Practice Questions

TLS Online TPP Program

#Question id: 2858

When lactose enters the cell, it is converted to allolactose. It is allolactose (rather than lactose itself) that controls the Lac repressor. Paradoxically, the conversion of lactose to allolactose is catalyzed by b-galactosidase, itself encoded by one of the lac genes. How is this possible? Which statement is incorrect according lac genes expressions?

#Unit 2. Cellular Organization

The expression of the lac genes is leaky, even when they are repressed, because, allolactose binds to a part of the Lac repressor distinct from its DNA-binding domain.
In the absence of allolactose, the repressor is present in a form that binds its site on DNA (and so keeps the lac genes switched off).
Allolactose has altered the shape of the repressor, the protein can no longer bind DNA, and so the lac genes are no longer repressed.
This leakiness ensures that there is a low level of b-galactosidase in the cell even in the absence of glucose, and so there is enzyme poised to catalyze the conversion of lactose to allolactose.