Q.31 In E.coli lac operon, the lac repressor binds to the operator region
__________ in the absence of lactose.
- Very weakly
- Very strongly
- Not binds
- None of the above
Answer: Very strongly
In the E. coli lac operon, the lac repressor binds very strongly to the operator region in the absence of lactose, preventing transcription of genes needed for lactose metabolism.
Option Analysis
Very weakly: Incorrect. The repressor has high affinity for the operator without lactose, blocking RNA polymerase effectively.
Very strongly: Correct. Without lactose (or allolactose), the repressor tetramer binds tightly to the operator sequence, repressing lacZ, lacY, and lacA transcription.
Not binds: Incorrect. Binding occurs specifically in lactose absence; lactose induces a conformational change releasing the repressor.
None of the above: Incorrect, as “very strongly” matches the mechanism.
Lac Operon Basics
The lac operon regulates lactose use in E. coli via negative control by the lac repressor (from lacI gene). In no-lactose conditions, repressor-operator binding halts unnecessary enzyme production, conserving energy.
The lac repressor binds operator in absence of lactose as a core feature of the E. coli lac operon, enabling efficient gene regulation. This inducible system, discovered by Jacob and Monod, controls β-galactosidase (lacZ), permease (lacY), and transacetylase (lacA) expression.
Repressor Binding Mechanism
Without lactose, the lac repressor tetramer attaches with high affinity (Kd ~10^-13 M) to the operator (O1 site, overlapping promoter). This sterically hinders RNA polymerase, stopping mRNA synthesis.
Lactose (via allolactose) binds repressor, reducing operator affinity >1000-fold, derepressing the operon. Glucose absence further boosts via CAP-cAMP positive control.
Why “Very Strongly”?
Binding is sequence-specific and tight, ensuring near-zero basal transcription. Mutations (lacI-) cause constitutive expression, proving repressor’s role.
Exam Relevance
For GATE Life Sciences or similar, recall: absence lactose → strong binding → no transcription. Presence lactose → weak/no binding → induction.
This regulation exemplifies prokaryotic operons, vital for molecular biology and genetics prep.
