The results of this study are reported in the journal Nature, in a letter entitled DNA Overwinds When Stretched, which is now available on-line. Coauthoring this letter with Bustamante were Jeff Gore, Zev Bryant, Marcelo Nöllmann, Mai Le and Nicholas Cozzarelli.
The magic of DNA replication and the transcription of genetic information into the production of proteins depends upon the mechanical properties of the double helix. This is why understanding these mechanical properties has been a scientific priority since the double-helix was first discovered by Watson and Crick more than 50 years ago. Bustamante has been one of the foremost pioneers in this area of research. More than a decade ago, he and his research group tethered DNA molecules to tiny beads and measured their elasticity. Among the many breakthroughs he and his group have achieved is the development of the technique called "rotor bead tracking."
In rotor bead tracking, a single DNA molecule is anchored to a surface and a magnetized bead is attached to the free end. A point along the double-helix is then biochemically "nicked" to create a single strand of DNA that acts as a free swivel. Immediately below this nick, a plastic bead is attached to the DNA to serve as a "rotor" that will spin in response to torque. Magnets are used to manipulate the magnetized bead, providing a measured and highly controlled amount of tension to stretch the DNA molecule. With the use of a fluorescent coating, the subsequent spinning of the rotor bead in response to the stretching can be recorded.
"When we apply tension to the DNA molecule, changes in the rotor bead angle reflect changes in the twist of the lower DNA segment," Bustamante said. "The overwinding observed upon stretching, implies that contrary to the held belief, the stretch-torsion coupling constant of DNA is a negative va
Source:DOE/Lawrence Berkeley National Laboratory