- Two experiments are described that examine the properties of a single DNA molecule. The experimental system is based on optical tweezers and on a protocol that allows to attach one end of the DNA molecule to the bottom of the sample and the other to a bead of micron size. First, the reaction of the RecA protein with DNA is monitored in a particular DNA molecule. This is made possible by the fact that the DNA is locally extended by a factor of 1.5 whenever a RecA molecule binds. Second, we monitor the dynamics of a DNA molecule as it relaxes from a fully stretched state to the coil configuration.