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Surprise finding for stretched DNA

Most of us are familiar with the winding staircase image of DNA, the repository of a biological cell's genetic information. But few of us realize just how tightly that famous double helix is wound. Stretched to its full length, a single molecule of human DNA extends more than three feet, but, when wound up inside the nucleus of a cell, that same molecule measures about one millionth of an inch across. Biologists have long believed that as a molecule of DNA is stretched, its double helix starts to unwind. As much sense as this makes from an intuitive standpoint, a recent experiment proved it not to be the case.

Researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley used a combination of microscopic beads and magnetic tweezers to observe that when a DNA molecule is stretched, it actually begins to overwind. This overwinding continues until the force being applied to stretch the DNA exceeds about 30 picoNewtons. (One picoNewton is about a trillionth the force required to hold an apple against Earth's gravity.) Beyond the 30 picoNewton threshold, the DNA double helix did begin to unwind in accordance with predictions.

"DNA's helical structure implies that twisting and stretching should be coupled, hence the prediction that DNA should unwind when stretched," said biophysicist Carlos Bustamante, who led this experiment. "That is why it was such surprise when we directly measured twist-stretch coupling to find instead DNA overwinds when stretched. The DNA molecule, when studied at close range, continues to surprise us!"

Bustamante is a leading authority on the use of single-molecule visualization and manipulation techniques to study the dynamics, structure and kinetics of molecular motors and nucleo-protein assemblies. He holds joint appointments with Berkeley Lab's Physical Biosciences Division and UC Berkeley's Departments of Molecular & Cell Biol
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Source:DOE/Lawrence Berkeley National Laboratory


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