Although the dependencies among cell cycle events and the coordination of growth and division have been recognized for a long time, it was not until Hartwell and Weinert
(1989) introduced the concept of ‘checkpoints’ that the underlying regulatory principles became clear. Hartwell and Weinert hypothesized the existence of ‘checkpoint pathways’ by which an uncompleted cell cycle event sends an inhibitory signal to laterevents. They provided evidence for such pathways by studying radiation-sensitive mutants of budding yeast. In response to DNA damage caused by radiation, wild-type cells stop dividing until the damage can be repaired. In contrast, mutations in the RAD9 gene
Secondary article
Article Contents
. Cell Cycle Logic
. What is a Checkpoint? . The CellCycle Engine . G1 Checkpoints . DNA Damage . G2 Checkpoints . Metaphase (Spindle) Checkpoint . Cell Cycle Checkpoints andCancer
permit irradiated cells to divide and consequently die. Hartwell and Weinert demonstrated that these mutants are not defective in DNA repair per se. Rad9 (the protein product of the RAD9 gene) is not essential for normal progression through the cell cycle but rather for the checkpoint mechanism that is responsible for blocking cell division after DNA damage.
The eukaryoticcell cycle isguarded at three checkpoints: at the G1/S boundary, the G2/M boundary, and the metaphase/anaphase boundary. Progress through the chromosome cycle can be halted at these checkpoints if the conditions for successful cell division are not met. To understand how these checkpoints work, we must first recognize the ‘accelerators’ and ‘brakes’ that control progression of the cell cycle engine, and then describe the surveillance mechanisms that sense unfavourable condi- tions and communicate halt signals to the engine (Figure1).