Question

Gene expresses itself in a cell system as a protein/enzyme. How an expression of gene occurs in a cell system, when it needs to occur and how the gene expression is regulated in prokaryotic cell system was studied by the combined efforts of Francois Jacob, a geneticist and Jacques Monod, a biochemist. For their work on lactose metabolism in E. coli and introducing the concept of "lac operon", they were awarded the Nobel Prize in 1965.

Why is lac operon said to be a transcriptionally regulated system ?

Hint:

Repressor and inducer control whether mRNA is made.

Answer 1

Step 1: Understanding the question:
Gene expression can be controlled at many levels, such as transcription, RNA processing or translation. We must show that the lac operon is controlled at transcription.

Step 2: Point 1: the control point:
Lactose decides whether the repressor sits on the operator. So it decides whether RNA polymerase can transcribe the structural genes.

Step 3: Point 2: what is switched:
When the repressor is bound, no mRNA is made for lac z, y and a. When the inducer removes the repressor, mRNA is made.

Step 4: Conclusion:
The amount of mRNA, and so of enzyme, depends on whether transcription starts. Hence the lac operon is a transcriptionally regulated system.

Final Answer:
Its genes are switched on or off at the level of mRNA synthesis, by the repressor, operator and inducer.