Which stage of cell cycle is characterised by DNA - replication, synthesis of Histones and formation of new nucleosomes :-

Options

A. S-phase
B. `G_(1)` phase
C. `G_(2)`-phase
D. M-phase

Answer

To solve the question regarding which stage of the cell cycle is characterized by DNA replication, synthesis of histones, and formation of new nucleosomes, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Cell Cycle Phases**: The cell cycle consists of several phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis). Each phase has distinct activities.

2. **Identify the S Phase**: The S phase is specifically known as the synthesis phase. During this phase, the cell replicates its DNA in preparation for cell division.

3. **DNA Replication**: In the S phase, the entire DNA content of the cell is duplicated. This is crucial for ensuring that each daughter cell receives an identical set of chromosomes.

4. **Synthesis of Histones**: Along with DNA replication, histone proteins are synthesized during the S phase. Histones are essential for packaging DNA into nucleosomes, which helps in organizing the DNA within the nucleus.

5. **Formation of New Nucleosomes**: As DNA is replicated, new nucleosomes are formed. Nucleosomes consist of DNA wrapped around histone proteins, and they play a vital role in the structural organization of chromatin.

6. **Review Other Phases**:
- **G1 Phase**: This phase involves cell growth and preparation for DNA synthesis, but no DNA replication occurs.
- **G2 Phase**: This phase involves further growth and preparation for mitosis, with no DNA replication.
- **M Phase**: This phase is where cell division occurs, not DNA replication.

7. **Conclusion**: Based on the characteristics of each phase, the correct answer to the question is the S phase, as it is the only phase where DNA replication, histone synthesis, and nucleosome formation occur.

### Final Answer:
The stage of the cell cycle characterized by DNA replication, synthesis of histones, and formation of new nucleosomes is the **S phase**.

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