Light microscopic structure of DNA in solution studied by the 4′,6-diamidino-2-phenylindole staining method

Abstract

Individual DNA molecules in solution can be visualized under a fluorescence microscope by using the DNA-binding dye 4′,6-diamidino-2-phenylindole and can be recorded on video as mobile structures (Morikawa & Yanagida, 1981). DNA in the presence of 10m m-MgCl 2 was found to adhere to the glass surface, so that 4′,6-diamidino-2-phenylindole-stained DNA can be filmed as still images. Fluorescence micrographs of DNA (bacteriophages T4, T3 and λ, yeast and chicken erythrocyte) taken by the present procedure are better in resolution than those obtained by video, showing structural details of DNA molecules hitherto not observed in solution. In the specimens prepared at the reduced shear stress, the folded particles and the short thick filaments were abundant. The shear stream extended them into the wavy and the straight thin filaments. The lengths of the thin filaments seen in viral DNA correlated well with those determined by electron microscopy. Our results suggest that a DNA molecule in solution forms a certain kind of supercoil of its own accord.