Question

Dr. Khorana and his colleagues synthesized an RNA molecule with repeating sequence of $U{G}_n$ bases (UG UG UG UG UG UG). It produced a tetrapeptide with alternating sequence of cysteine and valine. It proves that codons for 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

Answer: (B)

UGU and GUG

H. Gobind Khorana chemically synthesized polyribonucleotides with defined repeating sequences of U and G bases. The resultant tetrapeptides produced by these mRNAs had two amino acids in repeating patterns. The tetrapeptide (UG)${}_n$, has alternating UGU and GUG codons: UGU GUG UGU GUG. Therefore, the tetrapeptide synthesized by this messenger must have equal amounts of cysteine and valine. Since, a cycteine codon has one G and two Us, UGU must code for cysteine and GUG for valine. Option B is correct answer. The codons in option A could have encoded cysteine and valine if the mRNA sequence was "UGU GUU UGU GUU" wherein the U and G bases are not present in alternate sequence; option A is incorrect. The codons in option C could have encoded cysteine and valine if the mRNA sequence was "UUG GGU UUG GGU" wherein the three copies of U and G bases repeated, not the single base; option C is incorrect. The codons in option D could have encoded cysteine and valine if the mRNA sequence was "GUG UGU GUG UGU" wherein the first base is guanine not uracil as mentioned in question; option D is incorrect. The codons in option E could have encoded cysteine and valine if the mRNA sequence was "GUU UGU GUU UGU" wherein guanine and uracil are not present in alternate manner as mentioned in question; option E is incorrect.