Gene regulation is the process in which the expression of genes is activated or inhibited. This is an essential process within all organisms as they have finite resources and thus must use their resources economically by synthesising proteins only when they are needed. Moreover, not all proteins are required by all cells, and therefore gene regulation helps to inhibit their expression.
Before we begin, here are some terms which you should familiarise yourself with.
The Lac Operon is a group of genes in several species of bacterial cells which produces proteins necessary for the digestion of lactose only when it is present. Furthermore, as the favoured source of energy in these bacterial cells is glucose, therefore, if the availability of glucose is sufficient, then there is no need for proteins which can digest lactose and the expression of the genes are inhibited
When lactose is absent, the repressor protein is active and binds to the operator, thereby physically blocking the RNA polymerase from binding to the promotor region and therefore transcription of the gene is inhibited.
When lactose is present, it can bind to the repressor protein inducing a conformation change and thereby inactivating it so that it can no longer bind to the operator allowing for RNA polymerase to bind to the promotor region and catalyse transcription of the gene.