Question

How would you describe the effect a repressor has on the lac operon when lactose is present?

Answer 1

Hint: The lac repressor is encoded by the lacI gene, located upstream of the lac operon and has its own promoter. The lac repressor features a high affinity for lactose. When a little amount of lactose is present the lac repressor will bind it causing dissociation from the DNA operator thus freeing the operon for organic phenomenon.

Complete answer:
For the lac operon to be expressed, lactose must be present. This is sensible for the cell because it might be energetically wasteful to make the enzymes to process lactose if lactose wasn't available.
In the absence of lactose, the lac repressor is sure to the operator region of the lac operon, physically preventing RNA polymerase from transcribing the structural genes. However, when lactose is present, the lactose inside the cell is converted to allolactose.
Allolactose is an inducer molecule, binding to the repressor and changing its shape in order that it's not ready to bind to the operator DNA. Removal of the repressor within the presence of lactose allows RNA polymerase to maneuver through the operator region and start transcription of the lac structural genes.

Note:
Regulatory proteins bind to specific sequences within the DNA and control which genes to show on under any particular conditions. A classic case of a regulatory protein is Lac I, the lactose repressor. This protein binds DNA and prevents organic phenomenon when the tiny signal molecule (the inducer) is absent.