Functions of Lac Operon - In the presence of lactose, the lactose binds to the repressor and prevents it from binding to the promoter by inducing structural changes in the repressor. Then the RNA polymerase binds to the promoter and starts the transcription of the structural gene.

The structural gene is expressed as a single transcript. But different ribosomes bind to RNA transcript each at the starting of each cistron. Thus it makes the protein. These proteins then break down the lactose into glucose units. This gene expression continues as long as lactose is present in the media.