optics presentation
TRANSCRIPT
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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
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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
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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
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How do optical tweezers achieve this?
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Refracted photons are accelerated, causing the “trapped” particle to experience a force.
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Similarly, reflected photons also cause the particle to experience a force.
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Total forces for a particle in the center of the trap
will sum to zero.
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As the particle strays from the center of the trap, the magnitude of the individual vectors changes,
and no longer neccesarily sum to zero.
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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.
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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.
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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.
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Monitoring position:
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Monitoring position:
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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.
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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.
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Results
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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