Q29-22E - Organic Chemistry (Mcmurry)

Question

Human and horse insulin both have two polypeptide chains, with one chain containing $\mathbf{21}$ amino acids and the other containing   $\mathbf{30}$ amino acids. They differ in primary structure at two places. At the position data-custom-editor="chemistry" $\mathbf{9}$ in one chain, human insulin has Ser and horse insulin has Gly; at the position data-custom-editor="chemistry" $\mathbf{30}$ in the other chain, human insulin has Thr and horse insulin has Ala. How must the DNA for the two insulins differ?

Step-by-Step Solution

Answer

The horse and human insulin differ at the position $9 \mathrm{and} 30$ of the polypeptide chain.

Introduction

The genetic code stipulates that each nucleotide in a triplet generates a codon, which produces one amino acid, hence any alteration in a nucleotide triplet will affect the resultant amino acid. The RNA generated during transcription will be translated to produce amino acids, which will then be used to make proteins, therefore the triplet from the mRNA will code for a specific amino acid.

DNA differ for two insulin

The differences between horse and human insulin are attributable to nucleotide changes at positions $9 \mathrm{and} 30$ of the polypeptide chains. The ribosome reads the mRNA sequence in a different way, resulting in the polypeptide chain producing a completely new amino acid sequence.

The  data-custom-editor="chemistry" ${\mathbf{5}}^{\mathbf{\text{'}}}\mathbf{ }\mathbf{and}\mathbf{ }{\mathbf{3}}^{\mathbf{\text{'}}}$ ends of four complementary DNA sequences.

Serine: TCT, TCC, TCA, TCG, AGT, AGC

Glycine: GGT, GGC, GGA, GGG

Threonine: ACT, ACC, ACA, ACG

Alanine: GCT, GCC, GCA, GCG