Queen's University, Belfast, Northern Ireland

A chemist looks at DNA-based molecular logic

Logic gates — AND, OR and NOT gates — are used in all manner of electronic devices, for example computers, in which they are connected in huge arrays. Several research groups, including my own, have designed and built molecular logic gates since the early 1990s. But the usefulness of our efforts has been limited because linking these gates in series has proved difficult.

Recently, Reza Ghadiri and his colleagues at the Scripps Research Institute in La Jolla, California, constructed a full set of logic gates that release a single-stranded DNA sequence when provided with the correct combination of single-stranded DNA inputs (B. M. Frezza et al. J. Am. Chem. Soc. 129, 14875–14879; 2007). This means that the output of one gate can be the input for another, and that the gates can be 'wired together' into multi-level circuits using the solution containing the DNA as a communication medium.

The gates work as follows. When a single strand of DNA pairs with a longer strand, an 'overhang' of unpaired DNA is left. If another complementary strand then comes along that is the same length as the longer strand, the overhang provides a foothold, allowing the new strand to push the shorter one off and form a full-length hybridized pair. Ghadiri et al. attached the DNA to beads so that different gates could be kept apart until the correct input was ready. They also added a fluorescent part to the final output signal to make the result easy to monitor.

This may not seem like much of an achievement to a computer buff. Nevertheless, I think the principle that this paper describes could pave the way to more useful molecular logic gates. In the meantime, the simple molecular logic gates that are available can serve in real-life applications such as identification tags for small micrometric objects. Semiconductor identification devices are too big for this purpose.