The two polypeptides of human insulin are linked together by

Options

A. phosphodiester bonds
B. covalent bonds
C. disulphide bridges
D. hydrogen bonds

Explanation

To answer the question regarding how the two polypeptides of human insulin are linked together, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Insulin Structure**: Human insulin consists of two polypeptide chains known as the A chain and the B chain.

2. **Identifying the Linkage**: The question asks about the type of bond that links these two chains together.

3. **Types of Bonds**: There are several types of bonds that can link polypeptides:
- Phosphodiester bonds: Typically found in nucleic acids, not in proteins.
- Covalent bonds: A general term, but we need to specify which type.
- Disulfide bridges: A specific type of covalent bond formed between cysteine residues.
- Hydrogen bonds: Weaker interactions that can stabilize structures but do not link polypeptide chains in this case.

4. **Correct Answer**: In the case of human insulin, the A and B chains are specifically linked by **disulfide bridges**. These disulfide bonds are formed between the sulfur atoms of cysteine residues in the two chains.

5. **Conclusion**: Therefore, the two polypeptides of human insulin are linked together by **disulfide bonds**.

### Final Answer:
The two polypeptides of human insulin are linked together by **disulfide bonds**.

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