1. G2/M checkpoints Are imperfectly understood Monitor “completion” Assess genetic integrity Are comprised of signal, sensor, transducer and effector elements Regulate cell cycle, DNA repair and apoptosis

2. G2/M checkpoints The hallmark of checkpoints is cell-cycle delay in the face of damage Generally cannot be detected unless cell cycle is perturbed e.g., DNA damage by irradiation Cannot be identified in the absence of mutation e.g., mutation in checkpoint component leads to mitosis in spite of damage

3. Identification of G2/M Checkpoints Pioneered by Hartwell Sought mutations conferring “relief of dependence” Screened for mutants with the ability to undergo cell division after X-irradiation DNA repair mutants: arrest permanently (not transiently) in G2 Checkpoint (rad9) mutants: enter mitosis after X- irradiation with broken chromosomes and ensuing cell death

4. What do we now know about G2/M checkpoints? Gradually, signals, sensors, transducers and effectors are being unraveled. Experimentally, the DNA damage signal is best studied. Sensors and transducers that recognize the signal include ATM and ATR gene products. Transducers include CHK1 and CHK2 proteins that interact with CDC25C. Both p53 and 14-3-3 proteins are important. Apoptosis may be the final response in the face of overwhelming DNA damage.

5. Checkpoints intersect with kinases and phosphatases CHK1 --> CHK1-P ATM DNA damage