Making factories and computers with DNA

Puzzle pieces
DNA in nature is often just one long continuous chain, but researchers would prefer to have other shapes at their disposal.

More than three decades ago, biologists discovered that cells create cross-shaped DNA molecules during replication and repair. The side-arms, or branches, grow out of a genetic code whose letters read the same forwards and backwards, like the palindromes "racecar" and "rotator."

Seeman and others have modified the sequence of palindromic DNA to make a stable 4-armed molecule. They have also coaxed DNA to branch with 3, 5 and 6 arms.

These two-dimensional stick figures are only a few nanometers across, where a nanometer is one billionth of a meter. Researchers design them with "sticky ends"—single DNA strands that act as latches between molecules. Whole arrays of these connecting figures can be put together like pieces in a puzzle.

Earlier this year, LaBean and his collaborators built 4x4 lattices with 16 cross-shaped DNA pieces. By attaching a type of protein to specific "pixels" on these grids, the team spelled out "DNA."

The ability to attach particles to DNA pieces is a step towards fabricating nano-electronics. Scientists can hitch functional materials like metals, semiconductors and insulators to specific DNA molecules, which can then carry their cargo to pre-specified positions. Already this technique has been used to make a simple transistor, as well as metallic wires.

There is a problem, however, in making more complicated components. To keep negatively-charged DNA stable, researchers add positive ions to their solutions. But these ions can interfere with the functional materials needed to build electronics.

"It is difficult to keep all these things happy at the same time," LaBean says.

A solution might be to use a DNA-like molecule that is uncharged and yet has the same code as DNA. There are about 1000 "flavors" of DNA derivatives, Seeman says, so one of these might do the trick.

Trouble is these alternatives can be 10 times more expensive to make than regular DNA, according to LaBean. It could be worth it, however, as computer chip manufacturing techniques currently cannot go smaller than tens of nanometers.

Self-assembling arrays of DNA-like molecules could go beyond this limitation, by providing the scaffolds for nanometer-scale circuits. This would not only make our computers and other devices more compact, but faster as well.

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