6. In practice, optical tweezers are very expensive, custom-built instruments. These instruments usually start with a commercial optical microscope but add extensive modifications. In addition, the capability to couple multiple lasers into the microscope poses another challenge. High power infrared laser beams are often used to achieve high trapping stiffness with minimal photo-damage to biological samples. Precise steering of the optical trap is accomplished with lenses, mirrors, and acousto/electro-optical devices that can be controlled via computer.

7. Laser Tweezers and Transcription. A transcription complex is tethered between two streptavidin-coated beads and kept in a continuous buffer flow. As a transcribing polymerase moves along the DNA, it physically pulls the two beads closer together. One bead is held in place with laser tweezers (red funnel) and the other by a pipette. The separation of the beads is measured by video microscopy and used to determine the end-to-end distance of the DNA.

8. If some external object, like a molecular motor, were to pull the bead away from the center of the trap, a restoring force would be imparted to the bead and thus to the motor. An example trace of a single kinesin motor taking 8 nm steps against a 5-pN force is shown

11. Group Problem on Polarization A vertically polarized light beam is incident on two ideal polarizers P1 and P2 following one another with P1 oriented at 45 o to the vertical and P2 oriented horizontally. If the incident intensity is 1 unit, what is the transmitted intensity after P2? What happens in the same situation if the incident light of 1 unit is unpolarized?

13. actin (red), vinculin (an adhesion plaque protein; green), and the nucleus (blue)