Q.153 Zoology ⭐ 360° Option Elimination Active
NEET 2026 Zoology Question 153 Solution
Topic: Molecular Basis of Inheritance · Official NTA Booklet Code 50
Question
Which of the following statements about lac-operon is correct?
✓
Verified Against NTA Final Answer Key
Option (1) : The gene i synthesizes the repressor constitutively.
📚 Molecular Basis of Inheritance Medium
📋 Given Parameters & Problem Setup
Genetic System Lac Operon of Escherichia coli
Components i gene (inhibitor), promoter (p), operator (o), structural genes (z, y, a)
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⏱️ Save ~45 seconds The 30-Second Exam Hall Shortcut: ⚡ Lac Operon Diagnostic Filter
i-gene is ON 24/7 (Constitutive); Lactose is INDUCER (not co-repressor)!
1 Check Option (1): "The gene i synthesizes the repressor constitutively." -> TRUE! (NCERT verbatim: "The i gene codes for the repressor of the lac operon... synthesized constitutively (all the time)").
2 Check Option (2): "Lactose acts as a co-repressor..." -> FALSE (Lactose is an INDUCER; tryptophan is a co-repressor in trp operon).
3 Check Option (3): Jacob & Monod elucidated in 1961 (Nobel Prize in 1965) -> Misleading year.
4 Check Option (4): "z gene codes for transacetylase" -> FALSE (z = β-galactosidase, y = permease, a = transacetylase).
5 Confirm Option (1) with 100% confidence.
🛡️
Why You Can 100% Trust This Shortcut: NCERT Biology · Molecular Basis of Inheritance (DNA Fingerprinting: Variable Number of Tandem Repeats VNTRs as minisatellites).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Function of the i-Gene
The lac operon consists of one regulatory gene (the i gene, from the term inhibitor) and three structural genes (z, y, and a). The i gene codes for the repressor protein of the lac operon. It is synthesized constitutively (all the time) at a basal level.
i\text{-gene} \xrightarrow{\text{Constitutive Transcription & Translation}} \text{Lac Repressor Monomer} \rightarrow \text{Active Tetramer}
- • Constitutive expression means continuous, non-regulated transcription regardless of lactose presence.
Step 2
Action of Inducer vs. Structural Genes
Lactose (or allolactose) acts as an inducer by binding to the repressor tetramer, causing conformational change and inactivating it. The structural genes code for enzymes: z = β-galactosidase, y = permease, a = transacetylase.
- • Negative regulation: the repressor bound to operator blocks RNA polymerase; inducer lifts repression.
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 A
✓ 100% Correct Verdict & Scientific Reason:
100% correct. NCERT states verbatim: "The repressor of the operon is synthesised, all the time – constitutively – from the i gene."
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Why this is airtight: None; textbook definition.
2 B
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Lactose is an INDUCER that binds and inactivates the repressor. A co-repressor (like tryptophan in the trp operon) activates the repressor.
⚠️
Student Fallacy (Why candidates pick this trap): Confusing "inducer" with "co-repressor".
3 C
✗ Incorrect Verdict & Scientific Reason:
Incorrect. Jacob and Monod elucidated the lac operon in 1961. They received the Nobel Prize in 1965.
⚠️
Student Fallacy (Why candidates pick this trap): Confusing year of discovery (1961) with year of Nobel Prize award (1965).
4 D
✗ Incorrect Verdict & Scientific Reason:
Incorrect. The z gene codes for beta-galactosidase. The "a" gene codes for transacetylase.
⚠️
Student Fallacy (Why candidates pick this trap): Swapping z gene product (beta-galactosidase) with a gene product (transacetylase).
⚠️
NTA Trap Alert: Structural Gene Product Inversion (z, y, a)
Students frequently mix up the three structural genes. Use the mnemonic "Z-B, Y-P, A-T" (Zebra Bites, Yellow Panther, Always Tired) to remember z -> β-gal, y -> permease, a -> transacetylase.
🛡️ How to avoid the -1 mark: Write out "z = beta-gal, y = permease, a = transacetylase" before reading the choices.
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Core NEET Syllabus Standard
NEET Biology Core Syllabus · Molecular Basis of Inheritance Standard Benchmark
Core Scientific Principle Tested:
"The repressor of the lac operon is synthesized constitutively from the i gene."
Syllabus Focus: The Lac Operon
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
i-gene is an Island: It works Indefinitely and All the time (Constitutive)!
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