Atomic force microscopy of long DNA: Imaging in air and under water

We have obtained striking atomic force microscopy images of the intact λ bacteriophage genome and of several λ restriction fragments both in air and under water. The DNA is unstained and the images are stable under continuous scanning for up to 30 min. Measured contour lengths of fully imaged restriction fragments and intact λ DNA are accurate to within a few percent. The key to this development is the use of a process for binding unmodified double-stranded DNA to chemically treated mica surfaces. This procedure leads to strong DNA attachment and yields high-quality images that are stable under repeated scanning, even with the sample submerged in water. This allows normal hydration conditions to be maintained during scanning and in addition leads to a general improvement of image quality. Both the lateral resolution and the contrast increase by a factor of ≈3 under water.