References

H. (1988) The RAD9 gene controls the cell cycle response to DNA damage in Saccharomyces cerevisiae. Science 241, 317-322. 2 Nyberg, K. A., Michelson, R. J., Putnam, C. W., and Weinert, T. A. (2002) Toward maintaining the genome DNA damage and replication checkpoints. Annu. Rev. Genet. 36, 617-56. 3 Zhou, B. B. and Elledge, S. J. (2000) The DNA damage response putting checkpoints in perspective. Nature 408, 433-439. 4 Osborn, A. J., Elledge, S. J., and Zou, L....

Checkpoint Defects and Cancer

Genetic instability is one of the hallmarks of cancer, and its links to mutations in a number of checkpoint genes, aberrations in DNA repair machinery, and the cell cycle checkpoint pathways is well documented (1-10) However, the defects in checkpoints that result in predisposition to and diagnosis of cancer will be discussed elsewhere. Here, we focus on the significance of the defects with respect to cell cycle checkpoints and cancer. Except for ATR and Chk1, whose knockout causes embryonic...

DNA Damage Checkpoint Genes

P53 Dna Repair

The DNA damage checkpoint is a signal cascade that blocks the cell cycle at Gl, G2, or metaphase, or slows the rate of DNA replication in S phase. Besides cell cycle blockage, cells respond in different ways, including apoptosis, DNA repair, and activation of transcription. There are many genes involved in DNA damage checkpoint pathways that control both repair and cell cycle progression. However, it is beyond the scope of this article to discuss all their details. The most important genes and...

Spindle Assembly Checkpoint

How Does Meiosis Occur

Genetic studies in yeast as well as laser ablation and micromanipulation studies in animal cells have identified a checkpoint that delays sister-chroma-tid separation Fig. 4 163 . To ensure equal segregation of each homologous chromosome, the kinetochores in each sister chromatid pair must interact with microtubules, referred to as bipolar attachment. Several mitotic checkpoint proteins, including Madl, Mad2, Bubl, Bub3, BubRl, Mbsl, and CENP-E have now been shown to be kinetochore-associated...

The Molecular Basis of G2 Arrest

To understand how the cell cycle is regulated in response to DNA damage, mutants of budding and fission yeast were identified in which the cell cycle was no longer delayed in response to DNA damage 58 . This approach led to the concept of the cell cycle checkpoint, and uncovered many genes that form an important part of what we know about the workings of the DNA-damage response. Cell cycle checkpoints are regulatory mechanisms that ensure that cell cycle processes occur at the right time and in...

G2 Checkpoint

Checkpoint

G2 is the second restriction point in the cell cycle. This checkpoint assures that the replication machinery has completed DNA duplication. Simultaneously, it also makes sure that, in the replicated DNA, if any deletions or duplications of bases have occurred, they should be repaired before cells enter the prophase stage. Cdc25A is stable after phosphorylation by cyclin B-Cdc2 at G2 M in normal cells 140,141 . This phosphorylation allows this protein to remain functional during mitosis and...

SPhase Checkpoint in Response to DNA Replication Stress

Phase Trigger

Several types of agents are known to interfere with the function of replication forks and to elicit the S-phase checkpoint. These include agents that Fig. 4. Molecular mechanism of S-phase progression and the S-phase checkpoints. Upon initiation of DNA replication, DNA replication initiation factors such as MCM10, CDC45-Sld3 budding yeast and RPA and checkpoint complexes bind to the pre-replicative complex pre-RC on chromatin and trigger the unwinding of DNA. The hexameric MCM2-7 complex is...

The ATMATRp53Mediated Pathway

Spindle Checkpoint Pathway

The ATM ATR p53 pathway plays a pivotal role in one of the checkpoint mechanisms that arrest the cell cycle at G1 phase following DNA damage G1 checkpoint Fig. 3 . As described in Subheading 3, ATM is activated in response to IR, whereas ATR is activated in response to replication inhibition or UV-induced damage. The activated ATM or ATR then phosphorylates p53 on Ser-15 , and this phosphorylation causes MDM2 to dissociate from p53, which stabilizes p53 and leads to its accumulation 128,129 ....

The Function of p53

The p53 tumor suppressor was originally identified as a protein bound to the large T antigen of the SV40 tumor virus 104 . Early studies on p53 function suggested that it was an oncogene, because the first clones of p53 could immortalize primary cells and cooperate with other oncogenes such as Ha-ras to induce neoplastic transformation 105-108 . The true function of p53 as a tumor suppressor was established in a number of landmark studies showing that 1 transforming p53 alleles contained...

Spindle Check Point

Chk2 Chk1 Pathway

The lNK4a ARF locus encodes p16INK4a and p19ARF, two degenerated tumor suppressor proteins. Absence of p16INK4a composed of ANK activates CDK4 6 cyclin D kinase, which phosphorylates pRB, thereby activating the E2F DP1 transcription factor. Since the transcription of the ARF gene is regulated by E2F, expression of ARF is induced. ARF separates MDM2 from p53 by recruiting MDM2 into the nucleolus, possibly in collaboration with cyclin G1 and cyclin G2. This stabilizes p53 in the nucleus...

Defects in G1S Checkpoint and Cancer

Defects in the genome maintenance mechanisms, including DNA repair and cell cycle checkpoint pathways, are believed to enhance genetic instability and cause the accumulation of mutations and chromosomal aberrations that is a hallmark of cancer cells 155 . Most of the G1 S checkpoint transducers and effectors are classified as either tumor suppressors or proto-oncogenes, and their loss-of-function mutations or overexpression appear to play pivotal roles in many types of human tumors. Mouse...

Regulation of p53 Stability by ARF and MDM2

The lNK4a locus that generates p16INK4a also encodes a degenerated gene product called ARF after alternative reading frame 131 . Thus, the locus encodes two tumor suppressor proteins, p16INK4a and p19ARF p14ARF in humans , which activate the growth suppressive functions of pRB and p53, respectively 67 . ARF is a highly basic pI gt 12 , arginine-rich nucleolar protein 132 . Deletion of the ARF gene can inactivate p53 function in tumors where p53 itself remains intact. Transcription of the ARF...

The p53pRB Pathway Controls the G1S Transition

The p53 tumor suppressor gene TP53 is the most frequently mutated gene about 50 in human tumors, and encodes a 53 kDa transcription factor p53 that directly induces the expression of a substantial number of genes that are important for cell cycle regulation, DNA damage repair, and apoptosis 108,109 . Of the genes that are induced by p53, p21WAF1 plays a pivotal role in G1 arrest by inhibiting CDK4 6 cyclin D1 activity, thereby reducing the phosphorylation of pRB and promoting G1 arrest of the...