Checkpoint controls that couple mitosis to completion of DNA replication.

Cells treated with inhibitors of DNA synthesis do not normally enter mitosis. Incompletely replicated DNA apparently activates a regulatory mechanism that prevents activation of the mitotic inducer M-phase kinase by controlling the dephosphorylation of a critical tyrosine residue in the active site of the kinase. The control system may also target a second mitotic inducer, possibly the NIMA protein kinase. Unreplicated DNA may be detected and signalled by a complex of RCC1, a DNA-binding protein, and Ran, a Ras-related protein. This article reviews these recent developments and discusses the possibility that the control system also operates in the normal cell cycle, to ensure that mitosis strictly follows S phase.