SCANNING-TUNNELING-MICROSCOPY STUDY OF A DNA FRAGMENT OF KNOWN SIZE AND SEQUENCE

Assays to visualize a DNA fragment that may serve as DNA test for furt her experiments were performed by scanning tunneling microscopy (STM). We present observations on a 350 bp restriction fragment from pBR322 deposited on natural graphite single crystal surfaces and imaged in va cuum. Measurement of the length and of helical parameters of the molec ule in conjunction with the fact that no features of the substrate or electrical characteristics were suggestive of a graphite artifact, all owed unambiguous identification of the molecule. Imaging of the entire DNA molecule has been achieved and high,resolution details are shown. Their correspondence to precise DNA sequences may be deduced from the ir localization on the image since the DNA sequence and orientation of the molecule is known. Therefore, both global and local sequence-dire cted variations of structural features along a DNA molecule are now ac cessible to analysis with STM imaging. The ability of STM ton investig ate DNA structures involved in gene activity appears thus to be very p romising.