CBSE Class 12-science Answered

• The DNA segments that code these enzymes are lac Z, lac Y and lac A. These are called structural genes that are located in a linear sequence in the bacterial chromosome.
• The action of these genes is controlled in a coordinated manner by regulatory genes which form an operon along with the structural genes.
• The operon model for lactose catabolism is called lac operon. 

Mechanism of lac operon

• In the absence of inducer lactose, the regulator gene R produces a repressor protein which binds to the operator site and prevents transcription of structural genes.
• When inducer lactose is introduced in the medium, it binds to the repressor and prevents it from binding to the operator. The operator then induces the RNA polymerase to bind to the promoter and transcribe mRNA by the structural genes and enzymes. produced.