TOPOGRAPHICAL STRUCTURE OF PBR322 DNA STUDIED BY SCANNING TUNNELING MICROSCOPE AND ATOMIC-FORCE MICROSCOPE

Plasmid pBR322 DNA (0.5 mg/ml) isolated from Eschericha coli HB101 was suspended in Tris-HC1/EDTA (1/0.1 mol/L, pH 8.5), deposited on a high ly oriented pyrolytic graphite surface, then imaged with scanning tunn eling microscope (STM) in air and under paraffin liquid in constant cu rrent mode. The methods for preparing DNA samples used in the STM stud y are discussed. Dynamic changes of apparent width of supercoiled pBR3 22 DNA were captured during an STM imaging course. The pBR322 DNA is a lso visualized with an atomic force microscope (AFM) built by this lab oratory after the sample was absorbed on the freshly cleaved mica surf ace. Characteristically relaxed circular structures of pBR322 DNA have been observed in the AFM experiment.