Direct visualization of DNA decondensation on mica by atomic force microscopy

Lambda DNA/Hind III solution in the presence of cations was gradually dilut ed into four different concentration, namely 50, 25, 10, 1 ng/mu l. The mor phology change during this process was observed by atomic force microscopy. Images reveal that at the highest concentration, a flower-like structure w ith multiple loops crossing at the same point was exhibited on mice. Follow ing it are some irregularly planer "layers" accompanied by occasional toroi ds. When DNA concentration was diluted to 10 ng/mu l, a large complex netwo rk structure appeared, which was eventually replaced by the extended and is olated DNA molecules at 1 ng/mu l. This structural transition may be helpfu l for us to understand the molecular mechanism of DNA decondensation.