Q.115 Botany ⭐ 360° Option Elimination Active

NEET 2025 Botany Question 115 Solution

Topic: Biotechnology: Principles and Processes · Official NTA Booklet Code 45

Question

In the above represented plasmid an alien piece of DNA is inserted at EcoRI site. Which of the following strategies will be chosen to select the recombinant colonies?

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Verified Against NTA Final Answer Key

Option (3) : White color colonies will be selected.

📚 Biotechnology: Principles and Processes Medium

📋 Given Parameters & Problem Setup

Plasmid Features Ori, ampR, tetR (with BamHI), β-Galactosidase (with EcoRI)

Cloning Site Used EcoRI site within the β-Galactosidase gene

Objective Selection strategy for recombinant colonies

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The 30-Second Exam Hall Shortcut: ⚡ Blue-White Screening Fingerprint

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Insertional inactivation of β-galactosidase = Loss of enzyme activity = No blue product on X-gal = WHITE COLONIES are Recombinants!

1 Identify the insertion site: Alien DNA is cloned into the EcoRI site inside the β-galactosidase gene.

2 Recall the insertional inactivation principle:

3 - Recombinant plasmid: Gene for β-galactosidase is disrupted -> No functional β-galactosidase enzyme produced -> Inability to hydrolyze chromogenic substrate (X-gal) -> Colonies remain WHITE.

4 - Non-recombinant plasmid: Gene remains intact -> Active β-galactosidase produced -> Cleaves X-gal to form BLUE colonies.

5 Therefore, recombinant colonies are identified as WHITE colonies!

6 Select Option (3) White color colonies will be selected!

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Why You Can 100% Trust This Shortcut: Restriction Endonucleases & Palindromic Cleavage (NCERT Biology · Biotechnology: Principles and Processes: EcoRI cuts at G|AATTC).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Mechanisms of Selectable Markers and Insertional Inactivation

Selection of recombinants due to inactivation of antibiotics requires simultaneous replica plating on two different antibiotic plates, which is cumbersome:

• Alternative selectable markers differentiate recombinants from non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate.
• Here, a recombinant DNA is inserted within the coding sequence of the enzyme $\beta$ -galactosidase (encoded by the *lacZ* gene).

Step 2

Phenotypic Outcome on Chromogenic Substrate (X-gal)

Evaluating the two bacterial populations:

• Non-recombinant colonies: The plasmid does not have an insert; the $\beta$ -galactosidase gene remains intact and produces active enzyme, which hydrolyzes the chromogenic substrate X-gal (5-bromo-4-chloro-3-indolyl- $\beta$ -D-galactopyranoside) to produce an insoluble blue dye -> Blue colonies.
• Recombinant colonies: Insertion of alien DNA at the EcoRI site disrupts the $\beta$ -galactosidase reading frame (insertional inactivation). No functional enzyme is made, so no blue pigment is formed -> White colonies.

Step 3

Selection of Recombinants

Because recombinant bacteria fail to produce color due to insertional inactivation, researchers select the white-colored colonies.

\text{Alien DNA in } \beta\text{-galactosidase} \implies \text{No Enzyme} \implies \text{White Colonies (Recombinants)}

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360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. The insertion is made at the EcoRI site inside the β-galactosidase gene, not within the tetR or ampR genes; both resistance genes remain intact, so antibiotic dual screening does not distinguish recombinants.

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Student Fallacy (Why candidates pick this trap): Confusing insertional inactivation of β-galactosidase with pBR322 antibiotic inactivation at BamHI.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Blue colonies are non-recombinants containing intact β-galactosidase.

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Student Fallacy (Why candidates pick this trap): Inverting the color interpretation of the blue-white screening assay.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. Recombinants suffer insertional inactivation of β-galactosidase and appear as white colonies.

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Why this is airtight: None; textbook mastery of chromogenic selection.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Blue colonies are non-recombinants, regardless of ampicillin resistance.

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Student Fallacy (Why candidates pick this trap): Assuming blue indicates successful recombinant expression.

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NTA Trap Alert: Blue vs White: Which is Recombinant?

A classic NEET exam trap: "Blue = Good / Has Insert" vs "White = Empty". In reality, the insert DESTROYS the enzyme, causing LOSS of color (White = Recombinant, Blue = Non-recombinant)!

🛡️ How to avoid the -1 mark: Remember: "Break the gene, lose the dye -> White is the recombinant guy!"

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Core NEET Syllabus Standard

NEET Biology Core Syllabus · Biotechnology: Principles and Processes Standard Benchmark

Core Scientific Principle Tested:

"Insertion of alien DNA within the β-galactosidase gene inactivates it, resulting in white recombinant colonies in the presence of chromogenic substrate."

Syllabus Focus: Cloning Vectors · Selection of Recombinants (Insertional Inactivation)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

WHITE = Recombinant (Gene was Wiped out); BLUE = Non-recombinant (Enzyme is Blooming)!

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Vector Anatomy & Features (Ori, Selectable Marker, Cloning Site, Transformation)

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NEET 2025 Botany Question 115 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Blue-White Screening Fingerprint

Step-by-Step Rigorous Workflow & Mathematical Proof

Mechanisms of Selectable Markers and Insertional Inactivation

Phenotypic Outcome on Chromogenic Substrate (X-gal)

Selection of Recombinants

360° Option Elimination Engine Key Feature

NTA Trap Alert: Blue vs White: Which is Recombinant?

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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