Figure 2.

Photograph of the top view of the square cell shown in the schematic in Fig. 1B. The dark object shown at the right is the surface of the round capillary. The dark dots are the magnetic beads tethered to the surface by DNA. The image was taken after applying a constant force of 15 pN during 45 min. The arrows at the top of the figure show the fully zipped and fully unzipped lengths of the molecules.