The metaphase scaffold is helically folded: sister chromatids have predominantly opposite helical handedness.

We have studied the three-dimensional folding of the scaffolding in histone H1-depleted chromosomes by immunofluorescence with an antibody specific for topoisomerase II. Two different types of decondensed chromosomes are observed. The majority of the chromosomes are expanded, and the central fluorescence signal is surrounded by a large halo of chromatin. A much smaller number of chromosomes are more compact in length; they contain a smaller halo of chromatin and their scaffolds are not extended but folded into a genuine, quite regular helical coil. This conclusion is based on a three-dimensional structural analysis by optical sectioning. The number of helical coils is related to chromosome length. Surprisingly, sister chromatids have predominantly opposite helical handedness; that is, they are related by mirror symmetry.