DNA nanotechnology is a subfield of nanotechnology which seeks to use the unique molecular recognition properties of DNA and other nucleic acids to create novel, controllable structures out of DNA. The DNA is thus used as a structural material rather than as a carrier of biological information, making it an example of bionanotechnology. This has possible applications in molecular self-assembly and in DNA computing.
Introduction: DNA crossover molecules
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Left: A schematic. Right: A more realistic model.[1]
Each of the four separate DNA single strands are shown in different colors.
![A double-crossover (DX) molecule. This molecule consists of five DNA single strands which form two antiparallel double-helical domains, on the left and the right in this image. There are two crossover points where the strands cross from one domain into the other. Image from Mao, 2004. [1]](http://en.wikipedia.org/wiki/Image:Mao-DX-schematic.jpg)
DNA nanotechnology makes use of branched DNA structures to create DNA complexes with useful properties. DNA is normally a linear molecule, in that its axis is unbranched. However, DNA molecules containing junctions can also be made. For example, a four-arm junction can be made using four individual DNA strands which are complementary to each other in the correct pattern. Due to Watson-Crick base pairing, only portions of the strands which are complementary to each other will attach to each other to form duplex DNA. This four-arm junction is an immoble form of a Holliday junction.
Junctions can be used in more complex molecules. The most important of these is the "double-crossover" or DX motif. Here, two antiparallel DNA duplexes lie next to each other, and share two junction points where strands cross from one duplex into the other. This molecule has the advantage that the junction points are now constrained to a single orientation as opposed to being flexible as in the four-arm junction. This makes the DX motif suitible as a structural building block for larger DNA complexes.[2]
Tile-based arrays
![Assembly of a DX array. Each bar represents a double-helical domain of DNA, with the shapes representing comlimentary sticky ends. The DX molecule at top will combine into the two-dimensional DNA array shown at bottom. Image from Mao, 2004. [2]](http://en.wikipedia.org/wiki/Image:Mao-DXarray-schematic.gif)
DX arrays
DX molecules can be equipped with sticky ends in order to combine them into a two-dimenstional periodic lattice. Each DX molecule has four termini, one at each end of the two double-helical domains, and these can be equipped with sticky ends that program them to combine into a specific pattern. More than one type of DX can be used which can be made to arrange in rows or any other tessellated pattern. They thus form extended flat sheets which are essentiallt two-dimensional crystals of DNA.[3]
DNA nanotubes
In addition to flat sheets, DX arrays have been made to form hollow tubes of 4-20 nm diameter. These have been dubbed DNA nanotubes by analogy with the similarly-shaped carbon nanotubes.[4]
Other tile arrays
Two-dimensional arrays have been made out of other motifs as well, including the Holliday junction rhombus array as well as various DX-based arrays in the shapes of triangles and hexagons.[5] Another motif, the six-helix bundle, has the ability to form three-dimensional DNA arrays as well.[6]
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