Lactose operon or lac operon is a gene system customized by E.coli bacteria to be used in the hydrolysis of lactose. Lac operon consists of structural genes such as lacZ, lacY and lacA. It also has a promoter gene and operator gene. LacI, a regulatory gene that codes for repressor protein is located outside the operon. When lactose is absent, there will be no inducer. Hence, repressor protein binds to the operator which physically blocks the progression of RNA polymerase.
The inability of transcription by the RNA polymerase results in the corresponding protein not made. So there will be no break down of lactose. In the presence of lactose, some are converted into allolactose which binds to the repressor protein and changes its shape. In the new confirmation, the repressor protein can no longer bind to the operator. RNA polymerase is now able to transcript the structural genes, lacZ, lacY and lacA into mRNA. Then, mRNA is translated by ribosome to produce ?-galactosidase.
Lactose is broken down into glucose and galactose.