What Cyclin/Cyclin Dependent Kinases combinations regulate the cell cycle and at what stage?
The correct answer and explanation is:
The regulation of the cell cycle is primarily controlled by cyclin-cyclin-dependent kinase (CDK) complexes. These complexes drive the progression through different phases of the cell cycle by phosphorylating target proteins. Several cyclins and CDKs work in tandem at specific checkpoints to ensure proper cell division.
- Cyclin D/CDK4/6 (G1 Phase):
- Cyclin D binds with CDK4 or CDK6 to regulate the G1 phase. This complex is critical for the transition from G1 to the S phase. The activation of Cyclin D/CDK4/6 leads to phosphorylation of the retinoblastoma protein (Rb), which releases E2F transcription factors. These factors promote the transcription of genes necessary for DNA replication, marking the cell’s commitment to enter the S phase.
- Cyclin E/CDK2 (Late G1 Phase to S Phase):
- Cyclin E associates with CDK2 and prepares the cell for DNA replication. The Cyclin E/CDK2 complex is responsible for the phosphorylation of Rb and further activation of E2F, ensuring that the cell enters the S phase and begins DNA synthesis.
- Cyclin A/CDK2 (S Phase):
- Cyclin A binds with CDK2 to maintain the S phase. This complex regulates the initiation and progression of DNA replication. Cyclin A/CDK2 also ensures the proper replication of the genome without errors.
- Cyclin B/CDK1 (G2 Phase to M Phase):
- Cyclin B binds with CDK1 to regulate the G2 to M phase transition. The Cyclin B/CDK1 complex, also known as maturation-promoting factor (MPF), plays a pivotal role in the initiation of mitosis. It promotes the condensation of chromosomes, nuclear envelope breakdown, and the formation of the mitotic spindle.
These cyclins and CDKs are tightly regulated by various mechanisms, including synthesis and degradation of cyclins at specific points in the cell cycle, as well as the phosphorylation and inhibition of CDKs by specific CDK inhibitors (CKIs). Any disruption in these regulatory mechanisms can lead to uncontrolled cell division and potentially contribute to cancer development.