Question

26. The lac operon in E. coli consists of genes that code for enzymes necessary for the breakdown of lactose. When lactose is absent, the operon is inactive because a repressor protein binds to a specific site in the lac operon. When lactose is present, lactose molecules bind to the repressor protein, causing the repressor protein to dissociate from the binding site. In the absence of glucose (a preferred energy source for bacteria), the protein CAP binds to a regulatory site near the lac promoter to activate transcription of the lac operon. The following symbols represent the macromolecules involved in regulation of the lac operon. Repressor Lactose САР, RNA polymerase mRNA In the diagrams below, the horizontal line represents the lac operon and the bent arrow represents the transcription start site of the lac operon. Which of the following diagrams best represents the scenario in which lactose is available to the cell and glucose is absent?

Answer 1

glucose inhibits the transcription of the lac operon in the presence of the glucose, glucose prevents the CAP from binding to the CAP binding site.

( CAP binds to DNA only when it is bound to cAMP, glucose inhibit the adenylate cyclase and decreases the levels of cAMP in the cell)

lactose inhibits the lac repressor and prevents its binding to the operator region.

in the absence of glucose and presence of lactose, CAP binds to the CAP site and recruits RNA polymerase and lactose removes repressor from the operator so the RNA polymerase can transcribe the lac operon.

so the answer is C)