Visualization of the intermediates in a uniform DNA condensation system bytapping mode atomic force microscopy

In this work, we report observation of the fine structure of cobalt-hexammi ne-induced DNA condensates by tapping mode atomic force microscopy (AFM). A uniform system with minimum fluctuations of concentration was obtained by introducing a slow-evaporation method, which enables investigation of the i ntermediate structures in DNA condensation. Some toroid condensates had wel l-defined small globule subunits; some other condensates were composed of n odular sections with gauze-like complexes spreading around at the outskirts ; typical images revealed badly assembled toroidal condensates composed of small globules with a fraction of loosely associated globules dotted in the gauze-like complexes spreading around the badly-assembled part of the toro id. These observations indicate that DNA condensation seems to involve five major stages: initial rapid association of chains to form gauze-like compl exes; collapse of these gauze-like complexes at some critical sizes; growth of the critical nucleus by accretion, formation of rods by aggregation of condensed particles; and association of rod-like condensates to form toroid s. The results may shed light on uncovering the physical mechanism of DNA c ondensation by multivalent cations. Copyright (C) 2001 John Wiley & Sons, L td.