Main Page | See live article | Alphabetical index An operon is a collection of inter-related genes including one which acts as a switch that governs the expression of the structural genes in the collection. Structural genes are those that encode a product. Operons, which occur primarily in prokaryotes and nematodes, allow cells to respond to their environment. They were first described by François Jacob and Jacques Monod in 1961. Table of contents 1 The operon as a unit of transcription 2 Example: Lac operon 3 See also The operon as a unit of transcription An operon is a unit of transcription consisting of one or more structural genes, and two associated segments of DNA: an operator (the switch) a promoter (a binding site for the transcription enzyme). The switch of an operon (that is, the "operator") is turned on unless a specific substance is bound to the operator. This substance is therefore called a repressor. In summary: The "operator" is a DNA binding site for a repressor; transcription of the structural genes takes place unless a repressor is bound to the operator. Example: Lac operon The lac operon governs the production of enzymes for metabolizing lactose. In the absence of lactose, the Lac repressor substance binds to the operator, inhibiting the production of three enzymes. Lactose, however, represses the repressor, allowing the enzymes to be produced. See also gene regulatory network This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.