Deep-etched, replicated DNA toruses held on edge in ice demonstrate a complex 3-D substructure by extensive tilting along shadowing direction
Torus shaped spermidine-DNA structures formed in vitro are thought to have a DNA organization similar to certain double stranded DNA bacteriophage and viruses. For this reason we have used freeze-etch low Pt-C metal replica TEM to visualize toruses formed from a variety of condensed DNAs. Both Calf Thymus DNA and circular and linear ϕX-174 RFII DNA freeze-etch TEM and biochemical data support a circumferential DNA winding model for hydrated spermidine-condensed DNA toruses. Heretofore, we utilized stereoscopic measurements from tilt views of thin replicas (60-90 Å) of toruses to yield information about the DNA topology and thickness of these objects. In a companion paper in these proceedings we examine the precision of such stereoscopic measurements on tilt micrographs of a torus. In our attempts to visualize DNA toruses we have noted that deep-etched toruses are generally suspended above the surface by virtue of a residual attachment to the ice.