Q.2 In the lac operon, the lac repressor prevents the transcription initiation by binding to
1. lac inducer
2. lac terminator
3. open reading frame
4. lac operator
In the lac operon, the lac repressor binds to the operator to block transcription initiation. This SEO-optimized guide explains the correct answer and all options for biology exams like NEET.
Correct Answer
The lac repressor prevents transcription initiation by binding to 4. lac operator. This DNA sequence, located downstream of the promoter, acts as a switch; repressor binding physically blocks RNA polymerase access, repressing lacZ, lacY, and lacA genes without lactose. Allolactose (inducer) alters repressor conformation, releasing it to allow transcription.
Option Breakdown
| Option | Explanation | Correct/Incorrect |
|---|---|---|
| 1. lac inducer | Allolactose or IPTG; binds repressor to inactivate it, promoting transcription—not the repressor’s target. | Incorrect |
| 2. lac terminator | Rho-independent sequence causing transcription termination; unrelated to repressor binding or initiation block. | Incorrect |
| 3. open reading frame | Protein-coding DNA stretch (e.g., lacZ ORF); repressor doesn’t bind here, targets regulatory operator instead. | Incorrect |
| 4. lac operator | Specific 21-26 bp sequence (O1 primary site); repressor tetramer binds via helix-turn-helix motif, looping DNA for tight repression. | Correct |
Mechanism Overview
Without lactose, active lac repressor (LacI tetramer) binds operator O1 (and auxiliary O2/O3), forming DNA loops that sterically hinder promoter access. Inducer binding shifts repressor to low-affinity state, freeing operator for CAP-cAMP activation. This negative control exemplifies inducible operons in E. coli.
Key for exams: Operator mutations (O^c) cause constitutive expression.
