DNA in the centromeric heterochromatin of polytene chromosomes is topologically open.

Heterochromatin differs from euchromatin by a set of specific features. We suggested earlier that specific features of heterochromatin result from differences in DNA topology of these two chromatin types and provided explanations for the majority of them (Gruzdev 2000). We proposed that, unlike topologically closed euchromatic DNA, the DNA of heterochromatin is topologically open, i.e. it likely contains single- or doublestrand breaks. In this work, we studied the topological state of DNA in a block of centromeric heterochromatin and in a euchromatic banded region of Chironomus melanotus polytene chromosomes by microfluorimetric methods using the fluorescent intercalating dye ethidium bromide (EB). It was demonstrated that the fraction of topologically closed DNA in heterochromatin blocks is five-fold smaller than in the banded region. The data obtained support the hypothesis proposed.