Positive Regulator Molecules

Cyclins and cyclin-dependent kinases (CDKs) form complexes that drive cell cycle progression. CDKs remain inactive by default until they bind to a specific cyclin. Once bound, the CDK-cyclin complex phosphorylates target proteins, altering their function and signaling the cell to advance to the next phase. When the cyclin is degraded, the CDK becomes inactivated, marking the end of that phase.

CDKs remain at relatively constant levels throughout the cell cycle because their activity is controlled by cyclin availability rather than CDK production. Cyclins are synthesized and degraded at specific checkpoints, allowing cells to regulate which target proteins CDKs phosphorylate. This design ensures precise timing of cell cycle events without requiring constant CDK production and destruction.

During G1 phase, cyclin D is synthesized and binds to a CDK, forming a complex that promotes the cell's transition into S phase. As the cell enters S phase, cyclin D levels decline and cyclin E peaks, forming a new CDK complex that promotes DNA replication. This sequential cyclin activation ensures orderly progression through cell cycle checkpoints.

Cyclin A concentrations increase throughout S phase and remain high into G2, promoting entry into M phase when bound to CDK. After cyclin A is degraded, cyclin B levels peak in M phase, activating the different stages of mitosis. When cyclin B levels drop, the cell exits mitosis and completes cell division mitosis and cytokinesis.

The specific cyclin bound to a CDK directs it to different target proteins needed for that cell cycle stage. Each cyclin-CDK complex recognizes and phosphorylates distinct target proteins appropriate to its phase. This specificity ensures that only the correct proteins are modified at each checkpoint, maintaining proper cell cycle regulation.

Cytoplasmic enzymes degrade cyclins at specific points in the cell cycle, causing their levels to decline and inactivating their associated CDKs. This degradation signals the end of a particular phase and allows new cyclins to accumulate for the next checkpoint. The predictable pattern of cyclin synthesis and degradation ensures forward momentum through the cell cycle.

Positive regulators including cyclins and CDKs actively promote cell progression through regulatory checkpoints by phosphorylating target proteins that advance the cycle. Without these positive regulators, cells would stall at checkpoints. They work alongside negative regulators cell cycle to ensure cells advance only when conditions are appropriate for division.