Class 12 NEET Biology Molecular Basis of Inheritance Questions Part 3

Concept:In the lac operon, the repressor protein is always made (constitutive expression) and binds to the operator only when lactose is absent. An inducer like allolactose inactivates the repressor, allowing transcription.Explanation:Galactose is a monosaccharide, not an inducer of the lac operon. The actual inducer is allolactose, formed from lactose. So option A is false.Gene $i$ codes for the repressor protein. It is expressed continuously (constitutive) because its promoter is always active. This is a key fact — the repressor is always present unless inactivated. So option B is correct.Lactose itself does not activate the repressor. Instead, when lactose is present, it is converted to allolactose, which binds to the repressor and prevents it from attaching to the operator. Thus option C is false.Genes $z$, $y$, and $a$ are structural genes that share a common promoter (the lac promoter). Gene $i$ has its own separate promoter. Therefore option D is false.Answer:Option B: Gene $i$ is constitutively expressed is the correct statement.