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This "optical trap" reveals the dynamics of DNA's replication molecular machinery. Scientists use the device to measure the force that's produced as DNA is converted from its single-stranded form (right) to its double-stranded form (left). Two beads hold the DNA in place. The bead on the left rests atop a pipette, while the bead on the right sits in a laser beam. The DNA stretches between the two beads, producing a measurable amount of force. When the polymerase molecule in the middle adds base pairs to the DNA, this force changes, causing the bead on the right to move slightly within the laser beam. Researchers can measure this movement optically. The more the bead moves, the more force the DNA produced. Knowing the amount of force given off at each moment helps scientists understand the "mechanics" of DNA synthesis. Adapted from Fig. 1a, Nature 404:103, 2000 © Macmillan Magazines Ltd.
Illustration: George Eade
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