Mitosis vs. Meiosis and the Role of Cyclin-CDK Complex in Cell Division
Part A: Differences Between Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, and asexual reproduction | Sexual reproduction (producing gametes) |
| Number of Cell Divisions | One | Two (Meiosis I and II) |
| Daughter Cells | Two genetically identical diploid cells | Four genetically different haploid cells |
| Chromosome Number | Remains the same as parent cell (2n → 2n) | Reduced by half (2n → n) |
| Crossing Over | Does not occur | Occurs during Prophase I |
| Homologous Chromosomes | Do not pair up | Pair and segregate during Meiosis I |
Part B: Cyclin and Cyclin-Dependent Kinases (CDKs) in Cell Cycle Regulation
What are Cyclins and CDKs?
- Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle.
- Cyclin-dependent kinases (CDKs): Enzymes that are activated by binding to cyclins. They phosphorylate target proteins to progress through different phases of the cell cycle.
Major Phases Regulated by Cyclin-CDK Complexes:
- G1 Phase: Cyclin D binds CDK4/6 to drive the cell toward DNA synthesis (S phase).
- S Phase: Cyclin E-CDK2 initiates DNA replication.
- G2 Phase: Cyclin A-CDK2 helps the cell prepare for mitosis.
- M Phase: Cyclin B-CDK1 (also called MPF – Maturation Promoting Factor) initiates mitosis.
Regulatory Checkpoints
Cyclin-CDK complexes are tightly regulated by checkpoints to ensure DNA is correctly replicated and cells are ready to divide. If damage or errors are detected, cell cycle progression is paused.
CDK Inhibitors
Proteins like p21, p27, and p53 inhibit cyclin-CDK complexes when there is DNA damage, preventing faulty cell division and promoting repair or apoptosis.
Conclusion
Mitosis and meiosis are two essential processes with distinct roles in organism growth and reproduction. Cyclins and CDKs play a pivotal role in ensuring that cells divide at the right time and under the right conditions. Any disruption in this regulation can lead to uncontrolled cell growth or cancer.