Thermal Denaturation of Plasmid DNA Observed by Atomic Force Microscopy

Abstract

(50 ng/ml) Plasmid DNA solutions were heated at different temperatures, and thermal denaturation of plasmid DNA was directly observed by atomic force microscopy (AFM). At room temperature DNA shows some open circular and supercoiled structure on the surface of new cleaved mica. When temperature was increased above 50°C, DNA molecules were partly denatured and the AFM images show globule structure inside the DNA chains. The globule structure resulted from a separation of double-strands into single ones with a subsequently coil-to-globule transition or intrachain recombination of base pairs. Above 80°C DNA appeared as granular structure, which came from complete denaturation and collapse of single strands. At 100°C DNA took place degradation into pieces. AFM studies also show that thermal denaturation of heterogeneous DNA is a multistep process with A–T base pairs separated first and G–C ones latter.