Which event marks the commitment to cell division?
Late G1 restriction point defines irreversible commitment to divide independent of external growth factors. Its molecular basis revolves around retinoblastoma protein Rb and E2F transcription factors. In quiescence, hypophosphorylated Rb binds E2F-DP1 heterodimers, recruiting histone deacetylases, SWI/SNF remodelers and H3K9 methyltransferases to repress genes needed for S phase. Mitogen signaling induces Cyclin D synthesis via Ras-MAPK and PI3K pathways. Cyclin D-CDK4/6 complexes accumulate, phosphorylate Rb at multiple C-terminal residues, partially reducing repression and derepressing limited E2F target transcription including Cyclin E gene. Cyclin E then binds CDK2, forming potent kinase that hyperphosphorylates Rb at additional pocket domain residues, creating complete disruption of Rb-E2F interaction. This generates positive feedback loop producing large burst of E2F activity transcribing DNA polymerase subunits, dihydrofolate reductase, Cdc6, Cyclin A and MCM proteins. After this switch, growth factor removal no longer halts progression; cells complete division autonomously, highlighting Rb phosphorylation as commitment event essential for proliferative control and cancer relevance. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.
Ref: Weinberg, Retinoblastoma Protein and Cell Cycle Control, Cell 1995; Alberts et al., Molecular Biology of the Cell, Chapter 17, Rb-E2F Switch.