Question

The diagram below illustrates the LAC operon in its OFF state when the inducer molecule —lactose—is absent. Predict the ways in which the following conditions will affect the transcription of the lactose-utilization genes. OPERON Regulatory Promoter Operator_ gene Lactose-utilization genes DNA mRNA RNA polymerase cannot attach to promoter Active repressor Protein If a mutation in the regulatory gene results in a misfolding of the repressor protein so that it can no longer bind DNA, the lactose-utilization genes O Will be transcribed only in the presence of lactose O Will not be transcribed O Will be transcribed constitutively

Answer 1

In LAC operon , in the absence of lactose ,the repressor which is coded by the regulatory Gene binds to the operator this preventing the binding of the RNA polymerase to the promoter. In this condition, the transcription of the genes required for lactose utilisation does not occur. In the presence of lactose, lactose binds to the repressor and changes it's conformation, thus reducing it's affinity for the operator region. Thus the repressor falls off from the operator region,RNA pol binds to the promoter ,lactose utilisation genes are transcribed and lactose is utilised.

However, if the regulatory Gene is mutated,causing misfolding of the repressor,the repressor which can no longer bind to DNA cannot bind to the operator region. Therefore, the promoter constantly remains free for RNA pol to bind.Therefore ,the lactose utilisation genes will be constitutively transcribed irrespective of the presence or absence of lactose .