Q.127 Botany ⭐ 360° Option Elimination Active

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NEET 2021 Botany Question 127 Solution

Topic: Biotechnology: Principles and Processes · Official NTA Booklet Code M1

Question

Which of the following is a correct sequence of steps in a PCR (Polymerase Chain Reaction)?

(1)Denaturation, Annealing, Extension
(2)Denaturation, Extension, Annealing
(3)Extension, Denaturation, Annealing
(4)Annealing, Denaturation, Extension

✓

Verified Against NTA Final Answer Key

Option (1) : Denaturation, Annealing, Extension

📚 Biotechnology: Principles and Processes Easy

📋 Given Parameters & Problem Setup

Technique Polymerase Chain Reaction (PCR)

Target Correct chronological order of steps per cycle

⚡

The 30-Second Exam Hall Shortcut: ⚡ "D - A - E" (Denature $\to$ Anneal $\to$ Extend)

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Each thermal cycle of PCR follows the immutable sequence: 1. Denaturation of dsDNA ( $94^\circ\text{C}$ ) $\to$ 2. Annealing of primers ( $50-60^\circ\text{C}$ ) $\to$ 3. Extension by Taq polymerase ( $72^\circ\text{C}$ )! Remember the acronym D-A-E. Match Option (1)!

1 Step 1: Denaturation — High temperature melts the hydrogen bonds between double-stranded DNA.

2 Step 2: Annealing — Lower temperature allows synthetic oligonucleotide primers to bind to complementary 3' template ends.

3 Step 3: Extension — Thermostable Taq DNA polymerase synthesizes new strands using dNTPs at ~72°C.

4 Order: Denaturation

\to

Annealing

\to

Extension.

5 Match Option (1).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Biotechnology Principles on Polymerase Chain Reaction Steps.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

The Three Successive Phases of PCR

In vitro enzymatic gene amplification relies on cyclic repetition of three thermal phases:

\mathbf{1.\text{ Denaturation } (\sim 94^\circ\text{C})} \longrightarrow \mathbf{2.\text{ Annealing } (\sim 54^\circ\text{C})} \longrightarrow \mathbf{3.\text{ Extension } (\sim 72^\circ\text{C})}

Step 2

Mechanisms of Each Phase

- Denaturation: Thermal disruption converts double-stranded target DNA into two separate single strands. - Annealing: Two sets of oligonucleotide primers hybridize to their complementary flanking sequences at the $3'$ ends of each template strand. - Extension: The thermostable enzyme *Taq* DNA polymerase (isolated from *Thermus aquaticus*) extends primers along the templates in the $5' \to 3'$ direction by adding deoxynucleotides ( $dNTPs$ ).

⭐

360° Option Elimination Engine Key Feature

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4-Option Deep Scan

1 1

✓ 100% Correct

Verdict & Scientific Reason:

The exact and only functional chronological sequence of events in each thermal cycle of PCR.

💡

Why this is airtight: None. Textbook standard sequence.

2 2

✗ Incorrect

Verdict & Scientific Reason:

DNA polymerase cannot extend before primers have annealed to provide a free 3'-OH group.

⚠️

Student Fallacy (Why candidates pick this trap): Placing extension prior to primer annealing.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Cannot begin with extension on native double-stranded DNA.

⚠️

Student Fallacy (Why candidates pick this trap): Inverting the cycle starting point.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Primers cannot anneal to target sequences while DNA is double-stranded prior to denaturation.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming primers can bind before melting the double helix.

⚠️

NTA Trap Alert: Permutation Traps in Process Steps

NTA routinely scrambles the order of biological cycles (PCR, Calvin cycle, translation, prophase stages).

🛡️ How to avoid the -1 mark: Anchor: D-A-E (Denaturation

\to

Annealing

\to

Extension).

📖

Core NEET Syllabus Standard

NEET / NCERT Biology · Biotechnology: Principles and Processes Standard Benchmark

Core Scientific Principle Tested:

"Each cycle of PCR has three steps: (i) Denaturation, (ii) Primer annealing, and (iii) Extension of primers."

Syllabus Focus: Polymerase Chain Reaction (PCR)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Botany Question 127 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "D - A - E" (Denature →\to→ Anneal →\to→ Extend)

Step-by-Step Rigorous Workflow & Mathematical Proof

The Three Successive Phases of PCR

Mechanisms of Each Phase

360° Option Elimination Engine Key Feature

NTA Trap Alert: Permutation Traps in Process Steps

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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The 30-Second Exam Hall Shortcut: ⚡ "D - A - E" (Denature $\to$ Anneal $\to$ Extend)