Stretching DNA with Optical Lasers

M.D. Wang, H. Yin, R. Landick, J. Gelles, S.M. Block

Biophysical Journal

Volume 72, Issue 3, March 1997, Pages 1335–1346

Why do we want to stretch DNA?

- Elasticity and mechanical flexibility of DNA is an important factor in cellular functions

- Researchers sought to gather data for comparison to several preexisting models for the elasticity and flexibility of organic polymers

Why optical tweezers?

- Optical tweezers are one of the only options for researchs who want to reproducibly manipulate delicate matter on the scale of DNA strands.

- Forces applied are in the picoNewton range (~.1 – 50 pN)

- Position is monitored with a minimum resolution of 1 nanometer

How do optical tweezers achieve this?

Refracted photons are accelerated, causing the “trapped” particle to experience a force.

Similarly, reflected photons also cause the particle to experience a force.

Total forces for a particle in the center of the trap

will sum to zero.

As the particle strays from the center of the trap, the magnitude of the individual vectors changes,

and no longer neccesarily sum to zero.

Hooke's Law

- In many cases (but not all) the restorative force imparted by the laser can be modeled with good using Hooke's law:

F(x) = kx

where the laser is incident parallel to the y-axis.

- In this model k is no longer a “spring constant” but is a function of the intensity of the laser and optical properties of the particle being trapped. It is common to refer to the “stiffness” of an optical trap.

Some important points:

- It is possible to trap asymetric particles with the tweezers (ex. DNA molecules), but spherical particles and the resulting symmetry of the forces experienced is optimal.

- Trapped particles are usually dielectrics.

- To work with in these limitations, the end of a strand of DNA is attached to a polystyrene particle. The other end of the strand is then attached to a moveable glass stage via RNA polymerase.

Controlling position:

- Piezo stages are used to alter and monitor the stretch of each DNA strand with sufficient resolution.

- Displacement in x is a function of input voltage. Similar in concept to a solenoid, but with far greater resolution.

Monitoring position:

Monitoring position:

AOM and Position Detector operation

- The position detector send two output voltages, one each for the x and y axis, to the AOM unit.

- The AOM unit then adjusts the laser's voltage in accordance with the total displacement of the trapped particle. Voltage is increased until total displacement falls to zero.

Using the data

- Researchers can track the force experienced by particle at any given displacement, as it is a function of the laser's intensity.

- Comparing the force needed to keep the partice in the trap as the piezo stage stretches the DNA, researchers are able to calculate the elasticity.

Results

Sources:

- M.D. Wang, H. Yin, R. Landick, J. Gelles, S.M. Block. “Stretching DNA with Optical Tweezers”, Biophysical Journal. Volume 72, Issue 3, March 1997

- Steve Wasserman, Steven Nagel. “An Introduction to Optical

Trapping”, http://scripts.mit.edu/~20.309/wiki/index.php?title=Optical_trap, MIT Bioinstrumentation Teaching Lab. October 8, 2013