Progression of cell cycle is regulated by the concentration of which type of molecule

Options

A. Centrosomes
B. Cyclin dependent kinases
C. Cyclins
D. Microtubules.

Answer

To determine which type of molecule regulates the progression of the cell cycle, we need to analyze the options provided:

1. **Centrosomes**: Centrosomes play a role in organizing the microtubules that form the spindle fibers during cell division. However, they do not regulate the progression of the cell cycle itself. Their function is more about supporting the process once it has begun.

2. **Cyclin-dependent kinases (CDKs)**: CDKs are enzymes that play a crucial role in regulating the cell cycle. They are activated by cyclins and are responsible for phosphorylating target proteins, which in turn signals the cell to proceed to the next phase of the cell cycle. Therefore, CDKs are essential for the regulation of the cell cycle's progression.

3. **Cyclin**: Cyclins are proteins that regulate CDKs. While cyclins are important for the activation of CDKs, they do not directly regulate the progression of the cell cycle. Instead, they serve as a regulatory partner for CDKs.

4. **Microtubules**: Microtubules are structural components that help in the formation of the spindle apparatus during cell division. Similar to centrosomes, they assist in the process but do not regulate the cell cycle's progression.

Based on this analysis, the correct answer is:

**Option 2: Cyclin-dependent kinases (CDKs)** are the molecules that regulate the progression of the cell cycle.

### Step-by-Step Solution:
1. **Identify the role of each option**:
- Centrosomes: Organize spindle fibers but do not regulate the cell cycle.
- Cyclin-dependent kinases (CDKs): Activated by cyclins and regulate progression by phosphorylating target proteins.
- Cyclin: Activates CDKs but does not directly regulate the cell cycle.
- Microtubules: Assist in cell division but do not regulate the cell cycle.

2. **Determine the regulatory molecule**:
- CDKs are essential for signaling the cell to move to the next phase of the cell cycle, making them the key regulators.

3. **Conclude with the correct answer**:
- The progression of the cell cycle is regulated by cyclin-dependent kinases (CDKs).