Why does the lac operon shut down sometime after the addition of lactose in the medium where E.coti was growing? Why low-level expression of the lac operon is always required?
After the addition of lactose, a complete breakdown of lactose to glucose and galactose takes place. Therefore, there is no more lactose to bind to the repressor protein and the lac operon shuts down. A very low level of expression of lac operon has to be present in the cell all the time, otherwise, lactose cannot enter the cells.
