In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
Answers
Explanation:
In lac operon, lactose acts as an inducer. ... After sometime, when the level of inducer decreases as it is completely metabolized by enzymes, it causes synthesis of the repressor from regulator gene. The repressor binds to the operator gene and prevents RNA polymerase from transcribing the operon.Lac operon refers to a segment of DNA which consists of one regulatory gene (i ) and three structural genes (y,z and a). Among the three structural genes, the z gene code for enzyme beta-galactosidase, that is responsible for the hydrolysis of the disaccharide lactose into its monomeric units, galactose and glucose. Gene y code for enzyme permease, which increases the permeability of the cell. The gene a encode for enzyme transacetylase. Lactose which is called the inducer is the substrate for enzyme beta-galactosidase and it regulates switching on and off of the operon. The regulatory gene, i code for the repressor of the lac operon. Lactose can bind to the repressor an inactivate it. When lactose is bound to the repressor, the RNA polymerase binds to the promoter of the lac operon. As a result of this, the transcription of three structural genes takes place and they form their respective enzymes. Further, these enzymes metabolize the lactose and lead to the formation of glucose and galactose. When the lactose metabolism is at its highest, the repressor protein is set free to bind with the operator gene. As a result of this, the transcription of lac operon stops. Therefore, lac operon shut down sometime after addition of lactose in the medium.
ANSWER :-
Lac-operon is switched on after the addition of lactose into the medium because lactose acts as an inducer and makes the repressor inactive. Due to this switching on of the lac-operon system, beta-galactosidase is formed which converts lactose into glucose and galactose. As soon as the lactose in the medium gets consumed, the repressor which was initially inactive, becomes active again and causes the shut down of the lac-operon system.
This is the reason why the lac-operon shuts down after some time after addition of lactose into the medium.