1. Internal Control System: This is a complex network of proteins and enzymes that act as checkpoints to ensure the cell is ready to proceed to the next phase. These checkpoints monitor:
* DNA replication: Ensuring all DNA is copied accurately before moving to mitosis.
* DNA damage: Stopping the cycle if DNA is damaged, allowing time for repair.
* Cell size: Ensuring the cell is large enough and has sufficient resources to divide.
* Attachment of chromosomes to microtubules: Guaranteeing chromosomes are correctly aligned before separation.
* Cyclin-dependent kinases (CDKs): These enzymes, activated by cyclins, phosphorylate target proteins to control cell cycle progression.
* Checkpoints: These points within the cell cycle where progress is halted until specific criteria are met.
2. External Signals: These are signals from the environment that influence the cell cycle. They include:
* Growth factors: Proteins that stimulate cell growth and division.
* Nutrients: Adequate nutrients are essential for cell growth and division.
* Density-dependent inhibition: Cells stop dividing when they reach a certain density.
* Contact inhibition: Cells stop dividing when they come into contact with other cells.
* Apoptosis: Programmed cell death eliminates damaged or unnecessary cells.
The interplay of these internal and external factors ensures that cell division occurs only when it is appropriate and safe for the organism.
