FIGURE 1.

Cartoon illustrating the experimental geometry (not to scale). A DNA molecule (green) is linked at one end to a surface. The opposite end of the DNA is attached to a bead held in an optical trap (orange). The trap center is offset a lateral distance x_DNA from the surface attachment point. The vertical distance of the trap center from the surface is z_trap. The bead displacement in both axes (x_bd and z_bd) is measured from the center of the trap (dashed line) to the center of the bead (solid line). The extension, x, of the DNA along the stretching axis can be determined from the geometry (24).