Assertion (A): Chromosome number is maintained constant at the end of S phase.
Reason (R ): S phase does not involve crossing over phenomenon.

Options

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

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**: The assertion states that "Chromosome number is maintained constant at the end of S phase." In the S phase (Synthesis phase) of the cell cycle, DNA replication occurs, leading to the duplication of genetic material. However, it is important to note that while the amount of DNA doubles, the number of chromosomes remains the same because each chromosome now consists of two sister chromatids.

2. **Understanding the Reason (R)**: The reason states that "S phase does not involve crossing over phenomenon." This statement is correct. Crossing over is a process that occurs during the prophase I of meiosis, not during the S phase of the cell cycle. Therefore, the S phase is focused solely on the replication of DNA without any exchange of genetic material between homologous chromosomes.

3. **Evaluating the Relationship**: The assertion is true because the chromosome number remains constant at the end of the S phase, even though the amount of DNA has doubled. The reason is also true, but it does not explain the assertion. The assertion is about the maintenance of chromosome number, while the reason discusses a different aspect of the cell cycle (crossing over).

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