ABSTRACT
This chapter focuses on a particular type of physical assay which allows readers to study the properties of biomolecules at a single-molecule level. This technique makes use of an important property of DNA—it is a flexible molecule. One needs to learn something about how to treat and characterize DNA as an elastic object. The configuration of the dangling DNA, or even more precisely the motion of the bead at the extremity of the DNA, resembles the motion of the tip of a top in the gravitational field. This is a problem of classical mechanics which was first studied systematically by Leonhard Euler. The two main methods are magnetic and optical tweezers in which a small bead is fixed to the DNA, which either responds to a magnetic field gradient, which is the case if a superparamagnetic bead is used, or to a focused laser beam in the optical case, in which electrical field gradients operate.
