Operon
Molecular Biology · Microbiology · Genetics
Say it: OP-uh-ron
An operon is a cluster of genes in bacteria that share one promoter, are switched on and off together and are copied into a single messenger RNA.
Also: operons
Definition
What is an operon?
An operon is a unit of gene control found mainly in bacteria and archaea. It contains several structural genes, usually for enzymes that work in the same pathway, lined up behind a single promoter where RNA polymerase binds. Between the promoter and the genes lies the operator, a short DNA sequence that a repressor protein can grip. Because all the genes are transcribed together into one messenger RNA, the cell can turn a whole pathway on or off with one switch.
François Jacob and Jacques Monod proposed the model in 1961 from their work on lactose use in E. coli. The lac operon is inducible: it is normally off because the lac repressor sits on the operator, but when lactose is present, a related sugar, allolactose, binds the repressor and lifts it away. Transcription is strongest when glucose is also scarce, because a complex of cAMP and the activator protein CAP then helps RNA polymerase bind. The trp operon is repressible: it is normally on, and plenty of tryptophan activates its repressor to shut it down.
Operons show negative control by repressors and positive control by activators in one small system.
Word parts
Where does the word “operon” come from?
- oper-
- from operator, the switch region; to workLatin operari, to work
- -on
- a unitscientific suffix, as in electron
Practice it
Where you'll meet it
Related
Terms that go with it
- Gene expressionGene expression is the use of a gene's information to make a working product, usually a protein or a functional RNA molecule.
- TranscriptionTranscription is the process in which a cell copies the DNA sequence of a gene into a matching strand of RNA.
- ProkaryoteA prokaryote is a usually single-celled organism, a bacterium or an archaeon, whose cell has no nucleus or other membrane-bound organelles.
- BacteriumA bacterium is a single-celled prokaryote with no nucleus, found in almost every habitat on Earth, including in and on the human body.
- Messenger RNAMessenger RNA is the RNA copy of a gene that carries protein-building instructions from DNA to the ribosomes, where it is read in three-base codons.
- GeneA gene is a stretch of DNA that carries the instructions for making a protein or a working RNA molecule, and is the basic unit of heredity.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
Last updated
Spotted a mistake? Tell us and we'll correct it.
Practice Molecular Biology
Free molecular biology quizzes, with an explanation for every answer.
See the Quizzes