““In 1957, Pardee, Monod, and Jacob discovered that the lactose operon was controlled by a single master switch-a protein eventually called the repressor. The repressor functioned like a molecular lock. When lactose was added to the growth medium, the repressor protein sensed the lactose, altered its molecular structure, and "unlocked" the lactose-digesting and lactose-transporting genes (i.e. allowed the genes to be activated), thereby enabling a cell to metabolize lactose. When another sugar, such as glucose, was present, the lock remained intact, and no lactose-digesting genes were allowed to be activated,””
