For the lactose operon to be “on” and actively transcribed, _ must be present and _ must be absent.

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Understand the lactose operon system: It is a set of genes involved in the metabolism of lactose in bacteria, primarily E. coli. The operon is regulated by the presence or absence of lactose and glucose.

Identify the role of the inducer molecule: Lactose (or its isomer allolactose) acts as the inducer that binds to the repressor protein, causing it to change shape and release from the operator region, allowing transcription to proceed.

Recognize the role of the repressor protein: The repressor binds to the operator to block transcription when lactose is absent. For the operon to be 'on,' the repressor must not be bound to the operator, which means it must be inactive or absent from the operator site.

Consider the effect of glucose presence: When glucose is present, it inhibits the activation of the operon by lowering cyclic AMP (cAMP) levels, which are necessary for the cAMP receptor protein (CRP) to bind and activate transcription. Therefore, glucose must be absent or at low levels for the operon to be fully active.

Summarize the conditions for the operon to be 'on': Lactose (or allolactose) must be present to inactivate the repressor, and glucose must be absent to allow cAMP-CRP complex formation and activation of transcription.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lactose Operon Structure and Function

The lactose operon (lac operon) is a set of genes in bacteria that enables the metabolism of lactose. It includes structural genes, a promoter, an operator, and regulatory elements that control transcription based on environmental signals.

Role of Allolactose (Inducer) in Operon Activation

Allolactose, a derivative of lactose, acts as an inducer by binding to the lac repressor protein, causing it to change shape and detach from the operator. This removal allows RNA polymerase to transcribe the operon genes.

Function of the Lac Repressor Protein

The lac repressor is a protein that binds to the operator region of the lac operon to block transcription when lactose is absent. Its absence from the operator is necessary for the operon to be 'on' and actively transcribed.

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