The two polypeptide chasins of insulin are cross linked at ……… places.

Explanation

**Step-by-Step Solution:**

1. **Understanding Insulin Structure**: Insulin is a protein hormone composed of two polypeptide chains known as the A chain and the B chain.

2. **Identifying the Chains**: The A chain consists of 21 amino acids, while the B chain consists of 30 amino acids.

3. **Linkage Between Chains**: The two polypeptide chains (A and B) are linked together by disulfide bonds.

4. **Counting the Disulfide Bonds**: There are two disulfide bonds that connect the A chain and the B chain together.

5. **Additional Disulfide Bond**: In addition to the bonds linking the two chains, there is also an additional disulfide bond that is formed within the A chain itself.

6. **Final Answer**: Therefore, the two polypeptide chains of insulin are cross-linked at **three places**: two disulfide bonds between the chains and one additional bond within the A chain.

**Final Answer**: The two polypeptide chains of insulin are cross-linked at **three places**.

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