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#G1 checkpoint

2 public questions tagged with this topic.

Cell Cycle Regulation

What is the role of p53 in the G1 checkpoint?

G1 progression toward S phase is driven by cyclin-dependent kinases CDK4/6-cyclin D and CDK2-cyclin E that phosphorylate retinoblastoma protein Rb, releasing transcription factor E2F to induce genes for DNA replication enzymes, nucleotide biosynthesis, and replication licensing factors such as Cdc6. DNA damage in G1 would risk copying lesions, necessitating arrest. Tumor suppressor p53 serves as guardian by acting as transcription factor stabilized upon ATM-CHK2 signaling after double-strand breaks. One of its principal targets is CDKN1A encoding p21CIP1/WAF1, a potent inhibitor of CDK2-cyclin E and CDK2-cyclin A complexes. p21 binds and inhibits kinase active site, preventing phosphorylation of Rb and downstream substrates like Cdc6 and keeping E2F repressed, thereby imposing G1 arrest and allowing time for repair. p53 also induces GADD45 and 14-3-3 sigma contributing to pause. p53 does not degrade Rb, directly phosphorylate Cdc25, or ubiquitinate securin; those actions belong to other regulators. Through p21 induction, p53 enforces G1 checkpoint, and loss of this axis contributes to unchecked proliferation and genomic instability in majority of human cancers.

Ref: El-Deiry et al., Cell 1993, p21 as p53 Target; Bertoli et al., Nature Rev Mol Cell Biol 2013, G1-S Regulation.

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Cell Cycle Regulation

Which checkpoint prevents cells with damaged DNA from entering S phase?

Cell cycle progression in eukaryotes is guarded by surveillance mechanisms that halt advance when conditions are unfavorable. At transition from G1 to S phase, cell must ensure adequate size, nutrients, growth factor signaling, and integrity of genome before committing to DNA replication, an irreversible step that duplicates entire genetic content. G1 checkpoint, often termed restriction point in mammalian cells, integrates signals from cyclin D-CDK4/6 and cyclin E-CDK2 that phosphorylate retinoblastoma protein Rb, releasing E2F transcription factors driving S-phase genes. If DNA damage, oxidative stress, or insufficient nucleotides are detected, pathways involving p53-induced p21 and CHK2 activate to inhibit CDK2-cyclin E, keeping Rb hypophosphorylated and preventing entry. Double-strand breaks sensed by ATM activate p53 stabilization. Failure of this checkpoint allows replication of damaged templates, increasing mutation rate and chromosomal instability predisposing to cancer. G2 checkpoint instead monitors completion of replication, M checkpoint monitors spindle attachment. Hence G1 checkpoint specifically blocks damaged cells from entering S, preserving genomic stability.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Cell Cycle Control and Checkpoints; Sherr, Science 1996, G1 Progression.

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