STRETCHED DNA STRUCTURES OBSERVED WITH ATOMIC-FORCE MICROSCOPY

Double-stranded DNA molecules are occasionally found that appear to be straightened and stretched in atomic force microscope (AFM) images. U sually pBS(+) plasmid and lambda DNA show relaxed structures with bend s and kinks along the strands and have measured contour lengths consis tent to about 5 - 7%; they also appear not to cross over each other, e xcept in very high concentrations. The anomalous molecules observed he re, compared with the majority of molecules in the preparation, show c ontour lengths increased by as much as 80% and have measured heights o f about half that of normal relaxed DNA. Some molecules also appear to be in transition between stretched and relaxed forms. These observati ons are consistent with an uncoiling of the DNA helix without breakage of the covalent bonds in the deoxyribose-phosphate backbone.