How did an American Company, Eli Lilly use the knowledge of r-DNA technology to produce human insulin?

The structure of insulin consists of two short polypeptide chains chain A and chain B linked together by disulphide bridges. In mammals, including human beings, insulin is synthesized as a pro-hormone which contains an extra stretch called the C peptide which is removed during maturation into insulin. The rDNA technique is used for assembling insulin into the mature form.

In 1983, Eli Lily, an American company, prepared two DNA sequences corresponding to A and B chains of human insulin and introduced them in the plasmids of E. coli to produce insulin chains. Chains A and B were produced separately, extracted and combined by creating disulphide bonds to form human insulin.