Question

The greatest expression of the lac operon occurs when lactose levels are _____. a) high and glucose levels are low b) low and glucose levels are low c) low and glucose levels are high d) high and glucose levels are high.

Answer 1

The lac operon is most highly expressed when lactose levels are high and glucose levels are low, which corresponds to option (a) high and glucose levels are low.

Step-by-step explanation:

The greatest expression of the lac operon occurs when lactose levels are high and glucose levels are low. This situation corresponds to option (a) of the provided choices. The lac operon is an inducible operon that is maximally expressed only under these conditions for a few reasons. First, the presence of lactose ensures that the inducer allolactose is present, which binds to the repressor and prevents it from blocking transcription. Second, when glucose levels are low, cAMP levels increase, which activates the catabolite activator protein (CAP) making the transcription of the lac operon more effective.

Thus, maximal activation of the lac operon, and therefore the highest level of enzyme production for the metabolism of lactose, occurs when cells detect the presence of lactose and the absence of glucose, signaling that lactose should be used as an alternate energy source.

Answer 2

The correct answer is a) high and glucose levels are low

Step-by-step explanation:

Lac operon is an inducible operon which is found in bacteria like E.coli. An inducible operon is the operon which is expressed in the presence of inducer. The inducer binds to the regulatory or repressor proteins, preventing the binding of repressor protein on the operator site thereby trigger gene transcription.

Here the inducer is lactose or allolactose so when lactose is present in high amount it binds to the repressor and allow the transcription of structural gene that are required in the metabolism of lactose.

When glucose is present at a high level in the medium than the lac operon is not expressed and cells use glucose as a primary source of energy. Therefore lac operon expresses more when lactose level is high and glucose level is low.