DNA logic gate components……Credit: Royal Publishing Society

Let’s come at computers from a different angle for a moment. An alien species lands on earth. Their spaceship doesn’t look like a spaceship. It looks like a very large blob, of sorts. It’s a blob because the whole thing is organic, not a scrap of metal on it or in it. The aliens are, of course, also organic. Their entire technology uses only organic compounds, as we call them (praise be to carbon). That means their on-board computers are also organic. No silicon chips. In fact, they don’t even use graphene, which is pure carbon but not organic. Instead they use computers based on DNA and related substances, which with minor variations is almost the same as our DNA. They have used DNA because on their long voyages through space only DNA is self-perpetuating, self-repairing. Their computers get old, but a new crop is always in preparation. Their organic computers are infinitely recyclable and require only the minimum of manufacturing capability.

Okay, that’s science fiction. Computers based on DNA are not science fiction, quite. Biologists and computer scientists have been attracted to the notion of using the combinatorial power of DNA to perform computer-like calculations for decades. It’s fair to say though that moving from what seems to be a logical use of DNA to the actual biological material has not been easy. No human made ‘biological computer’ exists; the problems are far too complicated. These days what most scientists choose to do is work on something comparatively simple – logic gates, the basic computer component, built from DNA.

Lulu Qian and Erik Winfree with their team at the California Institute of Technology (Caltech, Pasadena, USA) built the first such logic gate, or DNA circuit, in 2006. They then constructed a 5-layer 12-DNA molecule circuit from these gates – and the processing speed fell off a cliff, orders of magnitude slower. Back to the computer modeler, as they say. The result is a new design, published in the June 3, 2011 issue of Science and available in a version without paywall at Royal Society Publishing 6 June 2011, [A simple DNA gate motif for synthesizing large-scale circuits] One of the circuits built with the new approach used 74 different DNA molecules, the largest such circuit to-date, which can calculate the square root of numbers up to 15. Yes, obviously this is not even a crude silicon calculator, but that comparison misses the point entirely. More »