SCIENCE & ENGINEERING NEWS
San Diego, CA — Portending a possible role for DNA beyond biology, Andrew Pollack reports that a team of researchers at Bell Laboratories and Oxford University has fashioned molecular-sized motors out of the chemical that stores the genetic code.
The DNA motors resemble tweezers that are so small they could potentially pick up a single atom, with 30 trillion of the devices fitting into a drop of water. The molecular tweezers are opened and closed by other DNA that serves as “fuel.”
The development is part of an effort, still in its infancy, to harness DNA as a structural and electronic material that might one day be used to make ultratiny computers or mechanical devices, like a robot that could cruise through the bloodstream to repair an injury.
“As far as we know, DNA is not used this way in the body,” said Bernard Yurke, a physicist at Bell Labs who led the work, described in today’s issue of Nature. “We’re using DNA in a very nonbiological way, as a structural material and as a fuel in some sense.”
DNA could be attractive for such molecular construction because a strand of DNA will stick only to another strand with a corresponding sequence of bases, the chemical units that form the genetic code. So DNA strands in solution could automatically assemble themselves into a structure. “Think of it as a smart glue,” Dr. Yurke said.
He said the DNA tweezers themselves might one day be used to pick up particular ions for molecular construction or to provide motion to a tiny machine. But Bell Labs, which is based in Murray Hill, N.J., and is part of Lucent Technologies, is more interested in computing. For that, the tweezers are not of direct interest but do demonstrate the idea of self-assembly. In the future it might be possible to attach electronic components to DNA and have the DNA strands link together to form a computer with far more speed and information storage capacity than exists today, he said.
Nadrian C. Seeman, a professor of chemistry at New York University, has used DNA to make three-dimensional “stick figures” like cubes and also a mechanical switch that flips between two positions when a chemical is added to the solution.
Nanogen, a biotechnology company in San Diego, received a patent this year for a method of using DNA connected to fluorescent dyes to make an optical memory that could potentially hold far more information in a given space than can a compact disc.
But experts say it will be a decade or more before practical devices can be made of DNA. And some experts say other chemicals might prove to be more useful than DNA.
“It’s going to be cute little laboratory experiments,” James M. Tour, professor of chemistry and a molecular electronics researcher at Rice University, said of work using DNA. “I’m not sure how viable it’s going to be.” One problem, he said, is that DNA works best in solution, not solid form. DNA also does not conduct electricity well, limiting its direct use in electronic devices.
The number of electronic components that can fit on a silicon chip has been doubling every 18 months or so, greatly increasing computer speed and storage capacity. But physicists say there will eventually be limits to how small electronic components can be made using existing technology, so that new approaches will be needed.
DNA is nature’s method of storing huge amounts of information in a tiny space. The space between each letter in the genetic code is 0.34 nanometer, or billionth of a meter. Existing electronics technology makes features about 100 nanometers in size, more than 100 times as large.
For several years scientists have been experimenting with using DNA directly for computation. A problem to be solved can be encoded in a sequence of bases. The DNA is then put into a test tube, and the DNA strands match up in a way that produces a strand encoding the solution.
But some experts say such chemical computers will be slower and more cumbersome to use than are electronic ones. So Dr. Yurke and others are looking more at using DNA as a scaffolding to build electronic computers.
DNA is a double helix, a twisted ladder with each rung made of two of the four bases in the genetic code. The base represented by the letter A always pairs with T, and C with G, so when the ladder is split down the middle, each half will bind only with another half that has the complementary sequence of bases.
The molecular tweezers are made by putting three specific strands of DNA into a test tube. The strands stick together to form two stiff double-stranded arms connected by a short single strand that acts as a hinge. A single strand dangles off the end of each arm.
To close the tweezers, a single strand of DNA that is complementary to both dangling ends is put into solution. This “fuel” DNA binds to those ends and pulls them together, almost like tying a shoe. To open the hinge, a strand complementary to the fuel strand is put into the solution. It eventually out-competes the tweezers’ arms to bind to the fuel DNA. This double-stranded “waste” product floats away, like the exhaust from an engine.
Others taking part in the work were Andrew J. Turberfield, a physicist at Oxford; Allen P. Mills Jr. and Friedrich C. Simmel of Bell Labs; and Jennifer Neumann, a graduate student at Rutgers University.