If our genes are wired like circuits, does that mean nature is an electrician?
One of the most important sorts of jobs that genes do is to switch other genes on or off. The classic example comes from Escherichia coli, and how it eats milk. (I'm afraid Escherichia coli will be progressively infecting this blog in the months to come, as I finish my new book on this remarkable bug.) Escherichia coli can make the enzymes necessary for digesting lactose, the sugar in milk. But it normally doesn't, because it makes proteins that clamp onto its DNA next to these genes and prevent gene-reading enzymes from reaching them. If lactose should seep into the microbe, however, the molecule binds to the repressor proteins and causes them to change shape and fall off the DNA. If another protein, which signals a drop in other kinds of sugar, also grabs the DNA near the genes, Escherichia coli begins to crank out the lactose-digesting enzymes, and the milk feast begins.
