How many variable segments are present in the basic structure of antibody molecules?

Options

A. One
B. Two
C. Three
D. Four

Explanation

To determine how many variable segments are present in the basic structure of antibody molecules, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Antibody Structure**: Antibodies, also known as immunoglobulins, are protein molecules that play a crucial role in the immune response. They are composed of four polypeptide chains: two heavy chains and two light chains.

2. **Identifying the Chains**: In the structure of an antibody, the two heavy chains are larger and have a higher molecular weight, while the two light chains are smaller.

3. **Variable and Constant Regions**: Each chain has regions that can be classified as variable and constant. The variable regions are responsible for binding to specific antigens, while the constant regions determine the class of the antibody.

4. **Counting Variable Segments**:
- Each heavy chain has one variable region.
- Each light chain also has one variable region.
- Therefore, there are a total of:
- 2 variable regions from the heavy chains (1 from each heavy chain)
- 2 variable regions from the light chains (1 from each light chain)

5. **Calculating the Total**: Adding these together gives:
- 2 (heavy chain variable regions) + 2 (light chain variable regions) = 4 variable segments in total.

6. **Conclusion**: Thus, the total number of variable segments present in the basic structure of antibody molecules is **4**.

### Final Answer:
The correct answer is **4**.

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