Assertion :- Lactose is the substrate for the enzyme `beta`-galactosidase
& it regulates switching on & off of the operon.
Reason: A very low level of expression of lac operon is always
present in cell all the time.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are trur but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is Flase.
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Text Solution:

1. **Understanding the Assertion**: The assertion states that "Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on & off of the operon."
- **Explanation**: Lactose serves as the substrate for the enzyme beta-galactosidase (also known as lactase), which is responsible for breaking down lactose into glucose and galactose. In the context of the lac operon, when lactose is present, it binds to the repressor protein, leading to the activation of the operon.

2. **Understanding the Reason**: The reason states that "A very low level of expression of lac operon is always present in the cell all the time."
- **Explanation**: The lac operon is not completely turned off even in the absence of lactose. There is a basal level of expression due to the presence of a small amount of lactose or due to the leaky nature of the operon, which allows for a minimal expression of the genes involved in lactose metabolism.

3. **Evaluating the Relationship Between Assertion and Reason**:
- Both statements are true. However, the reason does not directly explain the assertion. The assertion focuses on the role of lactose in regulating the operon, while the reason discusses the constant low-level expression of the operon.

4. **Conclusion**: Since both the assertion and reason are true, but the reason is not the correct explanation of the assertion, the correct answer to the question is option B: "Both the Assertion and Reason are true, but Reason is not the correct explanation of the Assertion."