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Volume 23 / No. 1 / 2012
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Single Molecules Trapped for Study

Michelle D. Wang, Physics Wang
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To help us understand some of life’s mysteries, we have developed precision optical instruments and techniques to look at important molecules in biology—particularly DNA molecules and their associated proteins—one molecule at a time.

A Focused Beam of Light

When we highly focus a laser beam down to an extremely tight spot, it becomes very powerful. That tight spot becomes a trap for particles as small as a micron in diameter. If we move the trap, the particle follows.

We can see the trapped particles, but at this point, we can’t directly visualize molecules because they are around 10 nanometers. So we attach a molecule to a particle, trap the particle, and use the trapped particle as a handle to sense what’s happening at the molecule and control it. More specifically, we have developed ways to exert controlled force and torque on the molecule and precisely measure its response.

Our work opens up a new dimension for studying deoxyribonucleic acid (DNA), motor proteins that travel along DNA, and other motor proteins. Our particular ability to exert and measure torque has received tremendous attention, and many labs are duplicating it.

Looking Inside a Cell Nucleus

Take a look. If we were to line up end to end all the DNA molecules that are inside the nucleus of a single human cell, the lineup would be more than one meter long. But a cell nucleus is only a few microns in diameter. A micron is one millionth of a meter. All those DNA molecules are squeezed into this very tiny space, all tangled and twisted up.

That’s a lot of compaction. And a profusion of topological activity accompanies the compacting. What’s more, biological molecules are all about motion. Some can translocate and rotate along DNA like tiny “molecular motors.” So we wanted to study the issues of topology, torque, and rotation, all at the same time.

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