What is the Cell Cycle?

The cell cycle consists of interphase and mitosis. Interphase includes G1, S, and G2 phases, during which the cell grows and replicates its DNA. Mitosis is the division phase where the cell separates replicated chromosomes and divides into two daughter cells. This cycle repeats to enable cellular growth and reproduction.

Cyclins are regulatory proteins that bind to cyclin-dependent kinases (CDKs) to activate them at specific cell cycle stages. Different cyclin-CDK complexes control transitions between phases. Positive regulators cell cycle cyclins and CDKs work together to phosphorylate target proteins, driving the cell forward through checkpoints and ensuring orderly progression.

Checkpoints are control mechanisms that monitor cell cycle progression and ensure DNA integrity before advancing to the next phase. The cell cycle control system check points and regulator proteins halt progression if problems are detected, allowing time for repair or triggering cell death if damage is irreparable. This prevents the propagation of mutations.

Cells coordinate growth and proliferation by monitoring cell size and nutrient availability through checkpoint mechanisms. When cells reach appropriate size thresholds, they receive signals to proceed through the cell cycle. This coordination ensures cells don't divide prematurely and maintains proper cell size homeostasis within tissues.

Cells have a finite replication capacity determined by telomere length. With each division, telomeres shorten until they become critically short, triggering replicative cell senescence. This process prevents unlimited cell division and protects against cancer, though it also contributes to aging. Shortening telomere length and replicative cell senescence represents a natural cellular aging mechanism.

M-CDK (mitotic cyclin-dependent kinase) accumulates as cells progress through G2 phase and triggers the onset of mitosis. When M-CDK activity reaches a threshold, it phosphorylates proteins essential for chromosome condensation, nuclear envelope breakdown, and spindle formation. M-CDK drives transition into mitosis through a series of coordinated phosphorylation events.

S-CDK (S-phase cyclin-dependent kinase) initiates DNA replication by phosphorylating licensing factors and replication machinery proteins. This activation allows DNA polymerase and associated enzymes to begin synthesizing new DNA strands. S-CDK initiates DNA replication through precise temporal regulation, ensuring each chromosome replicates exactly once per cell cycle.