→ Negative control — The binding of a protein to DNA stops transcription. → Positive control — The binding of a protein to DNA increases transcription. → Inducible system — The system is normally “off” but can be made to turn “on,” given a particular signal. → Repressible system — The system is normally “on” but can be made to turn “off,” given a particular signal.

Note: Bacteria can digest lactose, but it is more energetically expensive than digesting glucose. Therefore, bacteria only want to use this option if lactose is high and glucose is low. The lac operon is induced by the presence of lactose; thus, these genes are only transcribed when it is useful to the cell.