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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Using optical traps to manipulate single DNA strands

Problem

An optical trap is a measurement device that allows minute forces to be exerted on cells and small tissues using a laser. In a standard setup (Figure 1), a collimated laser beam is passed through a lens that focuses it towards a single point. This results in a three-dimensional light field shaped like two inverted cones, which exerts a force that pushes transparent objects towards the focal point of the assembly. Photons will pass through the object from a particular angle and scatter, moving the object in a desired direction. The device works best when the object being confined is a transparent insulator, and so polystyrene beads are often used in the trap. These beads can, in turn, be connected to objects of diagnostic interest, allowing detailed manipulation of biological materials via purely optical means. To reduce the influence of environmental factors on the optical trap, experiments are performed in mechanical isolation through the use of air tables removed from heat and acoustic sources.
One such experiment is a “DNA pulling experiment,” in which a bead in an optical trap is affixed to the end of a DNA strand and tugged by moving the laser and trap. The displacement of the bead is then recorded as a function of the force exerted on it by moving the trap. The amount that the DNA strand resists pulling provides information about its secondary structures, like coils or folds, that require an applied force to disassemble.
One drawback of this technique emerges over long timescales, when the DNA strand accumulates damage due to photons from the trap passing into it and provoking chemical changes. The total photodamage to a given strand is proportional to the total number of photons that have reached it during the experiment.
Figure 1: A schematic of a simple optical trap acting on a transparent polystyrene bead.
Which of the following causes likely describes the origin of the force that pushes objects towards the center of the trap?
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