DNA Movement in Cells
By colliding two laser beams, scientists in LAS are measuring the movement of chromatin—tiny packets of DNA—in the nucleus of a living cell.
"Chromatin plays a key role in several important cellular reactions," says Christopher Bardeen, a professor of chemistry. "Understanding how its motility affects reactions, like the transcription of DNA into RNA for the production of proteins, is essential to extending our knowledge in such areas as cell reproduction, embryology, and genetic engineering."
Although scientists understand how chemical reactions work in a simple test tube, the dense environment in a living cell presents a far more complicated system.
"A living cell is a complex reaction vessel, crowded with proteins and other large molecules that must move around and interact," Bardeen says. "If we try to take a cell apart and examine its constituents, they no longer behave as they do in intact, living cells."
To non-invasively measure chromatin movement in a live frog skin cell, Bardeen and two graduate students treat the cell with a harmless fluorescent dye that selectively labels the DNA. Then, using two counter-propagating, near-infrared laser beams, they create a standing-wave interference pattern in the cell and excite fluorescence through a two-photon transition. Next, they turn up the laser power briefly, thereby bleaching some of the dye and creating a distinctive signal pattern. As the DNA wiggles around, this pattern gradually washes out and the fluorescence signal recovers.
Photo by Bill Wiegand