question_answer 1)

In medium where E. coli was growing lactose was added, which induced the lac-operon. But why does lac-operon shut down after sometime after addition of lactose in the medium? 

Answer:

Inducer is a chemical (substrate, hormone or some other metabolite) which after coming in contact with the repressor, changes the latter into non-DNA binding state so as to free the operator gene. The inducer for lac-operon of Escherichia coli is lactose (actually allolactose, or metabolite of lactose). RNA polymerase is recognized by promoter gene. It passes over the freed operator gene and then catalyses transcription of mRNAs over the structural genes. The mRNA pass into the cytoplasm and form particular proteins or enzymes. Out of the three enzymes produced by lac-operon, permease is meant for bringing lactose inside the cell. galactosidase ( = lactase) breaks lactose into two components, glucose and galactose. The enzyme like lactase or galactosidase which is formed in response to the presence of its substrate is often called inducible enzyme. Inducible operon systems generally occur in catabolic pathways. The lac-operon will not, however, remain operative indefinitely despite presence of lactose in the external environment. It will stop its activity with the accumulation of glucose and galactose in the cell beyond the capacity of the bacterium for their metabolism.