The two polypeptides of human insulin are linked together by

Options

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

Answer

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

### Step-by-Step Solution:

1. **Understand the Structure of Insulin**:
- Human insulin 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. **Identify the Types of Bonds**:
- The question provides four types of bonds: phosphodiester bond, covalent bond, disulfide bridges, and hydrogen bond.
- It is important to know what each bond type is and where it is typically found.

3. **Analyze the Bonding in Insulin**:
- The two chains of insulin (A and B) are linked together by specific types of bonds.
- In the case of insulin, the two polypeptide chains are connected by disulfide bonds, which are also referred to as disulfide bridges.

4. **Eliminate Incorrect Options**:
- Phosphodiester bonds are found in the backbone of DNA, not in proteins.
- Covalent bonds are strong but do not specifically refer to the type of linkage between the insulin chains.
- Hydrogen bonds are weak interactions found in various biological structures, such as between nitrogenous bases in DNA, but they do not link the chains of insulin.

5. **Conclude the Correct Answer**:
- Since the two polypeptide chains of insulin are linked by disulfide bridges, the correct answer to the question is option 3: disulfide bridges.

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

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