The lac operon is on when the repressor protein get fallen off the operator.
What is Lac operon?
An operon or a group of genes with a single promoter, which encodes genes for the metabolism and transport of lactose in E.coli and other bacteria. In the process, the group of three genes need to be turned on before the bacterium, that is, E.coli can utilize lactose as food.
In the absence of lactose, the lac repressor, that is, the DNA-binding protein combines with the region known as the operator that switches off the lac operon. When binding of lactose takes place with the repressor, it makes the repressor to fall off the operator, hence, turning the operon on.
Thus, when the repressor protein gets fall off the molecule, it shows that the lac operon is turned on.
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