Question

Read the following passage and answer the questions that follow :

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.

It is said that "lac operon" has to be operational at a very low level in the bacterial cell all the time. Justify.

Hint:

Without a few enzyme molecules, the cell could not bring in lactose or make the inducer.

Answer 1

Step 1: Think about what the cell needs when lactose first appears:
When lactose enters a cell that has never met it, the repressor is bound to the operator, so the operon is switched off.
To switch it on, lactose must first enter the cell and be changed into allolactose, the true inducer.

Step 2: Link it to the enzymes:
Permease carries lactose into the cell, and beta-galactosidase changes lactose into allolactose.
If the operon were completely off, there would be no permease and no beta-galactosidase to begin the process.

Final Answer:
A very low basal level of expression keeps a few molecules of permease and beta-galactosidase in the cell, so lactose can enter, form the inducer and switch the operon on.