Supramolecular organization of double-stranded DNA molecules in the columnar hexagonal liquid crystalline phase. An electron microscopic analysis using freeze-fracture methods.

I present an electron microscopical analysis of the columnar hexagonal liquid crystalline phase of DNA. Freeze-fracture methods reveal that this phase is a lamellar structure, each layer (30 to 40 A thick) composed of DNA molecules aligned in parallel. Numerous defects can be seen in the structure, and their nature is determined. I show that they are mainly screw dislocations of both handedness. By this method it is possible to follow individual double-stranded DNA molecules in this highly packed structure. I show, moreover, that there is a local twist between DNA molecules along the screw dislocation lines and that this twist can be either right-handed or left-handed. The interest of such ultrastructural analysis is discussed in relation to the understanding of chromatin structure.