An operon is a single unit of physically adjacent genes that function together under the control of a single operator gene. With respect to transcription and translation, the genes within an operon code for enzymes or proteins that are functionally related and are usually members of a single enzyme system. The operon is under the control of a single gene that is responsible for switching the entire operon "on" or "off." A repressor molecule that is capable of binding to the operator gene and switching it, and consequently the whole operon, off, controls the operator gene. A gene that is not part of the operon produces the repressor molecule. The repressor molecule is itself produced by a regulator gene. The repressor molecule is inactivated by a metabolite or signal substance (effector). In other words, the effector causes the operon to become active.

The lac operon in the bacterium...