The `G_2`-phase of the cell cycle is characterized by the presence of

Options

A. the number of chromosomes increases from n to 2n or 2n to 4n
B. single chromatids
C. formation of tubulin
D. double the amount of DNA (in chromosomes)

Answer

To solve the question regarding the characteristics of the G2 phase of the cell cycle, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Cell Cycle**:
- The cell cycle consists of two main phases: Interphase and M phase (mitosis/meiosis).
- Interphase is further divided into three phases: G1, S, and G2.

2. **Identifying the G2 Phase**:
- The G2 phase follows the S phase, where DNA replication occurs.
- During G2, the cell prepares for mitosis by synthesizing proteins and mRNA.

3. **Analyzing the Options**:
- **Option 1**: "The number of chromosomes increases from N to 2N or 2N to 4N."
- This is incorrect because the number of chromosomes remains the same during G2; only the amount of DNA doubles.
- **Option 2**: "Single chromatids."
- This is also incorrect as, during G2, the chromosomes consist of sister chromatids (duplicated chromosomes).
- **Option 3**: "Formation of tubulin."
- This is correct. Tubulin, a protein that forms microtubules, is synthesized during the G2 phase.
- **Option 4**: "Double the amount of DNA in chromosomes."
- This is incorrect because the doubling of DNA occurs in the S phase.

4. **Conclusion**:
- The correct answer is that the G2 phase is characterized by the formation of tubulin.

### Final Answer:
The G2 phase of the cell cycle is characterized by the presence of **formation of tubulin**.

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