DNA strand separation studied by single molecule force measurements

We have separately attached the two complementary strands of one end of a DNA double helix to a glass slide and a glass microneedle. Displacing the slide away from the needle, the molecule is progressively pulled open and the changing deflection of the needle gives the corresponding variation in the opening force. Force signals which are very rich in detail are reproducibly obtained. The average level and amplitude of the force signal is almost independent of the opening velocity in the interval 20 nm/s to 800 nm/s. A theoretical description based on the assumption of thermal equilibrium allows us to link the measured force curves to the genomic sequence of the DNA. A molecular stick-slip motion is revealed, which in contrast to the dynamics of macroscopic solid friction is a deterministic and reproducible process. This process is considered experimentally and theoretically.