Protein-free spreading technique for the electron microscopy of DNA and chromatin

A new protein-free technique of preparation of DNA molecules for electron microscopy is described. The technique is based on the phenomenon of formation of a thin liquid film of a high concentration buffer on the surface of the hypophase of the lower concentration buffer on depositing a droplet of the high concentration buffer on the hypophase. We found that under certain specific conditions most of all frequently used buffers form films of such a type. We have chosen an ammonium acetate buffer for the preparation of DNA molecules for electron microscopy. The diameter of the molecules prepared in compliance with the described technique ranges from 2.5 to 3 nm (according to shadow measurements made after one direction shadowing), this being in agreement with the actual diameter of DNA molecules. It is shown that for chromatin preparation Tris-HCl buffer can be used to advantage. Suitable buffers can be selected for different objects under investigation.