In which phase of the cell cycle DNA doubles and chromosomes replicate ?

Options

A. S phase
B. `G_1` phase
C. `G_2` phase
D. metaphase

Answer

**Step-by-Step Solution:**

1. **Understanding the Cell Cycle**: The cell cycle consists of several phases that a cell goes through in order to grow and divide. These phases include G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis).

2. **Identifying the Purpose of Each Phase**:
- G1 phase: The cell grows and prepares for DNA replication.
- S phase: This is the phase where DNA replication occurs.
- G2 phase: The cell continues to grow and prepares for mitosis.
- M phase: The cell divides into two daughter cells.

3. **Focusing on the S Phase**: The question specifically asks about the phase where DNA doubles and chromosomes replicate. This is the S phase (Synthesis phase) of the cell cycle.

4. **Conclusion**: In the S phase, the cell synthesizes a complete copy of the DNA in its nucleus. At the end of this phase, each chromosome consists of two sister chromatids, which are identical copies of the chromosome.

5. **Final Answer**: Therefore, the phase of the cell cycle in which DNA doubles and chromosomes replicate is the **S phase**.

---