Experimental Tools and Techniques

Magnetic Tweezers

Magnetic tweezers facilitate the highly controlled exertion of force on microscopic paramagnetic beads. By tethering these paramagnetic beads to proximate surfaces via single dsDNA molecules, we are able to manipulate the tension in the DNA double strand. Using video imaging of the bead’s movement in relation to the tethering surface, we can correlate the length of the DNA molecule with the force exerted on it. Analysis of the force-extension relationship for DNA illuminates the effects of various experimental conditions on the molecule’s structural integrity.

Two-Fiber Optical Trap

Two-fiber optical traps enable us to halt free objects in solution. By aligning two infrared laser fibers in diametrical opposition across a microfluidic channel, we create a “trapping zone” in which we can exert controlled force. Increasing the power of both incoming beams equally exerts pressure on any trapped objects. Because the force applied by each incoming beam can be individually adjusted, we’re also able to move objects translationally across the channel.

Inexpensive optical tweezers for undergraduate laboratories

Stephen P. Smith, Sameer R. Bhalotra, Anne L. Brody, Benjamin L. Brown, Edward K. Boyda, and Mara Prentiss
American Journal of Physics — January 1999 — Volume 67, Issue 1, pp. 26

Fluorescence Microscopy

By labeling single- and double-stranded DNA with fluorophores, we can directly image the molecules under fluorescence. After dyeing the respective molecules with contrasting colors, we can observe DNA strand exchange, and other biological phenomena in action. Utilizing fluorescence microscopy in conjunction with the techniques described above, we are able to visualize DNA amidst varying force conditions, rather than monitoring DNA extension through labeling with visible beads.

Imaging of Strand Exchange under Fluorescence
Strand Exchange on dsDNA
Tethered to Paramagnetic Bead