APPLICATIONS FOR ATOMIC-FORCE MICROSCOPY OF DNA

Tapping mode atomic force microscopy (AFM) of DNA in propanol, dry hel ium, and aqueous buffer each have specific applications. Resolution is best in propanol, which precipitates and immobilizes the DNA and prov ides a fluid imaging environment where adhesive forces are minimized. Resolution on exceptional images of DNA appears to be similar to 2 nm, sufficient to see helix turns in detail, but the smallest substructur es typically seen on DNA in propanol are similar to 6-10 nm in size. T apping AFM in dry helium provides a convenient way of imaging such thi ngs as conformations of DNA molecules and positions of proteins on DNA . Images of single-stranded DNA and RecA-DNA complexes are presented. in aqueous buffer DNA molecules as small as 300 bp have been imaged ev en when in motion. Images are presented of the changes in shape and po sition of circular plasmid DNA molecules.