FIG. 53.

Cryo-electron micrograph of a densely packed monolayer of bacteriophage T7 heads recorded at 2.5-μm defocus (56). The particles exhibit characteristic fingerprint patterns with 25-Å spacings associated with closely packed strands of dsDNA. The pattern motifs vary according to the viewing direction, with concentric rings revealed in axial views and punctate arrays revealed in side views. In the particular double mutant sample shown here, the T7 heads adopt a preferred axial orientation. Modeling analysis of these and other images established that the T7 chromosome is spooled around an axis in approximately six coaxial shells in quasi-crystalline packing (56). Adapted from reference 56 with the permission of the author and the publisher.