Editing Lac operon (section)

====Mechanism of induction====
[[File:Lac Operon.svg|thumb|250px|'''''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 allolactose to inhibit the ''lac'' repressor, so the repressor binds tightly to the operator, which obstructs the RNA polymerase from binding to the promoter, resulting in no ''laczya'' mRNA transcripts.
'''Bottom''': The gene is turned on. Allolactose inhibits the repressor, allowing the RNA polymerase to bind to the promoter and express the genes, resulting in production of LacZYA. Eventually, the enzymes will digest all of the lactose, until there is no allolactose that can bind to the repressor. The repressor will then bind to the operator, stopping the transcription of the LacZYA genes.]]

The repressor is an [[allosteric protein]], i.e. it can assume either one of two slightly different shapes, which are in equilibrium with each other.  In one form the repressor will bind to the operator DNA with high specificity, and in the other form it has lost its specificity.  According to the classical model of induction, binding of the inducer, either allolactose or IPTG, to the repressor affects the distribution of repressor between the two shapes.  Thus, repressor with inducer bound is stabilized in the non-DNA-binding conformation.  However, this simple model cannot be the whole story, because repressor is bound quite stably to DNA, yet it is released rapidly by addition of inducer.  Therefore, it seems clear that an inducer can also bind to the repressor when the repressor is already bound to DNA. It is still not entirely known what the exact mechanism of binding is.<ref>{{Cite book|last1=Griffiths|first1=Anthony JF|last2=Gelbart|first2=William M.|last3=Miller|first3=Jeffrey H.|last4=Lewontin|first4=Richard C.|date=1999|chapter=Regulation of the Lactose System|title=Modern Genetic Analysis|chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK21402/|place=New York|publisher=W. H. Freeman|isbn=0-7167-3118-5|language=en|url-access=registration|url=https://archive.org/details/moderngeneticana0000unse}}</ref>