The two polypeptides of human insulin are linked together by

Options

A. Phosphodiester bond
B. Covalent bond
C. Disulphide bridges
D. Hydrogen bonds

Answer

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 is a peptide hormone composed of two polypeptide chains known as Chain A and Chain B.
- Chain A consists of 21 amino acids, while Chain B consists of 30 amino acids.

2. **Identifying the Linkage**:
- The two chains (A and B) are connected to form a single functional unit.
- The type of bond that links these two chains is crucial to understanding how insulin functions.

3. **Evaluating the Options**:
- **Phosphodiester Bond**: This type of bond is found in nucleic acids (DNA and RNA) and is not applicable to proteins like insulin.
- **Covalent Bond**: While disulfide bridges are a type of covalent bond, the term "covalent bond" is too broad and does not specify the type of bond that links the insulin chains.
- **Disulfide Bridges**: This is a specific type of covalent bond that forms between the sulfur atoms of cysteine residues in the polypeptide chains. This is the correct linkage for insulin.
- **Hydrogen Bond**: These bonds are weaker and typically occur between polar molecules; they do not link the polypeptide chains in insulin.

4. **Conclusion**:
- The correct answer is that the two polypeptides of human insulin are linked together by **disulfide bridges**.

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

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