DNA unzipping and force measurements with a dual optical trap

In order to open the DNA double helix mechanically, a molecular construction is prepared which allows specific attachment of the two complementary strands of an individual molecule to two different μm-sized beads. The beads are separately captured by a dual optical trap, thus holding the DNA construction in solution. The opening of a molecule is obtained by increasing the distance between the traps, one trap being slowly moved while the other is held fixed. Force is measured to sub-piconewton precision by back focal plane interferometry of the bead in the fixed trap. The experiment allows us to measure base sequence-dependent force signal. In this chapter, important technical aspects of this type of single-molecule force measurements are considered.