Question

According to lac operon concept, an operator gene combines with
Co-repressor to switch off transcription of structural genes.
A. Inducer gene to switch on structural gene transcription.
B. Repressor gene to switch off transcription of structural gene.
C. Regulator protein to switch on structural gene transcription.
D. Regulator protein to switch off structural gene transcription.

Answer 1

Hint: The lac operon means lactose operon is an operon which is present in the E.coli and enteric bacteria for the transport and metabolism of lactose. Mostly glucose is the carbon source for most bacteria, the lac operon helps for the digestion of lactose when glucose is not available through the activity of beta-galactosidase.

Complete Answer:
- Regulator gene controls the expression of structural genes by transcription factor. These transcription factors bind to the operator region which is present in the binding site of the RNA polymerase and that is why it prevents the binding and transcription.
- The operator region acts as a repressor protein binding site, not for the repressor or regulatory gene itself. This makes statement C and D incorrect.
- The gene that codes for any RNA known as a structural gene. A corepressor prevents the expression of genes by indirectly regulating gene expression by binding to the repressor. This makes statement A incorrect.
- A molecule which regulates gene expression through binding to repressors or activators to stop or initiate the transcription respectively is inducer. Allolactose serves as an inducer in the lac operon and binds to the repressor, not to operator gene, and reduces the repressor's affinity for the operator site; thereby facilitating transcription. This makes statement B incorrect

So, the correct answer is Regulator protein to switch off structural gene transcription.

Note: In lac operon concept, genes are present which are expressed only in presence of lactose when glucose is absent. The lac operon has the 3 structural genes (: lacZ, lacY, and lacA), and a promoter, a terminator, regulator, and an operator.