Fig. 2

Magnetic tweezers set-up for measuring on a tethered DNA molecule. a Schematic of the set-up. A LED illuminates the flow cell through a lens and the magnet holder. Imaging is done with a 50x Nikon objective onto a CCD camera. Magnets manipulate a magnetic bead attached to the DNA. b A flow cell is constructed with 24x60mm coverslips. The bottom coverslip is amine-coated and has reference beads bound to it. The top coverslip has sandblasted holes to allow fluid flow. Parafilm is used to seal the coverslips and to create a ˜50 μl flow cell volume. c Schematic of a tethered DNA molecule. A DNA molecule is linked to a streptavidin-coated magnetic bead with biotin, and to azide groups on the surface with DBCO at the other end