Internal Control:
* Cyclins and Cyclin-Dependent Kinases (CDKs): These proteins act as a master regulator of the cell cycle.
* Cyclins: Proteins that fluctuate in concentration throughout the cell cycle. They activate CDKs.
* CDKs: Enzymes that phosphorylate (add phosphate groups to) target proteins, triggering their activation or deactivation.
* Checkpoints: These are critical control points that ensure the cell cycle progresses correctly. They monitor DNA replication, cell size, and other factors.
* G1 Checkpoint: Checks for cell size, nutrients, and growth factors.
* G2 Checkpoint: Ensures DNA replication is complete and undamaged.
* M Checkpoint (Spindle Checkpoint): Ensures all chromosomes are properly attached to the spindle fibers before mitosis.
External Control:
* Growth Factors: Signaling molecules that stimulate cell division.
* Nutrients: Essential for building the components needed for cell growth and division.
* DNA Damage: The presence of DNA damage can trigger cell cycle arrest to allow for repair.
How it Works:
1. Cyclin-CDK Complexes: Fluctuations in cyclin levels throughout the cycle form different cyclin-CDK complexes that trigger specific events. For example, the cyclin E-CDK2 complex promotes the transition from G1 to S phase.
2. Checkpoints: If a checkpoint detects an error, it can halt the cell cycle until the issue is resolved. This prevents the propagation of damaged cells.
3. External Signals: Growth factors and nutrients can activate signaling pathways that influence the expression of cyclins and CDKs, thus controlling the cell cycle progression.
In summary:
The cell cycle is a tightly regulated process orchestrated by a complex interplay of internal and external factors. Cyclins and CDKs act as the main internal drivers, while checkpoints ensure fidelity, and external signals like growth factors provide context-dependent regulation. This intricate system ensures proper cell division and maintains the integrity of the organism.
