Colonies of blue color are produced by cells with functioning copies of beta galactosidase, while on the other hand, colonies of white color are produced by cells with mutations in the lacZ or in the lacY genes. Lac operon consists of three structural genes, and a promoter, a terminator, regulator, and operator. The structural genes are lacZ, lacY, and lacA. lacZ encodes beta galactosidase, which is an intracellular enzyme that cleaves lactose into glucose and galactose
