What occurs to the LAC operon when lactose levels are low?

• A. It is overexpressed
• B. It undergoes mutations
• C. It is activated
• D. It lacks expression

Answer: (D)

When lactose levels are low, the LAC operon is not actively expressed. This is primarily due to the presence of a repressor protein that binds to the operator region of the operon, preventing RNA polymerase from transcribing the genes necessary for the metabolism of lactose. In the absence of lactose, the repressor remains bound to the operator, effectively blocking transcription and leading to a lack of expression of the operon’s genes.

The mechanism is tied to the system's efficiency; when lactose is scarce, it makes sense for the cell to conserve resources by not producing enzymes that are only needed for lactose metabolism. In this scenario, adenylyl cyclase activity is low, resulting in low levels of cyclic AMP (cAMP). As a consequence, the cAMP-CAP complex, which promotes expression of the operon when lactose is available, is also inactive.

This situation allows the cell to maintain energy efficiency and respond to the availability of lactose, facilitating a functional adaptation to environmental changes. Thus, when lactose levels are low, the operon's genes remain unexpressed, resulting in the lack of expression.