Molecular Combing For Stretching Single-Stranded Phage Genomes on Conductive Graphite Surfaces

We have developed a method for stretching phage deoxyribonucleic acids (DNAs) on a conductive graphite surface and imaging them with nanometer-scale resolution employing atomic force microscopy (AFM). This method depends on careful control of the graphite surface charge, appropriate selection of the ionic strength of the DNA solution, and controlled translation of a droplet of DNA solution along the surface. The single-stranded (ss)-lambda phage DNA (up to 23130 bases) and the PhiX174 DNA (5386 bases) molecules have been elongated by dragging the meniscus on the flat highly ordered pyrolytic graphite (HOPG) surface and manipulating the surface tension between the DNA solution and the substrate. The precise positioning and stretching of DNA molecules combined with ultrahigh-resolution methods such as scanning tunneling microscopy (STM) and AFM provide intriguing new opportunities for direct electronic sequencing of DNA.