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G1/S

The primary G1/S cell cycle checkpoint controls the commitment of eukaryotic cells to transition through the G1 phase and enter into

the DNA synthesis phase (S). Two cell cycle kinase-complexes, CDK4/6-Cyclin D and CDK2-Cyclin E, work in concert to relieve inhibition

of a dynamic transcription complex that contains the retinoblastoma protein, Rb, and E2F. In G1-phase uncommitted cells, hypo-

phosphorylated Rb binds to the E2F-DP1 transcription factors in a repressive complex containing HDAC, thus inhibiting key downstream

transcription events. Phosphorylation of Rb by Cyclin D-CDK4/6 and subsequently by Cyclin E-CDK2 dissociates the repressor complex

from Rb, permitting transcription of key S-phase-promoting genes required for DNA replication. CDK2 may also phosphorylate FoxO1,

which inhibits its transcriptional activity by nuclear export and allows for survival and proliferation. Importantly, a multitude of differentstimuli exert checkpoint control, including TGF-, DNA damage, replicative senescence, and growth factor withdrawal. These stimuli act

though transcription factors to induce specific members of the INK4 or KIP/CIP families of cyclin dependent kinase inhibitors (CKIs).

Mounting evidence implicates the polycomb protein BMI1 in negative regulation of INK4A/B in stem cells and cancer. In addition to

regulating CKIs, TGF- also inhibits cdc25A transcription, a phosphatase directly required for CDK activation. At a critical convergence

point with the DNA-damage checkpoint, cdc25A is ubiquitinated and targeted for degradation via the SCF ubiquitin ligase complex

downstream of ATM/ATR/Chk-pathway. However, timely degradation of cdc25A in mitosis (M-phase) via the APC ubiquitin ligase

complex allows progression through mitosis. Furthermore, growth factor withdrawal activates GSK-3, which in turn phosphorylates

Cyclin D, leading to its rapid ubiquitination and proteasomal degradation. Collectively, ubiquitin/proteasome-dependent degradation and

nuclear export are mechanisms commonly used to rapidly reduce the concentration of cell cycle control proteins.

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The G2/M DNA damage checkpoint serves to prevent the cell from entering mitosis (M-phase) with genomic DNA damage. Specifically, th

activity of the cdc2-Cyclin B complex is pivotal in regulating the G2-phase transition wherein cdc2 is maintained in an inactive state by th

tyrosine kinases Wee1 and Myt1. It is thought that coordinated action of the kinase Aurora A and the cofactor Bora activate PLK1 as cel

approach the M-phase, which in turn activates the phosphatase cdc25 and downstream cdc2 activity, establishing a feedback amplification loo

that efficiently drives the cell into mitosis. Importantly, DNA damage cues activate the sensory DNA-PK/ATM/ATR kinases, which relay two paral

cascades that ultimately serve to inactivate the cdc2-Cyclin B complex. The first cascade rapidly inhibits progression into mitosis: the Chk kinase

phosphorylate and inactivate cdc25, which then prevents activation of cdc2. The slower second parallel cascade involves phosphorylation of p5

and allows for its dissociation from MDM2 and MDM4, which activates DNA binding and transcriptional regulatory activity, respectively. Th

transcriptional ability of p53 is further augmented through acetylation by p300/PCAF. The second cascade constitutes the p53 downstrea

regulated genes including: 14-3-3, which binds to the phosphorylated cdc2-Cyclin B complex and exports it from the nucleus; GADD45, which bin

to and dissociates the cdc2-Cyclin B complex; and p21 Cip1, an inhibitor of a subset of the cyclin-dependent kinases including cdc2. In huma

cancer, p53 is commonly mutated indicating that this checkpoint is a critical barrier to tumor formation. In addition, sporadic as well as familia

mutations in the DNA-repair proteins such as the BRCA-family, ATM and the Fanconi Anemia proteins further highlight this key tumor suppresso

checkpoint.