Concept Breakdown
1. TITLE
How does the Lac Operon switch on? The NEET explanation you actually need
2. CONCEPT EXPLANATION
The Lac Operon is a genetic switch in bacteria (like E. coli) that controls the production of enzymes needed to digest lactose. It normally stays "OFF" to save energy when lactose is missing.
Here is the underlying principle: A regulatory gene (i) constantly produces a repressor protein. This repressor acts like a roadblock; it binds tightly to the operator region of the DNA, physically preventing the enzyme RNA polymerase from passing through to transcribe the structural genes (z, y, a).
The switch turns "ON" only when Lactose (or allolactose) enters the cell. Lactose acts as an inducer. It binds to the repressor protein and changes its shape (conformational change). This inactivated repressor can no longer hold onto the DNA and falls off. With the roadblock removed, RNA polymerase can finally access the promoter and transcribe the genes,,.
Common Misconception: Students often think the inducer activates the RNA polymerase directly. It does not. It works by disabling the "brake" (repressor), not by pushing the "accelerator."
3. MCQ
In the lac operon system, what is the primary molecular event that allows transcription to proceed in the presence of lactose?
A. The inducer binds to the operator region to displace the repressor.
B. The repressor protein undergoes a conformational change and detaches from the operator.
C. RNA polymerase binds to the inducer, increasing its affinity for the promoter.
D. The repressor binds to the promoter, recruiting RNA polymerase to the site.
4. ANSWER + EXPLANATION
Correct Answer: (B)
Explanation:
The lac operon is a negatively regulated system. Normally, the repressor protein binds to the operator and blocks transcription. When the inducer (lactose) is present, it binds specifically to the repressor protein. This interaction causes the repressor to change its shape so that it can no longer bind to the operator region. Once the repressor detaches, the path is clear for RNA polymerase to bind to the promoter and initiate transcription.
Why the traps are wrong:
Option A is a common error—the inducer never binds to the DNA (operator) directly; it binds to the protein. Option D describes the opposite of what happens (the repressor blocks polymerase, it doesn't recruit it).
5. KEY TAKEAWAY BOX
Remember:
The inducer switches the operon ON by inactivating the repressor (removing the brake), not by activating the polymerase directly.
6. NCERT REFERENCE
NCERT Class 12, Chapter 5 (Molecular Basis of Inheritance)
"In the presence of an inducer, such as lactose or allolactose, the repressor is inactivated by interaction with the inducer. This allows RNA polymerase access to the promoter and transcription proceeds."
7. CTA
Full chapter explanation → Genetics and Evolution Complete Guide