Figure 4. Two-input AND gate.

Figure 1. Two-Input AND gate.

The gate is composed of two gate strands and one output strand, and is initially double-stranded, and therefore inert, with the exception of the "toehold." The computation is initiated by the addition of single-stranded inputs. The first input binds to the toehold, displacing the first gate strand and exposing the toehold for the second input. The second input binds the toehold, releasing the output strand.

If one of the gate strands contains a fluorophore, and the output strand contains a quencher, the release of the output strand will result in an increase in fluorescence.

(Image from Seelig et al., 2006. Permission pending.)