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Laser Tweezer Technique Measures DNA MechanicsThe enormous amount of DNA required to define the structure, and hence function, of higher organisms means that nature has had to develop special DNA packaging techniques. In order for the DNA to be used, selective unfolding of it is required, but how strongly does the molecule resist this bending and stretching? In the February 9th issue of the journal Science, National Science Foundation (NSF)-funded scientist Carlos Bustamante of the Institute of Molecular Biology at the University of Oregon, reports on results of a novel measurement of the "mechanical" properties of DNA. The measurement involves essentially "gripping" the DNA at both ends, pulling, and determining the DNA's resistance to stretching. Bustamante's experiment involved fixing small polystyrene beads to each end of a DNA molecule to provide "handles." Next, a tiny but accurately measured force was applied to one bead while the other bead remained still. Using a highly controlled laser beam, this "laser tweezer" technique permitted the measurement of the elastic behavior of the DNA molecule over a wide range of pulling forces. An interesting observation was that at some pulling forces, the DNA molecule abruptly shifts to a new structure with a distinctly different resistance. Bustamante suspects that this "structural transition" has important consequences for the interactions of DNA with so-called DNA-binding proteins. These observations contribute to development of a picture of the interactions occurring in a cell's nucleus, and reveal the physical processes involved in molecular recognition, says Kamal Shukla of NSF's division of molecular and cellular biosciences, which supported Bustamante's research.
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