University of Toronto (UofT) BIO230H1 From Genes to Organisms Midterm Practice Exam

The option indicating low lactose levels is correct because the presence of a repressor protein is typically associated with the regulation of certain operons, such as the lac operon in E. coli. In the absence of lactose (or when lactose levels are low), the repressor protein binds to the operator region of the operon, preventing RNA polymerase from transcribing the downstream genes necessary for lactose metabolism. This binding effectively inhibits gene expression for enzymes that are involved in the breakdown of lactose, as they are not needed when lactose is scarce.

When lactose is present and available, some of it is converted into allolactose, which can bind to the repressor protein. This binding changes the conformation of the repressor, leading to its release from the operator and allowing transcription to proceed. Thus, in the context of low lactose, the repressor is active in inhibiting gene expression, preventing unnecessary production of enzymes that would be involved in lactose utilization.

High glucose levels and low glucose levels (where the preferred energy source would be present or absent, respectively) influence the activation of the lac operon through a different mechanism, involving catabolite activator protein (CAP) that interacts differently with the operon’s transcription process. High lactose levels