Cells use biochemical signaling mechanisms known as checkpoints to monitor the status of the cell so that cell division occurs only when conditions allow for successful mitosis. One such checkpoint allows cell division to occur only if DNA replication is complete and no active replication forks are present. However, Torres-Rosell et al. (see the Perspective by Weinert) describe experiments in which uncompleted replication of ribosomal DNA (rDNA) genes does not prevent cells from proceeding into anaphase. Thus, at least in the scenario studied, in which yeast bear mutations in the genes encoding the Smc5 and Smc6 proteins (which function as a heterodimer in DNA repair), the delayed replication of rDNA did not trigger a checkpoint that blocks progression of the cells into mitosis.
J. Torres-Rosell, G. De Piccoli, V. Cordon-Preciado, S. Farmer, A. Jarmuz, F. Machin, P. Pasero, M. Lisby, J. E. Haber, L. Aragón, Anaphase onset before complete DNA replication with intact checkpoint responses. Science 315, 1411-1415 (2007). [Abstract] [Full Text]