Nanotram
a DNA fragment moves along a self-organized DNA track
28-Sep-2004
One of the most interesting challenges in the area of nanotechnology is the
precise transport of nano-objects from one place on a nanostructure to another
by a defined route. Researchers have now begun to overcome this challenge: there
are three "stops" on the route of their new nanoscopic "tram" made of DNA.
DNA has already proven itself as a material for nanoconstruction in a number of
ways. Because of the specific base pairing of complementary DNA regions, this
material organizes itself into defined structures that can be predetermined by
the order of its nucleotide components. The team working with Hao Yan and John
H. Reif in Durham (USA) and Andrew J. Turberfield in Oxford (U.K.) also turned
to DNA: the "tracks" for their nanotram are made from a double-stranded DNA
backbone. Short, single-stranded "hinges" were used to connect three
double-stranded DNA fragments, which act like stiff rods, into "stops" on the
backbone. The tram was initially attached at the first stop, the tiny vehicle
consisting of two segments of three nucleotides, each attached to the ends of
the two strands of the tram-stop DNA. The crucial trick is that the six tram
nucleotides don't lie directly opposite each other, but are offset so that three
nucleotides jut out from one of the strands. These act as a sort of sticking
point for their counterparts. The second stop is also constructed so that three
precisely fitting nucleotides jut out. If the two stops then flip around on
their hinges so that they point toward each other, the overlapping ends stick
together. The Enzyme ligase then binds them together into a continuous double
strand. Now a second enzyme, a nuclease that cuts up double-stranded DNA like a
scissor, springs into action. However, this molecular "scissor" is choosy, and
only cuts when it finds a specific nucleotide sequence. It cuts up the assembly
of stop 1, tram and stop 2 so that the first stop is released and the tram only
remains attached to the second stop. The tram is then moved on to stop 3 in a
similar fashion. The system is entirely autonomous, requiring only ATP
(adenosine triphosphate) as "fuel".
If the right nucleases can be found, more extensive "rail networks" with many
"stops" and longer "trams" should be feasible. The trams could be hooked onto
nano-objects-a requirement for the construction of "intelligent" nanorobots.
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