. The polyribonucleotide synthesized by him had repeating sequences of U and G bases. Moreover, the tetrapeptides produced by the mRNAs had two amino acids in repeating patterns.

Complete answer:
The repeating sequence of the $U{{G}_{n}}$ bases (UG UG UG UG UG UG) produces two different types of codons. These two different types of codons are UGU and GUG in an alternating repeating pattern. The codon UGU codes for the amino acid cysteine in the mRNA sequence. On the other hand, the codon GUG codes for the amino acid Valine in the mRNA sequence. Therefore, the tetrapeptide produced by this mRNA sequence will have equal amounts of the amino acids cysteine and valine.
UGU and GUU –
The codon UGU codes for cysteine. The codon GUU codes for valine. The repeating sequence for these amino acids would be UG UG UU UG UG UU and so on.
UGU and GUG –
The codon UGU codes for cysteine. The codon GUG codes for valine. The repeating sequence for these amino acids would be UG UG UG UG UG UG and so on.
UUG and GGU –
The codon UUG codes for leucine. The codon GGU codes for glycine. The repeating sequence for these amino acids would be UU GG GU UU GG GU and so on.
GUG and UGU –
The codon GUG codes for valine. The codon UGU codes for cysteine. The repeating sequence for these amino acids would be GU GU GU GU GU GU and so on.
GUU and UGU –
The codon GUU codes for valine. The codon UGU codes for cysteine. The repeating sequence for these amino acids would be GU UG and so on.
Thus, based on the above information we can conclude that the tetrapeptide with an alternating sequence of cysteine and valine synthesized by Dr. Khorana and his colleagues using an RNA molecule with a repeating sequence of $U{{G}_{n}}$ bases are UGU and GUG.

Hence, the correct answer is an option (B).

Note: Dr. Har Gobind Khorana developed a chemical method that was very important in synthesizing the RNA molecules with defined combinations of bases. These defined combinations of bases are the homopolymers and the copolymers. He used the synthetic DNA to prepare the nucleotides with the known repeating sequences.
For example – The sequence CUCUCUCUCUCUCU produces two different kinds of amino acids Leucine (CUC) and Serine (UCU).