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

Options

A. `alpha,beta`
B. `beta,gamma`
C. `in,alpha`
D. `beta,in`

Answer

To solve the question regarding the cross-linking of the two polypeptide chains of insulin, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Insulin Structure**:
- Insulin is a protein that consists of two polypeptide chains known as the A chain and the B chain. The A chain contains 21 amino acids, while the B chain contains 30 amino acids.

2. **Identifying the Chains**:
- The A chain is referred to as the alpha chain, and the B chain is referred to as the beta chain.

3. **Cross-Linking Mechanism**:
- The two chains are connected by disulfide bridges, which are covalent bonds formed between the sulfur atoms of cysteine residues in the polypeptide chains.

4. **Types of Disulfide Bonds**:
- There are two types of disulfide bonds in insulin:
- Inter-chain disulfide bonds (between the A and B chains).
- Intra-chain disulfide bonds (within the A chain or the B chain).

5. **Identifying Specific Bonds**:
- For insulin, the inter-chain disulfide bonds occur between specific amino acids in the A and B chains.
- The bonds are formed between A7 (from the A chain) and B7 (from the B chain), and another bond between A20 (from the A chain) and B19 (from the B chain).

6. **Conclusion**:
- Since the question asks where the two polypeptide chains of insulin are cross-linked, the answer is that they are cross-linked at the alpha (A chain) and beta (B chain) places.

### Final Answer:
The two polypeptide chains of insulin are cross-linked at **alpha and beta** places.

---