Question

Khorana synthesized two RNAs (a) with repeat sequence of AB and (b) with repeat sequence of ABC, the polypeptides coded by (a) & (b) are respectively.

• A. Homopolypeptides in both (a) and (b)
• B. Heteropolypeptides in both
• C. Homopolypeptide in (a) & peptide heteropoly in (b)
• D. Heteropolypeptide in (a) & peptide homopoly in (b)

Answer 1

The correct option is D Heteropolypeptide in (a) & peptide homopoly in (b)

For cracking the genetic code, different RNA molecules were synthesized Marshall Nirenberg's cell free system for protein synthesis finally helped the protein to be deciphered. A triplet of three nucleotides is read as a codon and an specific amino acid is incorporated against each. For the three stop codons UAA, UAG and UGA no amino acid is incorporated and protein synthesis machinery is terminated.

In the case (a) With the RNA sequence as AB repeats, i.e., sequence (ABABABABABAB--) a heteropolypeptide would be synthesized as there are two codons ABA and BAB in alternation.

On the other hand in case (b) mRNA with repeat sequence ABC, i.e., (ABCABCABCABCABCABC---) has only a single codon in it which is ABC Therefore, a homopolypeptide with a single amino acid will be formed. Thus, in cases (a) and (b) a heteropolypeptide and a homopolypeptide will be formed respectively. Therefore, the correct answer for the given question is (d)