Disulphide linkage:
The correct answer is option C, disulphide linkage. Insulin is a hormone that plays a crucial role in regulating blood sugar levels. It is composed of two polypeptide chains, an A chain and a B chain, which are linked together by disulphide bonds.

Structure of insulin:
Insulin is a protein hormone that is synthesized in the pancreas. It consists of 51 amino acids in total, with the A chain containing 21 amino acids and the B chain containing 30 amino acids. These two chains are synthesized as a single polypeptide known as proinsulin, which undergoes post-translational modifications to form the active insulin hormone.

Formation of disulphide bonds:
Disulphide bonds are covalent bonds formed between two cysteine residues in a protein. Cysteine is an amino acid that contains a thiol group (-SH) on its side chain. When two cysteine residues come close to each other, the thiol groups can react to form a disulphide bond (-S-S-). This reaction is known as oxidation and is typically catalyzed by enzymes called oxidoreductases.

In the case of insulin, the A and B chains contain multiple cysteine residues. During the maturation process, proinsulin is transported to the endoplasmic reticulum, where it undergoes folding and disulphide bond formation. Specifically, there are two cysteine residues in the A chain (Cys7 and Cys20) and two cysteine residues in the B chain (Cys7 and Cys19) that form disulphide bonds. The Cys7 of the A chain is linked to the Cys7 of the B chain, while the Cys20 of the A chain is linked to the Cys19 of the B chain.

Importance of disulphide bonds:
Disulphide bonds play a crucial role in stabilizing the three-dimensional structure of proteins. In the case of insulin, the disulphide bonds between the A and B chains help maintain the overall structure of the hormone. They also contribute to the stability and longevity of insulin in the bloodstream. Disruption of these disulphide bonds can lead to the denaturation of insulin and loss of its biological activity.

In conclusion, the two polypeptide chains in insulin are linked via disulphide bonds. These covalent bonds between cysteine residues are essential for the proper folding, stability, and biological activity of the hormone.