Question

3. Explain what happens to the Lac operon when lactose is present and glucose is also present. Then explain what happens when lactose is present but glucose is absent. In your answer, include all of the components of the Lac operon and the molecules involved in its regulation. What is the protein product of the Lac operon? Explain why this mechanism of control is adaptive.

Answer 1

• Glucose present, lactose present

When glucose is present, it inhibits the enzyme adenyl cyclase and this will not form AMP to cAMP. This will not form the cAMP- CAP complex and won't bind to the CAP binding site. This complex stimulates the RNA polymerase activity but in presence of glucose rna polymerase is not stimulated. As the lactose is present it will remove the inhibitor attached to the operator site and very basal level of transcription can be observed. So, low amount of Z, Y and A genes are made.

• Lactose present, glucose absent

When glucose is absent cAMP - CAP complex attached to the CAP binding site and this stimulus the RNA polymerase activity and simultaneously lactose will remove the inhibitor from operator site, as both rna polymerase is stimulated and inhibitor is not present, high level of operon gene transcription can be observed. So, Lac z will code for beta galactosidase, lacY will code for lac permease and lac A will code for transacetylase.