[Genomic DNA as compact particles: morphology, biochemical analysis and optical properties].

Compact DNA particles have been obtained by a new method we developed to isolate high-molecular-weight DNA. The method includes exhaustive proteolysis while the cellular DNA is folded in compact state in polyethylene glycol-containing aqueous-salt media. The DNA particles derived from interphase nuclei have a diameter of 8-10 microns and each of them contains the genome of an individual cell. The DNA particles derived from isolated metaphase chromosomes present tightly packed DNA of individual chromosomes. The compact particles contain no proteins detectable by electrophoretic analysis and so appear to be formed by DNA only. These data have been confirmed by electron microscopy, when the particles investigated with the protein monolayer technique have revealed only spread DNA molecules. Simultaneously the microscopic observations of DNA particles of different origin and of their behaviour in the absence of compactizing agents support the idea that during formation of compact particles the intrinsic features of DNA folding in nuclei and chromosomes have been conserved.