Read the given passage and answer the following questions :

Managing diabetes is possible by administering insulin periodically. Insulin used for diabetes was earlier extracted from pancreas of slaughtered cattle and pigs. However, this insulin caused some people to develop allergies and other types of reactions. In mammals, including humans, insulin is synthesized as a prohormone and is later converted to insulin. Insulin used for diabetes was earlier extracted from pancreas of slaughtered cattle and pigs. Insulin from an animal source, though caused some patients to develop allergy or other types of reactions to the foreign protein. Insulin consists of two short polypeptide chains: chain A and chain B, that are linked together by disulfide bridges.

How were the A and B chains of human insulin combined to form the mature hormone?

By creating disulfide bonds

The correct answer is Option (4) - By creating disulfide bonds

Insulin is composed of two peptide chains referred to as the A chain and B chain. A and B chains are linked together by two disulfide bonds, and an additional disulfide is formed within the A chain. In most species, the A chain consists of 21 amino acids and the B chain of 30 amino acids.
In mammals, including humans, insulin is synthesised as a pro-hormone (like a pro-enzyme, the pro-hormone also needs to be processed before it becomes a fully mature and functional hormone) which contains an extra stretch,called the C peptide. 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.