The biological circuit works just like a digital one: It receives an input and makes a logic-based decision, using AND, OR, and NOT operations. But instead of the inputs and outputs being voltage signals, they are the presence or absence of specific chemicals or proteins. The process begins with the design of a DNA strand that codes for all the logic the system will need. The researchers insert the synthesized DNA into E. coli bacteria as part of a plasmid—a ring of DNA that can replicate as it floats around in the cell.The DNA serves as a template for the biological computer’s machinery. The cell’s molecular machinery translates the DNA into RNA, essentially copying the DNA code onto a different molecule for use by the cell. RNA links up with a cell’s ribosome and instructs it to produce a protein specified in the RNA’s code.
Here’s where the system behaves like a computer, rather than just a genetically engineered organism: The RNA only does its job when it receives an input that activates it. That’s because the engineered RNA contains codes not just for a protein, but also for logic functions. The logic portions must receive the right inputs in order to activate the RNA in a way that allows the ribosome to use it to produce the circuit’s output—in this case a protein that glows.