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

In this work, we report observation of the fine structure of cobalt‐ hexammine‐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 intermediate structures in DNA condensation. Some toroid condensates had well‐defined small globule subunits; some other condensates were composed of nodular 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 toroid. These observations indicate that DNA condensation seems to involve five major stages: initial rapid association of chains to form gauze‐like complexes; 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 toroids. The results may shed light on uncovering the physical mechanism of DNA condensation by multivalent cations. Copyright © 2001 John Wiley & Sons, Ltd.