DNA Computing Circuit This wiring diagram specifies a system of 74 DNA molecules that constitute the largest synthetic circuit of its type ever made. The circuit computes the square root of a number up to 15 and rounds down to the nearest integer (the discrete square root of a four-bit integer). Caltech/Lulu Qian

Their largest computer was a 74-molecule, four-bit circuit that could compute the square root of any number up to 15, rounding down the answer to the nearest whole number. To get the answer, the researchers would monitor the concentration of output molecules in the test tube, using fluorescent tagging.

The process takes a long time, but speed is not the point — using this method, scientists could eventually engineer biochemical pathways that are capable of making decisions. This type of control over chemical reactions could be useful for anything from pharmaceuticals to industrial processes. Imagine DNA-based computer chips embedded in your skin, releasing drugs when the time is right, or a DNA computer that can study the concentration of certain molecules in a blood sample and quickly diagnose a disease.

The circuit can be scaled up to larger DNA computers, the researchers say. They can also be customized by adjusting the types of DNA used or reconfiguring the circuit.