Gene expression is basically the control of the cell over which genes to make and which not to make at a specific time. Now why would a cell want to do this? The example we'll use to explore prokaryotic gene expression is the lac operon. The lac operon controls the transcription of genes to make an enzyme to catalyze lactose. It turns lactose into glucose and galactose, the glucose is used in cell respiration to make energy for the cell. The cell takes lactose from the environment and catalyzes it. But if there is already glucose in the environment whats the point of taking lactose when it requires an extra step to make it useful? The cell takes actions to conserve energy and protein by not making the enzyme required for lactose metabolism.