Disorder forces DNA molecules out of tight spaces

ITHACA, N.Y. -- A new understanding of how large biological molecules behave in tiny spaces could lead to a method for separating DNA strands by length. It also could throw light on the way molecules move in living cells.

Using a forest of nanofabricated pillars so small that DNA molecules can only slip through lengthwise, Cornell University researchers have demonstrated the existence of an entropic recoil force that causes the molecules to move from a tight space into a more open one.

The findings, published in Physical Review Letters (March 25, 2002), are by Stephen Turner, a postdoctoral research assistant at Cornell; graduate student Mario Cabodi; and Harold Craighead, the Charles W. Lake Jr. Professor of Engineering, professor of applied and engineering physics and interim dean of the Cornell College of Engineering.

This work follows previous advances by Turner, Craighead and others in the same field that shed new light on how DNA molecules are inserted into confined spaces. Now they are the first to demonstrate how DNA strands are ejected from confined spaces.

In water, strands of DNA or other long-chain molecules tend to coil into a roughly spherical shape. Previously the researchers found that when a DNA molecule in a spherical configuration comes up against an opening too small for the sphere to pass through, some small part of the chain is first pulled into the opening, causing the rest to uncoil and follow.

In these experiments the DNA molecules are pulled into the dense array of pillars by an electric field. If the field is removed before a molecule is all the way in, it will recoil back into the open space and resume its spherical shape. What is the force that causes this behavior? Physicists have theorized that it is an entropic force related to the confinement of the molecule in a narrow tube. Entropy is a measure of the amount of disorder in a system, and an entropic force would tend to move things toward

Contact: Bill Steele
Cornell University News Service

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