DNA topoisomerase II (EC 5.99.1.3) is necessary for chromosome condensation and disjunction in yeast but not for other functions. In mammalian cells, it has been reported to be necessary for progression toward mitosis but not for transit through mitosis. We have found, on the contrary, that specific inhibition of topoisomerase II (but not of topoisomerase I) interferes with mammalian mitotic progression. Metaphase is prolonged, and anaphase separation of chromatids is completely inhibited, in cells given high concentrations of topoisomerase II inhibitors; nevertheless these cells attempt cleavage, sometimes generating nucleate and anucleate daughters. Lower concentrations of inhibitors interfere with anaphase and produce abnormalities of segregation. DNA topoisomerase II activity is therefore necessary for mammalian chromatid separation, but it is not tightly coupled to the control of other mitotic events.In eukaryotic cells DNA topoisomerase II (EC 5.99.1.3) is a major component of the mitotic chromosome core (1), with sites of action adjacent to the attachment points of supercoiled DNA loops (2). This structural role is conserved in all known cases.In budding and in fission yeast, temperature-sensitive top2 mutants show no defect in normal interphase cell functions, including cell cycle progression, at the restrictive temperature; but they are arrested in mitosis (3, 4). In top2 mutants of the fission yeast Schizosaccharomyces pombe, chromosome condensation is partly achieved at the restrictive temperature, but it produces long entangled prophase-like structures, which become fully condensed when the topoisomerase II activity is restored. If topoisomerase II is inactivated after chromosome condensation, the cells remain blocked at metaphase, unable to achieve chromatid separation (4). After release from such arrest, yeast cells show chromosome nondisjunction (5); meiotic nondisjunction can be similarly produced (6). This requirement for topoisomerase II in mitotic condensation and in separation of sister chromatids may be due to the need to separate sister strands of DNA, which inevitably become concatenated during replication but are not decatenated until later (7). Other cellular DNA topoisomerases appear to be adequate for other necessary manipulations of DNA topology but cannot perform this decatenation activity; in top2 mutants, concatenated plasmids accumulate at the restrictive temperature (3,8).Though the topoisomerase II function is necessary for chromatid segregation in fission yeast, other aspects of yeast mitosis continue without it. Temperature-sensitive top2 cells at the restrictive temperature divide into two portions, joined by chromatid fibers that prevent total cytokinesis; in top2 cdcll double mutants, in which septum formation is also blocked, cells reform single active nuclei after an abortive mitosis and enter a new round of replication (8).In mammalian cells the role of topoisomerase II in cycle progression has been reported to be rather different from its role in yeast. No tempe...