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Putting a molecular race in reverse.

Putting a molecular race in reverse

Gel electrophoresis is a commonly used method for separating large organic molecules such as nucleic acids. This technique separates ions by size as they migrate through a gel under the pull of an electric field. Usually, smaller molecules travel more quickly. But it doesn't work for molecules larger than a certain size because these bigger ions all travel at the same rate. Now a team at the Washington University School of Medicine in St. Louis has discovered that periodically reversing the electric field separates much larger molecules than before. Their report appears in the April 4 SCIENCE.

Still puzzling, however, is why the method works. "The physical and chemical theory of electrophoresis is poorly developed," says geneticist Maynard V. Olson, who led the development of the new technique. Scientists have postulated that small molecules, which roughly match the size of gel pores, seem to shift from one pore to the next as they migrate in the direction of the electric field. Longer molecules apparently wind through the gel in a snakelike motion. Because a molecule's tail is just dragged along, the speed of this type of motion would be largely independent of a large molecule's total length or size.

This conventional view isn't quite right, says Olson. It doesn't distinguish between the front and back ends of a molecule; if the electric field were reversed, this theory would predict that the molecules would simply go backward. Instead, Olson found that by reversing the field at just the right rate, he could even stop the motion of particular molecules. "It's quite a dramatic phenomenon," he says.

What may be happening is that large molecules need time to rearrange themselves before traveling in a new direction. It's possible, says Olson, that these molecules form into a narrow wedge pointing in the direction of travel. That wedge has to reform when the electric field is reversed. Caught by such a reversal, some molecules react more quickly than others. Thus, periodic electric field reversals make it possible to spread out the rate at which large molecules of different sizes migrate through a gel.

Although how the field-reversal method works is still unclear, it has already been useful for separating components of mixtures that contain DNA segments made up of a million or more base pairs. Conventional electrophoresis works with nucleic acids made up of fewer than 20,000 or so base pairs.
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Title Annotation:gel electrophoresis research
Publication:Science News
Date:Apr 12, 1986
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