The distribution of topoisomerase II on mammalian chromosomes

The distribution of topoisomerase II (Topo II) has been studied using immunofluorescence on cytocentrifuged preparations of mammalian chromosomes. Immunolabelling of Topo II is affected by choice of fixative, by barriers to accessibility and by the lability of the enzyme. Chromosomes still embedded in cytoplasm remain unlabelled, while in contrast Topo II can easily be lost from some sites in chromosomes free of cytoplasm. The definitive distribution of Topo II consists of a line along the centre of each chromatid, corresponding to the chromosome core or scaffold, and quantities of Topo II elsewhere in the chromosomes which vary during the course of mitosis. A strong reaction for Topo II can be seen throughout prophase chromosomes, consistent with a role in condensation and/or segregation of the chromosome arms at this stage. At metaphase, Topo II is restricted to the centromeric regions, the only parts of the chromosomes that still have to be separated at this stage, while in anaphase, after segregation has occurred, this centromeric concentration of Topo II is lost. The distribution and quantity of Topo II in mammalian chromosomes is thus wholly consistent with the known functions of this enzyme in chromosome condensation and segregation.