Which of these is usually constant for a variety of cells?

Options

A. Time for S, `G_(2)` and M phases
B. Time for `G_(1)`
C. Time for `G_(2)`
D. Time for `G_(1), S and G_(2)`

Answer

To solve the question "Which of these is usually constant for a variety of cells?", we need to analyze the different phases of the cell cycle and their durations in various cell types.

### Step-by-Step Solution:

1. **Understand the Cell Cycle**: The cell cycle consists of two main phases: Interphase and M-phase. Interphase includes G1 (Gap 1), S (Synthesis), and G2 (Gap 2) phases, while M-phase includes mitosis (Prophase, Metaphase, Anaphase, and Telophase).

2. **Analyze Each Phase**:
- **G1 Phase**: This is the longest phase and its duration varies significantly between different cell types. Therefore, G1 is not constant across various cells.
- **S Phase**: This phase involves DNA replication. While the duration of S phase can vary depending on the amount of DNA, the rate of DNA replication is generally consistent within a species. However, it is not constant across all cell types.
- **G2 Phase**: This phase is relatively short and tends to take about 3 to 4 hours in most cell types. This duration is quite consistent and can be considered constant across various cells.
- **M Phase**: The duration of the M phase is also relatively constant across different cell types, as it involves the actual process of cell division.

3. **Evaluate the Options**: The options provided are:
- Time for S, G2, and M phase
- Time for G1
- Time for G2
- Time for G1, S, and G2

From our analysis, we see that G1 varies, S can vary, but G2 and M phases are relatively constant.

4. **Conclusion**: The best answer is that the time for G2 and M phases is usually constant for a variety of cells. However, since the options do not explicitly mention this, we can conclude that the time for G2 is the most consistent across various cell types.

### Final Answer:
The correct answer is **time for G2**.