Which of the following steps presented the main challenge in the prod…

The correct answer is Option (3) → Achieving the correct assembly of insulin into its mature form

Insulin has two short polypeptide chains, A and B, joined by disulphide bridges. In the body it is first made as pro-insulin, which has an extra stretch called the C peptide. The C peptide is removed during maturation, and only then is the hormone active. Getting this mature, correctly assembled insulin was the main challenge in making it by rDNA techniques. Eli Lilly solved it in 1983 by making chains A and B separately in E. coli and then joining them by disulphide bonds.

Option (1) Separating the A and B peptide chains — Incorrect. In the rDNA method chains A and B were made separately in E. coli and then combined by disulphide bonds. The difficulty lay in joining them into mature insulin, not in separating them.

Option (2) Incorporating the C-peptide into proinsulin — Incorrect. The C peptide is removed during maturation and is not present in mature insulin. Adding it was never the aim.

Option (3) Achieving the correct assembly of insulin into its mature form — Correct. The book says the main challenge was getting insulin assembled into a mature form.

NCERT Extract:
Genetically Engineered Insulin – “… This C peptide is not present in the mature insulin and is removed during maturation into insulin. The main challenge for production of insulin using rDNA techniques was getting insulin assembled into a mature form. In 1983, Eli Lilly an American company prepared two DNA sequences corresponding to A and B, chains of human insulin and introduced them in plasmids of E. coli to produce insulin chains. Chains A and B were produced separately, extracted and combined by creating disulfide bonds to form human insulin.”

Figure 10.3 Maturation of pro-insulin into insulin (simplified)