The two polypeptides of human insulin are linked together by

Question

Accepted Answer

C. 1. **Background**: human insulin is made up of **two separate polypeptide chains**, called the A-chain (21 amino acids) and the B-chain (30 amino acids), each held together internally by peptide bonds between amino acids.

2. For the hormone to become fully functional, these two separate chains must be joined together through a special type of bond formed after the protein is made (a post-translational modification).

3. This connection is achieved specifically through **disulphide bridges**, formed between cysteine residues — specifically linking positions A7 to B7, and A20 to B19.

4. **Hydrogen bonds** and simple **covalent bonds** are too weak or too generic respectively to describe this specific, well-defined linkage, and a **phosphodiester bond** is the type of bond found in nucleic acid backbones (DNA/RNA), not in linking protein chains.

5. Hence, the correct answer is **C: Disulphide bridges**.