1. Internal Control System:
* Cyclins and Cyclin-Dependent Kinases (CDKs): These are the core regulatory proteins. Cyclins are proteins whose levels fluctuate throughout the cell cycle, while CDKs are kinases (enzymes that add phosphate groups) that are always present.
* Cyclin-CDK Complexes: When cyclins bind to CDKs, they become active and phosphorylate (add phosphate groups to) target proteins. This phosphorylation triggers specific events in the cell cycle, like DNA replication or chromosome condensation.
* Checkpoints: These are "quality control" points that ensure the cell cycle progresses only when certain conditions are met. For example, the G1 checkpoint ensures that the cell has the necessary nutrients and that DNA is not damaged before entering S phase (DNA replication).
2. External Signals:
* Growth factors: These are proteins that stimulate cell division. They bind to receptors on the cell surface and activate signaling pathways that ultimately influence the activity of cyclins and CDKs.
* Nutrients: Sufficient nutrients are essential for cell growth and division. Cells will not enter the cell cycle without adequate nutrients.
* Cell density: Cells can sense the density of cells around them and regulate their own division accordingly. This helps to prevent uncontrolled growth.
* DNA damage: If DNA is damaged, the cell cycle can be halted to allow for repair. This prevents the transmission of mutations to daughter cells.
3. Other Factors:
* Telomere length: The ends of chromosomes, called telomeres, shorten with each cell division. When telomeres become too short, the cell may enter senescence (a state of irreversible growth arrest) or undergo apoptosis (programmed cell death).
* Oxygen levels: Cells require oxygen for respiration and energy production. Low oxygen levels can trigger cell cycle arrest.
In summary: The cell cycle is a tightly controlled process that is regulated by a complex interplay of internal and external signals. These signals ultimately influence the activity of cyclins and CDKs, which drive the cell cycle through its various phases.
