Exit from Mitosis

To the observer, one of the most remarkable aspects of the eukaryotic mitotic cell cycle is the precision with which numerous complex processes are temporally coordinated. Aberrant initiation of key cycle events prior to the completion of preceding events can have severe consequences, such as genetic instability or cell death. The coordination of cell cycle events is achieved in part by checkpoints, regulatory mechanisms that enforce proper order. When Hartwell and Weinert introduced the checkpoint concept in 1989, these mechanisms were mostly philosophical constructions. In the ensuing decade of cell cycle research, one of the major accomplishments has been the molecular description of several checkpoint regulatory mechanisms. Three recent publications (Bardin et al. 2000; Bloecher et al. 2000; Pereira et al. 2000) illuminate yet another checkpoint mechanism by which the choice to execute or delay a cell cycle transition, exit from mitosis, is achieved. In a nutshell, the budding yeast Saccharomyces cerevisiae recognizes that it has finished the complicated business of mitosis when the bud cell body receives a progeny spindle pole. The signal that this has occurred is generated when a GTP binding protein located on the outside of the spindle pole is brought into the vicinity of an activator confined to the bud. Thus, the outcome of a motile process, mitotic spindle-mediated segregation of replicated chromosomes, is monitored spatially.