A : The complete disintegration of the nuclear envelope marks the start of metaphase.
R : Chromosomes are distinct with two chromatids at this stage.

Options

A. IF both Assertion & Reason are true and the reason followed is the correct explanation of the assertion then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statement then mark (4).

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Assertion:**
- The assertion states that "the complete disintegration of the nuclear envelope marks the start of metaphase."
- This is a correct statement. During the cell cycle, specifically in the transition from prophase to metaphase, the nuclear envelope breaks down completely. This allows the spindle fibers to interact with the chromosomes.

2. **Understanding the Reason:**
- The reason given is "chromosomes are distinct with two chromatids at this stage."
- This statement is also correct. During metaphase, chromosomes are indeed visible as distinct structures, each consisting of two sister chromatids joined at a region called the centromere.

3. **Analyzing the Relationship:**
- While both the assertion and reason are true, the reason does not directly explain the assertion. The assertion is about the disintegration of the nuclear envelope marking the start of metaphase, while the reason describes the state of chromosomes during metaphase.

4. **Conclusion:**
- Since both the assertion and reason are true but the reason does not correctly explain the assertion, the correct answer is that both statements are true, but the reason is not the correct explanation of the assertion.

**Final Answer:**
- Option B: Both assertion and reason are true, but the reason is not the correct explanation of the assertion.

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