Chromosomes duplicate during `underline (A)` and increase in number of chromosomes is observed first during `underline (B)` . Fill the blanks correctly

Options

A. A- Interphase B - prophase
B. A - S-phase B - Telophase
C. A - Synthetic phase B - Gap 2 phase
D. A- Interphase B - Anaphase

Answer

To fill in the blanks in the question, we need to identify the correct phases of the cell cycle where chromosome duplication and the first observable increase in chromosome number occur.

### Step-by-Step Solution:

1. **Understanding Chromosome Duplication**:
- Chromosome duplication occurs during the S phase of the cell cycle. This is the phase where the DNA is replicated, resulting in two identical copies of each chromosome, known as sister chromatids.
- Therefore, for blank (A), the correct answer is **S phase**.

2. **Identifying the First Observable Increase in Chromosome Number**:
- After the S phase, the cell enters the mitotic phase. Within this phase, the chromosomes become visible and are aligned during metaphase. However, the actual separation of sister chromatids, which leads to an increase in the number of chromosomes, occurs during anaphase.
- In anaphase, the sister chromatids are pulled apart to opposite poles of the cell, effectively doubling the number of chromosomes in the daughter cells.
- Thus, for blank (B), the correct answer is **anaphase**.

### Final Answers:
- (A) S phase
- (B) anaphase

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