Insulin in the human body is secreted by pancreas as prohormone/proinsulin.The schematic polypetide structure of proinsulin is given below. This proinsulin needs to undergo processing before it becomes functional in the body. Answer the questions that follow.

(a) State the change the proinsulin undegoes at the time of its processing to become functional.

(b) Name the technique the American company Eli Lilly used for the commercial production of human insulin.

(c) How are the two polypeptides of a functional insulin chemically held together ?