The researchers began with pairs of enzymes they've been tweaking and designing for the past eight years. Each member of the pairs can only reproduce with the help of the other member.
"We have two enzymes, a plus and a minus," Joyce explains. "The plus assembles the pieces to make the minus enzyme, and the minus enzyme assembles the pieces to draw the plus. It's kind of like biology, where there is a DNA strand with plus and minus strands."