A POSTPROPHASE TOPOISOMERASE II-DEPENDENT CHROMATID CORE SEPARATION STEP IN THE FORMATION OF METAPHASE CHROMOSOMES

Metaphase chromatids are believed to consist of loops of chromatin anc hored to a central scaffold, of which a major component is the decaten atory enzyme DNA topoisomerase II. Silver impregnation selectively sta ins an axial element of metaphase and anaphase chromatids; but we find that in earlier stages of mitosis, silver staining reveals an initial ly single, folded midline structure, which separates at prometaphase t o form two chromatid axes. Inhibition of topoisomerase II prevents thi s separation, and also prevents the contraction of chromatids that occ urs when metaphase is arrested. Immunolocalization of topoisomerase II alpha reveals chromatid cores analogous to those seen with silver sta ining. We conclude that the chromatid cores in early mitosis form a si ngle structure, constrained by DNA catenations, which must separate be fore metaphase chromatids can be resolved.