In order to enter the cell cycle a cell must be stimulated from outside. What type of molecule provides this stimulation?

Options

A. Cyclins
B. Cyclin-dependent kinases
C. Cytokines and growth factors
D. Tyrosine kinases

Answer

To answer the question about what type of molecule provides stimulation for a cell to enter the cell cycle, we can break down the information as follows:

### Step-by-Step Solution:

1. **Understanding Cell Cycle Regulation**: The cell cycle is a series of phases that a cell goes through to divide and replicate. For a cell to progress through these phases, it requires specific signals.

2. **Role of External Stimuli**: Cells need external signals to initiate the cell cycle. These signals can come from various sources, such as growth factors or hormones.

3. **Checkpoints in the Cell Cycle**: The cell cycle has several checkpoints where the cell assesses whether it is ready to proceed to the next phase. These checkpoints are crucial for ensuring that the cell is prepared for division.

4. **Cyclin-Dependent Kinases (CDKs)**: At these checkpoints, cyclin-dependent proteins (also known as cyclin-dependent kinases or CDKs) are present. These proteins play a significant role in regulating the cell cycle.

5. **Activation of CDKs**: CDKs require binding to cyclins to become active. Cyclins are regulatory proteins that are synthesized and degraded at specific points in the cell cycle. When a cyclin binds to a CDK, it activates the kinase, allowing the cell to move to the next stage of the cell cycle.

6. **Conclusion**: The stimulation that allows a cell to enter the cell cycle is provided by cyclins, which activate cyclin-dependent kinases. Therefore, the correct answer to the question is that cyclins provide the necessary stimulation for the cell to enter the cell cycle.

### Final Answer:
The type of molecule that provides stimulation for a cell to enter the cell cycle is **cyclins**.