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.

(b) (i) How are the structural genes activated in the lac operon in E. coli ?

Hint:

Lactose acts as the inducer, binds the repressor, and the freed operator lets RNA polymerase transcribe the genes.

Answer 1

Step 1: State the resting condition:
Without lactose, the repressor protein (made by the i gene) binds the operator. RNA polymerase is blocked, so the structural genes z, y and a are silent.

Step 2: Role of the inducer:
When lactose is present, a little of it is changed into allolactose. Lactose or allolactose is the inducer.
The inducer binds the repressor and changes its shape, so the repressor can no longer sit on the operator.

Step 3: Result:
The operator is free. RNA polymerase moves over the promoter and transcribes the structural genes lacZ, lacY and lacA. A single polycistronic mRNA is made and the three enzymes are produced.

Final Answer:
The inducer (lactose/allolactose) binds the repressor and inactivates it, so RNA polymerase can transcribe the structural genes.