Lac operon is a group of genes that are related to the metabolism of lactose in the medium. The operon consists of 2 types of genes; the regulatory genes which control the expression of the structural genes and the structural genes which express the proteins like beta-galactosidase required for the breakdown of lactose to galactose and glucose. As long as the lactose is present in the medium the regulatory genes allow the expression of the structural genes (i.e. the production of beta-galactosidase enzyme). The moment lactose in the medium is exhausted, a protein called repressor protein binds to a region called the operator and blocks the expression of the structural genes (beta-galactosidase enzyme). So, with time, lactose in the medium is completely digested by the beta-galactosidase enzyme and absence of lactose in the medium blocks the expression of beta-galactosidase enzyme after sometime.