The idea of making DNA perform computational tricks is hardly new. These data-packed molecules are in many ways perfect for the job. One very public test of their potential came in 1994, when Leonard Adleman of the University of Southern California showed that DNA could tackle the famous "traveling salesman" problem. And now Nadrian C. Seeman and his colleagues at New York University have found a clever new way to tease DNA strands into mimicking exculsive OR (XOR) logic gates.

Binary logic gates, which turn 1's and 0's of input into 1's and 0's of output, form the central processing units in digital computers. Almost any complex calculation can be parsed into a series of smaller steps through logic gates. In the case of an XOR gate, the rule is simple: when the same two digits enter the gate, a 0 comes out; two different entering digits return a 1 (see diagram). In this latest case of DNA computing, inputs are replaced by single-stranded molecules, and how they bind with each other--base pair to base pair--dictates the operations. In essence, the collection of input molecules that are used set up the problem; once that's done, the answer self-assembles in a single step. Seeman's team estimates the error rate to be as low as 2 to 5 percent.

This sort of test-tube calculator won't replace your desktop any time soon, if ever. But the authors do suggest that it might find some specialized applications. Two sets of molecules, when mixed, might reveal a key to decrypt secret messages, and added security would come from the fact that the molecules could only be mixed that once. Also, they write, the DNA might be "programmed" to help self-assemble smart materials.