Editing Operon (section)

== General structure ==
{{anchor|General structure}}<!--anchor for redirect in case of header name change-->
[[File:Lac Operon.svg|thumb|300px|'''''1'': RNA Polymerase, ''2'': Repressor, ''3'': Promoter, ''4'': Operator, ''5'': Lactose, ''6'': lacZ, ''7'': lacY, ''8'': lacA.
''' '''Top''': The gene is essentially turned off. There is no lactose to inhibit the repressor, so the repressor binds to the operator, which obstructs the RNA polymerase from binding to the promoter and making lactase. 
'''Bottom''': The gene is turned on. Lactose is inhibiting the repressor, allowing the RNA polymerase to bind with the promoter, and express the genes, which synthesize lactase. Eventually, the lactase will digest all of the lactose, until there is none to bind to the repressor. The repressor will then bind to the operator, stopping the manufacture of lactase.]]

An operon is made up of 3 basic DNA components:

* [[promoter (biology)|Promoter]] – a [[nucleotide]] sequence that enables a gene to be [[transcription (genetics)|transcribed]]. The promoter is recognized by [[RNA polymerase]], which then initiates transcription.  In RNA synthesis, promoters indicate which genes should be used for messenger RNA creation – and, by extension, control which proteins the cell produces.
*{{anchor|Operator}} '''Operator''' – a segment of [[DNA]] to which a [[repressor]] binds.  It is classically defined in the [[lac operon]] as a segment between the promoter and the genes of the operon.<ref name=blewin>{{cite book |last=Lewin |first=Benjamin |name-list-style=vanc |title=Genes IV |publisher=Oxford University Press |location=Oxford |year=1990 |pages=[https://archive.org/details/genesiv00lewi/page/243 243–58] |isbn=978-0-19-854267-4 |edition=4th |url=https://archive.org/details/genesiv00lewi/page/243 }}</ref> The main operator (O1) in the ''lac'' operon is located slightly downstream of the promoter; two additional operators, O2 and O3 are located at -82 and +412, respectively.  In the case of a repressor, the repressor protein physically obstructs the RNA polymerase from transcribing the genes.
* [[Structural gene]]s – the genes that are co-regulated by the operon.

Not always included within the operon, but important in its function is a [[regulatory gene]], a constantly expressed gene which codes for [[repressor proteins]]. The regulatory gene does not need to be in, adjacent to, or even near the operon to control it.<ref>{{cite book |last=Mayer |first=Gene | name-list-style = vanc | chapter = Bacteriology – Chapter Nine Genetic Regulatory Mechanisms| chapter-url = http://pathmicro.med.sc.edu/mayer/geneticreg.htm|title=Microbiology and Immunology Online|publisher=University of South Carolina School of Medicine|access-date=30 December 2012}}</ref>

An [[inducer]] (small molecule) can displace a repressor (protein) from the operator site (DNA), resulting in an uninhibited operon.

Alternatively, a [[corepressor]] can bind to the repressor to allow its binding to the operator site.  A good example of this type of regulation is seen for the [[trp operon]].