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--d