ATOMIC FORCE MICROSCOPY OF SINGLE-STRANDED AND DOUBLE-STRANDED DNA

A method has been developed for imaging single-stranded DNA with the atomic force microscope (AFM). PHI-X174 single-stranded DNA in formaldehyde on mica can be imaged in the AFM under propanol or butanol or in air. Measured lengths of most molecules are on the order of 1-mu, although occasionally more extended molecules with lengths of 1.7 to 1.9-mu are seen. Single-stranded DNA in the AFM generally appears lumpier than double-stranded DNA, even when extended. Images of double-stranded lambda DNA in the AFM show more sharp kinks and bends than are typically observed in the electron microscope. Dense, aggregated fields of double-stranded plasmids can be converted by gentle rinsing with hot water to well spread fields.