(c) : Insulin is now being commercially produced by genetic engineering. Insulin consists of two short polypeptide chains: chain $A$ and chain $B$, that are linked together by disulphide bonds. Insulin, in mammal is synthesised as a prohormone which contains an extra stretch called the $C$-peptide. During maturation this $C$-peptide is removed. The production of insulin could only have been commercially possible if somehow the maturation process of $C$-peptide been skipped.This problem was solved in $1988$ by Eli Lilly, an American company which prepared functionable insulin from two $DNA$ sequences corresponding to $A$ and $B$ chains of human insulin and introduced them in plasmids of E.coli to produce insulin chains. In this way, chains $A$ and $B$ were produced separately which was extracted, combined by creating disulfide bonds to get human insulin.