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DNA robot takes its first steps
DNA, as most of us already know, is the abbreviation for deoxyribonucleic acid, a chromosome molecule which carries genetic coding unique to each person with the only exception of identical twins. Through laboratory process, DNA can be extracted from body tissue such a strand of hair, semen or blood. Nanotechnologists all over the world have undertaken innumerable experiments with DNA and every now and then they come up with some amazing as well as shocking results.
One excellent example can be the recently invented microscopic biped which has been formed from fragments of DNA. Having legs just 10 nanometres long, the little nano-walker has finally taken its first steps and is being acknowledged as a major breakthrough in nanotechnology.
The biped’s inventors, chemists Nadrian Seeman and William Sherman of New York University, claim that while many scientists have been trying to build nanoscale devices capable of bipedal motion, theirs is the first one to actually succeed.
“It’s an advance on everything that has gone before,” says Bernard Yurke of Bell Labs.
Bernard is part of the team that made one of the best-known molecular machines to date: a pair of “tweezers” also constructed from DNA strands. He and his colleagues at the lab made a tweezers-like structure out of three strands of DNA. When a fourth strand was added to the mixture, it joined the loose ends of the tweezers, pulling them shut. Adding yet another strand of DNA popped the tweezers open. It was then said that the technology could one day be used to make self-assembling electronic components or chemical factories-on-a-chip.
“What is new is the use of DNA as a fuel,” says Bernard’s team member Andrew Turberfield of the University of Oxford.
He further adds: “To get controllable motion on a nanometer scale is really quite fantastic.”
However, the main drawback of Bernard’s nano-machine was that like similar molecular-scale efforts, the tweezers’ arms merely opened and closed and they couldn’t move around either. It was then said that for nanoscale manufacturing to become a realistic possibility, mobile microscopic robots would be needed to assemble other nanomachines and move useful molecules and atoms around.
So Nadrian Seeman and William Sherman sorted out a plan and invented the first ever walking DNA robot.
Now the first question that comes to mind is why DNA? Well, there were two main reasons; first, unlike other chemical compounds with many repeating structural units, DNA chains like to pair up. However, it is not as simple as it seems.
Two DNA strands will only zip together or pair up if the sequences of bases or foundations in each strand complement each other in the right way — so by making minor changes in the those sequences chemists get a high degree of control over where each strand attaches. Second, researchers hope that cells can one day be engineered to manufacture these DNA-based machines. So because of these two reasons Seeman and Sherman created the nano-walker with strands of DNA. Each of the legs in the walker is 36 bases long and is made from two strands of DNA that pair up to form a double helix.
Double Helix is known as the structural arrangement of DNA, which looks something like an immensely long ladder twisted into a helix or coil. The sides of the ladder are formed by a backbone of sugar and phosphate molecules, and the steps consist of nucleotide foundations joined weakly in the middle by hydrogen bonds.
At the top, an elastic portion of each DNA strand runs across from the left leg to the right, linking them together. At the bottom, one of the two strands pokes out of the helix to serve as a sticky foot.
“Our biped can walk because its DNA-based legs are able to detach themselves from a DNA-based track, move along a bit and then reattach them selves”, explains Seeman. (refer figure 1)
The track or footpath the walker walks on is also made of DNA. It is designed so that the un-paired sections of DNA strands stick up like spikes along its length and act as footholds for the walker. The feet attach to the grips using “anchor” strands of DNA that match up with the foot sequence at one end and with the grips at the other. Because the left and right foot/grip sequences are unique, each requires a different anchor.
In order to make the walker take a step, a free piece of DNA called an “unset strand” is introduced to peel away one of the anchors, releasing the foot. The anchor has a handle at the top — a short length of the DNA strand which does not bind to the foot or grip. The unset strand sticks to this handle and then binds with the anchor all the way down. The anchor comes away easily because it prefers to have partners for all its foundation pairs — including the sequence in the handle. The free foot grabs a new anchor sequence, which bolts onto the next grip, stepping the foot forward. Repeating the procedure to move the back foot forward completes the walker’s shuffle. (See figure 2)
“What we’ve done is to build a sidewalk to accommodate one step and we’ve demonstrated quantitatively that the robot can take a second step,” says Seeman.
The walker takes its nano-stroll in a bath of a liquid called a “non-denaturing buffer,” which stops the DNA falling apart. To start with, millions of walkers and tracks are floating around freely in this liquid. Only when the researchers add the DNA anchors do the nano-walker’s feet fix onto the footpaths. Then the unset strands can be added to begin the walking process.
The researchers were able to confirm that the nanowalkers had taken their first steps by taking small samples of the solution after each DNA addition.
“By feeding the material through a gel which separates DNA molecules by size and shape, we confirmed where the feet were attached”, explains Sherman.
After all that explanation the whole thing might still sound rather factious to most of us but yes, it has been done. Seeman and Sherman have finally succeeded in inventing this wondrous DNA walker and persuading the walker to carry a load, such as a metal atom, is the team’s next challenge.
The writer contributes regularly to Sci-tech World on science-related issues
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