Well-known structure with surprises
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The experiments reveal that a classic model for measuring the elasticity of double-stranded DNA leads to errors when the molecules are short. For instance, measurements are off by up to 18 percent for molecules 632 nanometers long, and by 10 percent for molecules about twice that length.
The old elasticity model assumes that polymers are infinitely long, whereas the most popular length for high precision single-molecule studies is 600 nm to 2 microns, biophysicist Tom Perkins says. Accordingly, several university collaborators developed a new theory, the finite worm-like chain (FWLC) model, which improves accuracy by incorporating three previously neglected effects, including length.
The work builds on expertise in measuring positions of microscopic objects. A DNA molecule is linked at one end to a moveable stage and at the other end to a polystyrene bead trapped by an infrared laser. While moving the stage to extend the DNA molecule, scientists measure changes in bead position using custom electronics and a second laser. By calculating the force exerted on the bead, based in part on the intensity of the laser, and comparing it to the position of the bead in the optical trap, which acts like a spring, scientists can measure DNA elasticity.
The work is part of a NIST project studying possible use of DNA as a picoforce standard, because enzymes build DNA with atomic precision. DNA already is used informally to calibrate atomic force microscopes. An official standard could, for the first time, enable picoscale measurements that are traceable to internationally accepted units.
COMPAMED.de; Source: National Institute of Standards and Technology (NIST)