Question

ASAP

In E.coli the lac operon is used to control the expression of the genes involved in the use and metabolism of lactose. Describe how the genes are not expressed in the absence of lactose. Describe how the genes are expressed when lactose is present, both in the presence and absence of glucose. What would happen if there were an I- mutation? What would happen if there were an Oc mutation? Which one of those two mutations is recessive?

Answer 1

Wild-type lac Operon

When lactose is absent, repressor protein is active without lactose so it will bind to the operator region and will not allow the RNA polymerase to pass results in turn OFF the operon.

When lactose is present, some lactose will be converted into allolactose and will bind to the lac repressor making it inactive. So repressor can no longer bind to the operator results in turn ON the operon.

presence of Glucose as well as lactose

catabolite repressor protein (CRP) is only active when cAMP binds to it and makes cAMP-CAP complex.

This complex is necessary for binding of RNA polymerase to the promoter at transcription initiation. Without bindig of this complex to the promoter site, RNA polymerase cannot bind and Operon remains OFF.

In the cell, when glucose level is high, the level of cAMP islower so no cAMP-CAP complex will be formed and Operon remains OFF. Even though there is lactose present, glucose level can regulate lac Operon. (cAMP concentration in cell is inversely proportional to glucose concentration)

When glucose level is lower, cAMP level will be higher, complex will be formed, RNA polymerase will start transcription.

Mutation description

Oc --> mutation on operator site which doesn't allow the binding of lac repressor protein. Genes under this operator are constitutive. Means will always be expressed in any condition (lactose absent or present)

lacI- /lacZ-/lacY- --> these mutation produce non-functional proteins so we can condider as no products will be formed by these genes.

So Oc mutation is dominant (cannot be complemented) and I- is recessive (can be complemented)