Wilhelm bernhard workshop on the cell nucleus

PERSISTENT DNA DECATENATIONS BETWEEN SISTER CHROMATIDS INDUCE CHECKPOINT DELAYS IN G2 AND EARLY MITOSIS

De la Torre C, Moreno Díaz de la Espina S, Clarke DJ* and Giménez-Abián JF.

Centro de Investigaciones Biológicas, CSIC, Madrid, Spain, and *University of Minnesota, Department of Genetics, Cell Biology and Development, Minneapolis, USA.
Chromosomal assembly and condensation in the cell cycle require topoisomerase II α. When inhibiting it, DNA catenations are kept unresolved. We show that topo II tasks are surveyed by checkpoints during G2 and early mitosis, in Allium cepa L. meristem cells. Checkpoints are signal transduction pathways that brake irreversible phase transitions when intracellular conditions make them undesirable, while licensing them when requirements are fulfilled. Progression from reversible to irreversible chromosome condensation, nuclear envelope breakdown and segregation of sister centromeres are the irreversible transitions in early mitosis. When inhibiting topo II with ICRF-193, undue override of these transitions took place after a delay. This override, known as checkpoint adaptation, allows us to define the unfulfilled requirement that activated the checkpoint. The topo II-dependent individual-ization of chromosomes, brought about by the resolution of randomly produced non-replicative catenations, proved to be a prerequisite for the reversible to irreversible chromosome condensation transition, taking place in midprophase. The resolution of replicative catenations linking sister chromatid arms was a prerequisite for the checkpoint that licensed nuclear envelope breakdown, at the onset of prometaphase. The resolution of residual replicative catenations between sister centromeres was required for the licensing of chromosome segregation by the checkpoint known as the spindle checkpoint, which also surveys whether all duplicated centromeres are bound to both spindle poles via microtubules. While the checkpoint controlling the midprophase transition co-surveys unreplicated DNA, unrepaired DNA and cell size, those, if any, co-surveyed by the checkpoint controlling initiation of prometaphase are unknown.