THE LACTOSE OPERON
The term operon refers to a gene complex consisting of a group of structural genes with related fuctions, plus the closely linked DNA sequences responsible for controlling them. For example, the structural genes of the lactose operon—lac Z, lac Y, and lac A—code for -galactosidase, galactose permease, and galactoside transacetylase, respectively.

Because all three enzymes are translated from the same mRNA molecule, a single molecular "switch" must coordinate their synthesis. The switch that controls mRNA synthesis is called the operator; it is a sequence of bases that is upstream from the first structural gene in the operon and that overlaps part of the promoter region.

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