Cell Cycle: The series of stages between one cell division and the next.
Four phases: G1 (Gap 1) , S (Synthesis), G2 (Gap 2) and M (Mitosis)
Average length of cell cycle is 16 hours (can widely vary)
G1 phase is the most variable in duration between cell types
The passage of the cell through the various stages is regulated by: 1. Cyclins and 2. Cyclin dependent kinases.
Cyclins bind to Cdk and induce a conformational change in Cdk that reveals its active site. These complexes help progress the cell though cell cycle checkpoints.
Inhibitors of Cdk such as p16 and p21 bind to Cdk and inhibit progress through the cell cycle.
Cyclins levels fluctuate throughout the cell cycle. Cdk levels are constant.
EGFR signalling pathway leads to Cyclin D transcription (linking cell cycle and growth factor pathways)
Cyclin-Cdk complexes act of multiple proteins including transcriptional regulator RB (and others such as cytoskeletal proteins, nuclear envelope proteins, Golgi apparatus, histones). Thus they control cell division through multiple end pathways.
Exposure to stressors (such as DNA damage) increase p53 levels in a cell. p53 increases transcription of p21 and hence causes cell cycle arrest at G1-S, thereby allowing time for DNA repair.