FIG. 2.

Relaxation responses of individual DNA molecules in (a) dilute and ((b) and (c)) concentrated (entangled) solutions. (a) Relaxation of a single DNA molecule, where the DNA is stretched around 40 μm in a flow and its relaxation is measured after cessation of the flow. Subsequent images are separated by 4.5 s. Reproduced with permission from Perkins et al., Science 264, 822 (1994). Copyright 1994 The American Association for the Advancement of Science.⁵⁶ ((b) and (c)) Series of images showing reptating tube-like DNA motion in a concentration solution of DNA molecules ((a): 12 molecules/μm³, (b): 7 molecules/μm³). The fluorescently labeled DNA was tethered to a fluorescent microbead, which was controlled by optical tweezers. The sequence of images starts at time 0 s (top row, left) after stopping the movement of the microbead. In (b) the image sequence is shown up to 128 s (bottom row, right). Note that the topological constraints imposed by the background polymers persisted in excess of 120 s, as shown by the persistence of the small loop of DNA near the microbead. In (c) the images are separated by 1.5 s. Reproduced with permission from Perkins et al., Science 264, 819 (1994). Copyright 1994 The American Association for the Advancement of Science.⁵⁷