Metabolically active cells, of the adult animal's heart, are remain in which phase ?

Options

A. S-phase
B. `G_(0)` phase
C. `G_(2)` phase
D. M phase

Answer

### Step-by-Step Solution:

1. **Understanding Cell Phases**: Cells go through various phases in the cell cycle, which include G1, S, G2, and M phases. Additionally, there is a phase known as G0, which is often referred to as the resting phase.

2. **Identifying the G0 Phase**: The G0 phase is a stage where cells are metabolically active but do not undergo division. This phase is crucial for cells that have differentiated and are not actively preparing to divide.

3. **Metabolic Activity in Heart Cells**: The heart cells, or cardiomyocytes, are known to be highly specialized and metabolically active. However, they do not frequently divide, which is characteristic of cells in the G0 phase.

4. **Comparison with Other Phases**:
- **S Phase**: This is the synthesis phase where DNA replication occurs, preparing the cell for division.
- **G2 Phase**: This phase involves the synthesis of proteins and RNA, preparing the cell for mitosis.
- **M Phase**: This is the mitotic phase where actual cell division occurs.

5. **Conclusion**: Since the question asks about metabolically active cells in the adult animal's heart that do not proliferate, the correct answer is the G0 phase.

### Final Answer:
The metabolically active cells of the adult animal's heart remain in the **G0 phase**.

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