Question

In the Lac operon model, the repressor protein binds to which site?
a. Structural genes
b. Promotor
c. Operator
d. None of the above

Answer 1

Hint: Operon concept was given by Francis Jacob (a geneticist) and Jacques Monod (a biochemist). This model given in bacteria is a model of gene regulation. The operon includes a coordinated group of genes such as structural gene, operator gene, promoter gene, regulator gene. These function together and regulate a metabolic pathway as a unit. Some examples of operon are lac operon, trp operon, ara operon, his operon, val operon etc.

Complete answer:
The lac operon (lac refers to lactose) comprises one regulatory gene or inhibitor gene (i), one promoter gene, one operator gene and three structural genes.

The bacterium that Jacob and Monod used was E. Coli. In E. Coli, breakdown of lactose requires three enzymes. This system is called an inducible system because the addition of lactose itself stimulates the production of required enzymes.

Lac operon genes:
• Structural genes- Three structural genes are:-
• Lac z: These genes code for beta-galactosidase which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units glucose and galactose.
• Lac y: This gene codes for permease, which increases permeability of the cell to beta-galactosidase.
• Lac a: The gene codes for transacetylase which can transfer acetyl group to beta-galactosidase.

• Operator gene: It binds with a protein molecule or regulator molecule, which prevents the transcription of structural genes.
• Promoter gene: The gene possess site for RNA polymerase attachment.
• Regulator gene (i): This gene codes for a protein known as repressor protein, it is synthesized continuously from the i gene and the functions always.

The operon is switched off when the repressor protein produced by regulatory or inhibitory gene binds to the operator gene. Here the RNA polymerase gets blocked and thus there would be no transcription.

Hence, the correct answer is option (C).

Note: Regulation of lac operon by repressor is referred to as negative control or regulation.
• Constitutive genes: These are those genes which are constantly expressing themselves in a cell because their products are required for the normal cellular activities, e.g., i gene as lac operon.
• Non constitutive genes or Luxury genes: These are not always expressing themselves in a cell. They are switched on or off according to the requirement of cellular activities, e.g., Lactose system in E. Coli. (z, y, a gene).