The transcription of genes in the lac operon is due to the lac repressor. We can summarize its activity as follows:
1. Lac repressor can be active or inactive. It is active if no lactose is present in the system.
2. The active lac repressor binds to the lac operator site to prevent the transcription of genes since the active lac repressor blocks it.
3. The lac repressor is inactive if lactose is present in the cell. Lactose is transformed into allolactose which is now called the lac inducer. The lac inducer binds to the lac repressor thus changing its conformation making it "inactive".
4. The inactive lac repressor cannot bind to the lac operator site. Consequently, the transcription of genes will now be possible since the operon is not blocked.