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Institut de Biologie Structurale Jean-Pierre Ebel (CEA/CNRS), 38027 Grenoble cedex 1, France
Here we report that DNA decatenation is
not a physical requirement for the formation of mammalian chromosomes containing a two-armed chromosome scaffold. 2-aminopurine override of G2 arrest imposed by VM-26 or ICRF-193, which inhibit topoisomerase II (topo II)-dependent DNA decatenation,
results in the activation of p34cdc2 kinase and entry into
mitosis. After override of a VM-26-dependent checkpoint, morphologically normal compact chromosomes
form with paired axial cores containing topo II and
ScII. Despite its capacity to form chromosomes of normal appearance, the chromatin remains covalently
complexed with topo II at continuous levels during G2
arrest with VM-26. Override of an ICRF-193 block,
which inhibits topo II-dependent decatenation at an
earlier step than VM-26, also generates chromosomes
with two distinct, but elongated, parallel arms containing topo II and ScII. These data demonstrate that DNA
decatenation is required to pass a G2 checkpoint, but
not to restructure chromatin for chromosome formation. We propose that the chromosome core structure is
templated during interphase, before DNA decatenation, and that condensation of the two-armed chromosome scaffold can therefore occur independently of the
formation of two intact and separate DNA helices.
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