The expression of the lac structural genes is _______ when lactose is absent from the culture medium and is _______ when lactose is added, because lactose binds to the _______ and inactivates it.

Question

Quizplus · Accepted Answer

The lac structural genes are under control of the lac operon. When lactose is absent, the lac repressor is active and binds to the lac operator, preventing transcription of the structural genes. When lactose is added, it binds to the lac repressor, causing it to be released from the operator and allowing transcription of the structural genes to occur. Therefore, the expression of the lac structural genes is low when lactose is absent (because the lac repressor is active) and high when lactose is present (because the lac repressor is inactive). The lac inducer is actually allolactose, which is formed from lactose by the action of β-galactosidase in the cell. The lac promoter is the site where RNA polymerase binds to initiate transcription, and is not directly affected by lactose or the lac repressor.

The Expression of the Lac Structural Genes Is _______ When