Optical tweezers

"Optical tweezers" is the name associated with the use of a focused laser beam to trap and pull a dielectric particle. The effect is somewhat similar to that described in equations (3.7-3.9), but the inhomogeneous electric field is at optical frequency and is most intense at the focus ("waist") of the laser beam. The optical tweezer is typically applied to dielectric spheres, such as polystyrene, of about 1 ]im size, which are chemically attached to some biological system of interest. The laser power can be adjusted so that the force on the sphere is appreciable, while the force on single molecules, which are smaller, is negligible. The dielectric sphere will be stably positioned at the center of the trap (waist of the focused laser beam) and as it is displaced from the center, a restoring force proportional to the displacement will be exerted by the trap. An application of this biological technique is indicated in Figure 3.12 [34]. In the left panel (A) the bead, attached to the free DNA strand (template) by the RNA polymerase engine in its copying function, is shown at succeeding times tlt h, t3. In panel (B) the increasing length of the RNA copy, calibrated in nucleotides (nt), is plotted vs. time [34].

The enzyme RNA polymerase (RNAP) transcribes a DNA template into messenger RNA. In doing so it moves like an engine along the DNA template. (In the experiment shown, the RNAP engine is fixed to the glass slide, and the DNA is pulled to the left through it, in the process of making the copy (RNA, shown as coiling upward in (A) of Figure 3.12). In the experiment the trap position was fixed, and

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Figure 3.12 Use of optical tweezers to observe single RNAP molecule pulling DNA [34]

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Figure 3.12 Use of optical tweezers to observe single RNAP molecule pulling DNA [34]

the motion of the bead was monitored by an optical interferometer. As the motion proceeded, the restoring force of the trap increased, and at positions marked by arrows in Figure 3.12 (B) the motor stalled, allowing a measurement of the maximum force available. The forces are in the range 21 - 27 pN. After stalling, the trap was repositioned and transcription continued. The upper trace in (B) was constructed by removing the stall phenomena artificially, to determine (from this graph) a replication rate of about 26 nucleotides per second.

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