Internal Factors:
* Cyclins and Cyclin-Dependent Kinases (CDKs): These proteins act as internal "clocks" of the cell cycle.
* Cyclins are regulatory proteins whose levels fluctuate throughout the cell cycle.
* CDKs are enzymes that are activated by cyclins. They phosphorylate (add phosphate groups to) other proteins, which triggers specific events within the cell cycle.
* Checkpoints: These are surveillance mechanisms that monitor the cell's progress through the cycle and ensure that each phase is completed correctly before moving on.
* G1 checkpoint: Checks for DNA damage and sufficient nutrients before entering the S phase.
* G2 checkpoint: Ensures DNA replication is complete and there are no errors before entering mitosis.
* M checkpoint (Spindle checkpoint): Confirms that all chromosomes are properly attached to the spindle fibers before proceeding with anaphase (separation of chromosomes).
* DNA Replication and Repair Mechanisms: The cell cycle is tightly coupled with DNA replication and repair processes. These mechanisms ensure that the genetic material is accurately copied and any errors are corrected.
External Factors:
* Growth Factors: These are signaling molecules that stimulate cell division and promote progression through the cell cycle.
* Nutrients: Cells require a sufficient supply of nutrients to support growth and division.
* Oxygen: Many cells require adequate oxygen levels for energy production and normal cell function.
* Cell-Cell Contact: This can inhibit cell division and regulate the growth of tissues.
* DNA Damage: External factors, such as radiation or chemicals, can damage DNA and trigger cell cycle arrest or apoptosis (programmed cell death).
Together, these internal and external factors create a sophisticated system that ensures proper cell division and prevents uncontrolled growth. This regulation is essential for maintaining tissue homeostasis, preventing cancer, and ensuring the proper development of multicellular organisms.
