operon

An operon is a set of genes wired to one switch, like several appliances on a single power strip — flip it and they all turn on or off together. The genes sit in a row on the DNA and are transcribed as one long RNA, so the cell makes the whole group as a package.

This arrangement is common in bacteria and makes sense when the genes do related jobs. If several enzymes are needed to digest one nutrient, grouping their genes under shared control lets the cell produce them all at once, only when that nutrient is around. The control region — a promoter plus an operator — sits at the front and governs the entire cluster.

Operons are largely a feature of bacteria and some simpler organisms; most genes in animals and plants are transcribed individually, each with its own promoter. Still, the operon was the first clear picture of gene regulation and remains the textbook model for how a cell turns genes on and off in response to conditions.