When constructing computer circuits, most folks start with silicon and metal, but not the researchers at Stanford. The boffins in Palo Alto want to build computers out of living tissue, and to that end they’ve created a biological transistor, called the transcriptor. Transcriptors substitute DNA for semiconductors and RNA for the electrons in traditional transistors — essentially, the transcriptor controls the flow of a specific RNA protein along a DNA strand using tailored combinations of enzymes.

Using these transcriptors, researchers built logic gates to derive true/false answers to biochemical questions posed within living cells.

More details can be found here:

http://www.engadget.com/2013/03/31/stanford-genetic-transistor-transcriptor/