Structural Gene - There are three structural genes in the lac operon. These genes are named as cistron-Z, cistron-Y and cistron-A. Cistron-Z codes for an enzyme called galactosidase. This enzyme cleaves the lactose into galactose and glucose.

Cistron- Y codes for an enzyme called galactoside permease. This protein is located on the cell wall and facilitates the transfer of lactose from outside into the cell. Cistron- A codes for galactoside transacetylase