The hormone, insulin, is a polypeptide made up of 2 chains that are connected together by disulphide bridges.

In biosynthesis in the pancreas pro-insulin is first made.
This consists of one long chain. The active insulin is
produced by some enzymatic tailoring-work in which a
peptide fragment, i.e. the amino acids 31 to 65, split off
(right).

The same product can be made in the laboratory by organic synthesis, this time on the basis of two partial chains which are oxidatively bonded together using disulphide bridges (left).