Figure 1.

Biological applications of optical tweezers to kinesin motor stepping and RNA folding. a, Record of motion for a single kinesin motor under force-clamped conditions, displaying discrete, 8 nm steps (blue trace) as it walks along a microtubule (inset, not to scale). The trap position is servoed under computer control to maintain a fixed distance behind the bead, thereby imposing a load of a few piconewtons in a direction that hinders movement (red trace). b,c, Unfolding of a structured RNA molecule (red, inset, not to scale) using a dual-beam optical trap arrangement (pink), producing out-of-equilibrium transitions (‘rips’) as the structure unfolds under a force ramp (b) or reversible, thermally driven fluctuations in extension when clamped near equilibrium (c). To exert forces on the ends of the RNA molecule, it is hybridized at one end to a DNA ‘handle’ (blue), which is chemically (yellow) linked to the left bead (blue). The RNA emerges at its other end as a transcript from RNAP (green), which is chemically attached to the right bead (blue). The RNAP molecule is transcriptionally stalled at a roadblock (yellow) placed on the DNA template (blue).