DNA makes tiny tweezers

By BBC News Online's Jonathan Amos

Scientists have built a pair of tweezers from DNA, the molecule that carries the "code of life".

Each arm on the tiny tool is just seven nanometres (millionths of a millimetre) in length.

The researchers, based at the Bell Laboratories (Lucent Technologies) in New Jersey, US, say their tweezers are like a little engine that will open and close when "fuelled" by yet more DNA.

Their device is just the latest addition to the miniature construction kit scientists hope will one day allow them to build useful objects atom by atom, such as electronic circuits 1,000 times more powerful than today's silicon chips.

Programmable molecule

The tweezers exploit the complementary nature of the two strands that make up the famous double helix that is DNA.

Each rung on the twisted ladder, first described by Crick and Watson in 1953, is a link between two chemical bases - adenine links with thymine; cytosine links with guanine. It is these bases that form the "letters" (A, T, C and G) in the code that describes how a human being is made.

A stretch of single-stranded DNA will stick firmly to another single strand only if their sequences of bases match up correctly.

Dr Bernard Yurke and co-workers used this feature not only to build a tiny tool but also to program it to snap open and shut.

'Exhaust fumes'

The tweezers comprise three single strands of synthetic DNA. Two strands act as the arms; one strand straddles the others and acts as a kind of backbone and hinge holding the whole V-shaped structure together.

The arms extend far enough to leave a number of unpaired bases dangling free beyond the backbone.

When a fourth DNA strand is added to the test tube, it grabs the unpaired bases and zips the tweezers shut. Again, just a few bases are allowed to hang unpaired, which permits a fifth strand to rip away the first fuel unit and open the tweezers.

The two fuel units, now paired, then drift away as if they are the "exhaust fumes" from this molecular motor.

'Seeing' the tweezers

"Its very clever in two ways," Dr Andrew Turberfield, a co-researcher on the project from Oxford University's Department of Physics, told BBC News Online.

"It's clever in the way the tweezers are self-assembled in the first place, and, secondly, in the way they are powered.

"It is trivial to design DNA sequences that are complementary. The tricky bit is making sure the bits that aren't supposed to stick together don't. So it was basically done by screening different sequences to ensure the interactions that weren't desired were minimised."

The first "fuel" unit closes the DNA tweezers

Because the researchers cannot observe the DNA tweezers with available microscopic techniques, they rely on the phenomenon of fluorescence to detect the closing and opening actions.

A pair of dye molecules is attached to the ends of the DNA motors, and when laser light "excites" the dyes, the amount of fluorescent light will indicate the distance between the two ends.

Carbon nanotubes

Tiny tweezers have been built before using tubes of carbon atoms, but the DNA tool described in the journal Nature is a thousand times smaller again.

Scientists are very keen to exploit the interesting and sometimes unusual properties that arise in materials when their constituent atoms or molecules are precisely arranged. They believe nanotechnology also offers the possibility of making electronic circuits yet smaller and more powerful.

But it is extremely difficult, not to say expensive, to build on the nanoscale. Hence, the search for special tools that can do the job.

Dr Yurke said of his team's DNA tweezers: "This may lead to a test-tube based nanofabrication technology that assembles complex structures, such as circuits, through the orderly addition of molecules." The Bell Laboratories are already working to attach DNA to electrically conducting molecules to assemble rudimentary molecular-scale electronic circuits.

"Of course it's all very speculative," said Dr Turberfield, "but you can imagine, for instance, little factories on chips doing chemistry or simple assembly. You can think of production lines made up of little motors with different reactants being passed from one place to the next."