Direct Visualization of DNA Decondensation on Mica by Atomic Force Microscopy

Lambda DNA/Hind III solution in the presence of cations was gradually diluted into four different concentration, namely 50, 25, 10, 1 ng/(mu)l. The morphology change during this process was observed by atomic force microscopy. Images reveal that at the highest concentration, a flower-like structure with multiple loops crossing at the same point was exhibited on mica. Following it are some irregularly planer “layers” accompanied by occasional toroids. When DNA concentration was diluted to 10 ng/(mu)l, a large complex network structure appeared, which was eventually replaced by the extended and isolated DNA molecules at 1 ng/(mu)l. This structural transition may be helpful for us to understand the molecular mechanism of DNA decondensation.