Assertion (A): Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the lac operon.
Reason ( R): The lac operon consists of one regulatory gene and three structural genes.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not the correct explanation of A
C. A is true but R is false
D. A is false but R is true

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the lac operon."
- This is correct because lactose indeed serves as the substrate for beta-galactosidase, which is an enzyme that breaks down lactose into glucose and galactose. Furthermore, the presence of lactose is what activates the lac operon, allowing for the expression of genes involved in lactose metabolism.

2. **Understanding the Reason (R)**:
- The reason states that "The lac operon consists of one regulatory gene and three structural genes."
- This is also correct. The lac operon is composed of one regulatory gene (lacI) and three structural genes (lacZ, lacY, and lacA). The structural genes encode proteins necessary for the metabolism of lactose.

3. **Evaluating the Relationship Between A and R**:
- While both the assertion and the reason are true, the reason does not directly explain the assertion. The assertion discusses the role of lactose in regulating the operon, while the reason describes the structure of the operon itself.
- Therefore, while both statements are true, the reason does not serve as a correct explanation for the assertion.

4. **Conclusion**:
- The correct answer is that both A and R are true, but R is not the correct explanation of A.

### Final Answer:
Both A and R are true, but R is not the correct explanation of A.

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