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When lactose is present, allolactose binds to the lac repressor, causing an allosteric change in its shape. In its changed state, the lac repressor is unable to bind tightly to its cognate operator. This effect is referred to as induction, because it induces, rather than represses, expression of the metabolic genes.
Sugars like lactose are âbackupâ carbohydrates. This means that the cell only metabolizes lactose if glucose is low and lactose is plentiful. The cell uses a negative control system (the lac operon) to respond to the availability of lactose in the environment.Negative control. means that itâs a system in which the active substance acts to turn off function.perons. Control of Gene Activity in Prokaryotic Cells. The activity of genes is controlled by the cell and the environment. Inducible. genes are inactive unless circumstances cause them to be activated (âturned onâ). Repressible. genes are active unless circumstances cause them to be inactivated (âturned offâ).