This document describes how researchers used optical tweezers to stretch and measure the elasticity of DNA strands. Optical tweezers use refracted and reflected laser light to apply tiny forces (piconewtons) to manipulate microscopic objects like DNA. The researchers attached one end of a DNA strand to a bead trapped by the tweezers and the other end to a movable stage. By varying the stage position and measuring the counteracting force from the tweezers, they could determine how much force was required to stretch the DNA and analyze its mechanical properties.

1. 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

2. 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

3. 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

4. How do optical tweezers achieve this?

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

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

7. Total forces for a particle in the center of the trap will sum to
zero.

8. As the particle strays from the center of the trap, the magnitude
of the individual vectors changes, and no longer neccesarily
sum to zero.

9. 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 y­axis.
­ 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.

10. 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.

11. 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.

13. Monitoring position:

14. Monitoring position:

16. 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.

17. 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.

18. Results

19. 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