Laser technique examines movement in nucleus of living cell

August 29, 2001

CHICAGO -- By colliding two laser beams head-on, scientists at the University of Illinois can measure the movement of chromatin (tiny packets of DNA) in the nucleus of a living cell.

"DNA, in the form of chromatin, plays a key role in several important chemical reactions that occur in living cells," said Christopher Bardeen, a UI professor of chemistry.

"Understanding how chromatin 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."

While 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 very complex reaction vessel, crowded with proteins and other large molecules that must move around and interact," Bardeen said. "If we try to take a cell apart and examine its constituents, we find 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 graduate students Sara Davis and Andrew Stout combine a two-photon laser fluorescence technique with a standing-wave, counter-propagating geometry.

First, the cell is treated with a harmless fluorescent dye that selectively labels the DNA. Then, two counter-propagating, near-infrared laser beams are used to create a standing-wave interference pattern in the cell and excite fluorescence through a two-photon transition.

Next, the researchers 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.

"If the DNA wasn't moving, we could bleach a pattern and it would remain frozen in the interference signal forever," Bardeen said. "By monitoring the decay of the bleached pattern, we can tell that the DNA is moving, and we can measure that movement to a precision of about 20 nanometers."

Preliminary measurements have hinted at the occurrence of subdiffusion within the cell nucleus, Bardeen said. "The chromatin is wobbling around, apparently bumping into neighboring molecules and not moving as far as it should have in the time elapsed." This indicates that molecular crowding is extremely important at the nanometer length scale, and suggests a major difference between life and death, Bardeen said. "When a cell is dead, we don't see any diffusion occurring. In fact, we don't see any movement in the cell at all."

Cellular motion is not just a simple mechanical operation, Bardeen said. "Motion is somehow connected with life itself. It's one of the things that differentiates a living cell from a lump of DNA."

The researchers will describe their experimental technique and present preliminary data on chromatin movement at the 222nd American Chemical Society national meeting in Chicago. The presentation will take place in room N138, McCormick Place.
-end-


University of Illinois at Urbana-Champaign

Related DNA Articles from Brightsurf:

A new twist on DNA origami
A team* of scientists from ASU and Shanghai Jiao Tong University (SJTU) led by Hao Yan, ASU's Milton Glick Professor in the School of Molecular Sciences, and director of the ASU Biodesign Institute's Center for Molecular Design and Biomimetics, has just announced the creation of a new type of meta-DNA structures that will open up the fields of optoelectronics (including information storage and encryption) as well as synthetic biology.

Solving a DNA mystery
''A watched pot never boils,'' as the saying goes, but that was not the case for UC Santa Barbara researchers watching a ''pot'' of liquids formed from DNA.

Junk DNA might be really, really useful for biocomputing
When you don't understand how things work, it's not unusual to think of them as just plain old junk.

Designing DNA from scratch: Engineering the functions of micrometer-sized DNA droplets
Scientists at Tokyo Institute of Technology (Tokyo Tech) have constructed ''DNA droplets'' comprising designed DNA nanostructures.

Does DNA in the water tell us how many fish are there?
Researchers have developed a new non-invasive method to count individual fish by measuring the concentration of environmental DNA in the water, which could be applied for quantitative monitoring of aquatic ecosystems.

Zigzag DNA
How the cell organizes DNA into tightly packed chromosomes. Nature publication by Delft University of Technology and EMBL Heidelberg.

Scientists now know what DNA's chaperone looks like
Researchers have discovered the structure of the FACT protein -- a mysterious protein central to the functioning of DNA.

DNA is like everything else: it's not what you have, but how you use it
A new paradigm for reading out genetic information in DNA is described by Dr.

A new spin on DNA
For decades, researchers have chased ways to study biological machines.

From face to DNA: New method aims to improve match between DNA sample and face database
Predicting what someone's face looks like based on a DNA sample remains a hard nut to crack for science.

Read More: DNA News and DNA Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.