`G_(1) to S` transition is regulated by

Options

A. Cyclins only
B. Cyclin independent kinases
C. Mitotic cyclin and cdc2 kinase
D. `G_(1)` cyclin cdc2 kinase

Answer

To answer the question "G1 to S transition is regulated by," we will break down the information step by step.

### Step-by-Step Solution:

1. **Understanding the Cell Cycle**:
- The cell cycle consists of several phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis). The G1 phase is the first phase where the cell grows and prepares for DNA replication.

**Hint**: Remember the phases of the cell cycle and their order.

2. **Role of G1 Phase**:
- During the G1 phase, the cell not only grows in size but also synthesizes mRNA and proteins necessary for DNA replication, which occurs in the S phase.

**Hint**: Focus on the activities that occur during the G1 phase.

3. **Transition from G1 to S Phase**:
- The transition from G1 to S phase is crucial for the cell to proceed with DNA replication. This transition is tightly regulated to ensure that the cell is ready for replication.

**Hint**: Think about what mechanisms control the progression of the cell cycle.

4. **Role of Cyclin-Dependent Kinases (CDKs)**:
- The transition from G1 to S phase is primarily regulated by cyclin-dependent kinases (CDKs). These are enzymes that, when activated by binding to cyclins, phosphorylate target proteins to drive the cell cycle forward.

**Hint**: Identify the key proteins involved in regulating the cell cycle.

5. **Conclusion**:
- Therefore, the correct answer to the question is that the G1 to S transition is regulated by cyclin-dependent kinases (CDKs).

**Final Answer**: Cyclin-dependent kinases (CDKs) regulate the G1 to S transition.

### Summary of the Correct Answer:
- The G1 to S transition is regulated by cyclin-dependent kinases (CDKs).