Electron crystallographic studies of DNA

The conformation of DNA in thin, platelet crystals is being studied by electron microscopy and diffraction, and by other techniques including circular dichroism spectroscopy. Specimens are prepared in the frozen-hydrated state for the electron microscope. Electron diffraction patterns from the crystals extend to 3.5 A, and lattice images with resolution to 6.5 A have been obtained. Electron diffraction patterns from fibers in the same preparations show a stacking distance between base pairs which is larger than in most DNA conformations. The data so far available can be interpreted to indicate that the DNA in the crystals is in a conformation which has not previously been reported and which is in the B family, unwound to have 12 base pairs per turn. Model calculations have been performed which support this interpretation and which show that a three-dimensional reconstruction, with even very limited data, could confirm the helix parameters.