lac operon, genetics question?

Predict whether the structural genes of the lac operon are:
A) Induced
B) Repressed

Any good cell biology book would explain this with a few diagrams. The lac-operon is designed to produce the enzymes necessary to break down lactose into glucose and galactose. Making proteins (including enzymes) requires alot of energy, so if there is no need for them, the cell regulates the production so that they aren’t made (e.g lac operon repressed)

4) If there is no lactose to break down, there is no need for the enzyme so the operon is switched off (REPRESSED)
1) and 2) The point of breaking down lactose is to provide glucose for energy. If the glucose is already there, there is no need to break down lactose. (REPRESSED)
3) Situation 3 is the only case where the breakdown of lactose would be useful. (INDUCED)

Thats the basics. Here are the specifics:—

———–I gene——–promoter—operator——z——-y——a—

The i gene (regulatory gene) produces the repressor protein. Without lactose, the repressor binds to the operator. This prevents RNA polymerase from binding to the promoter which is a necessary step to allow the z y a genes to be transcribed. (z codes for B-galactosidase and the other 2 code for other enzymes associated with lactose transport etc.).

When lactose is present, it binds to the allosteric site on the repressor which changes it’s shape so that it no longer binds to the operator. This allows RNA polymerase to bind to the promoter and thereore allows transcription of the genes.

When lactose and glucose are present (or just glucose), glucose inhibits the production of cAMP which is needed to make the CAP-cAMP complex. Therefore the CAP-cAMP complex does not bind to the promoter and this is required for transcription. So when there is glucose, the genes don’t get transcribed and the proteins don’t get made.

Hope that’s not too confusing.

Lac Operon