Regulation of the lac operon. The I gene continually makes repressor. (a) In the absence of lactose, the repressor binds to the O (operator) region and blocks transcription. (b) The binding of lactose changes the shape of the repressor so that the repressor no longer binds to O and falls off the DNA. The RNA polymerase is then able to transcribe the Z, Y, and A structural genes, and so the three enzymes are produced.
ANIMATED ART: Assaying lactose presence or absence through the Lac repressor