Question

According to the lac-operon concept, which functional unit of the bacterial gene material is responsible for suppressing the activity of the operator gene in the absence of lactose
A. Promoter gene
B. Repressor protein
C. Regulator gene
D. Structural gene

Answer 1

Hint: Lac operon is a famous lactose operon, which is essential for the utilization of the lactose in E. coli as the source of energy. Lac operon is a sequence of genes coding for a single mRNA. The bacteria uses lactose as a source of carbon dioxide.

Complete answer: The lac operon functions as a polycistronic gene. The normal source for carbon dioxide in E.coli is glucose. When glucose is not available, the bacteria has to shift to lactose as the substrate for energy.
When lactose is absent in the medium, the repressor gene binds to the operator of the lac operon. This blocks the transcription of the lac operon.
Lactose is the inducer for lac operon. When lactose is available in the medium, the operator cannot be blocked by the repressor. Now the RNA polymerase can bind to the promoter thus, resulting in transcription.
In this process, three genes are encoded – lac Y, lacZ and lac A. LacZ codes for beta galactosidase which breaks lactose into monosaccharides-glucose and galactose. Lac Y codes for permease and lac A codes for galactosidase transferase.
Thus, the repressor gene is the gene that prevents the transcription of lac operon and prevents the binding of the inducer that is lactose to the operator blocking the utilization of lactose.
So, the correct answer is “Option b”.

Note:
The mechanism of lac operon was the first gene regulation to be clearly understood. This operon is a negative regulator. Lac A, lacZ and Lac Y are the structural genes.