What happens in synthesis phase during cell cycle?

Options

A. DNA synthesis
B. Chromosome number becomes double
C. Formation of two nuclei
D. Synthesis of protein & RNA

Answer

### Step-by-Step Text Solution:

1. **Understanding the Cell Cycle**: The cell cycle is a series of events that cells go through as they grow and divide. It consists of various phases that are cyclic in nature, meaning they repeat continuously.

2. **Phases of the Cell Cycle**: The cell cycle is divided into two major parts:
- **Interphase**: This is the longer phase, accounting for about 95% of the cell cycle. It includes three sub-phases: G1 (Gap 1), S (Synthesis), and G2 (Gap 2).
- **M Phase (Mitotic Phase)**: This is the shorter phase, making up about 5% of the cell cycle, where actual cell division occurs.

3. **Focus on the S Phase**: The S phase, or synthesis phase, is crucial for DNA replication. During this phase, the cell prepares for division by duplicating its genetic material.

4. **DNA Replication**: In the S phase, each chromosome is replicated. Initially, a chromosome consists of one chromatid, but after replication, each chromosome will consist of two identical chromatids joined at a region called the centromere.

5. **Chromatid Count**: Although the number of chromosomes remains the same (as they are still counted based on the centromere), the number of chromatids doubles. For example, one chromosome with one chromatid becomes one chromosome with two chromatids after the S phase.

6. **Conclusion**: The key event that occurs during the S phase is DNA synthesis, which is essential for the proper distribution of genetic material during cell division.