Q.77 Zoology ⭐ 360° Option Elimination Active

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NEET 2020 Zoology Question 77 Solution

Topic: Molecular Basis of Inheritance · Official NTA Booklet Code E1

Question

Which of the following statements is correct?

(1)Adenine pairs with thymine through two H-bonds.
(2)Adenine pairs with thymine through one H-bond.
(3)Adenine pairs with thymine through three H-bonds.
(4)Adenine does not pair with thymine.

✓

Verified Against NTA Final Answer Key

Option (1) : Adenine pairs with thymine through two H-bonds.

📚 Molecular Basis of Inheritance Easy

📋 Given Parameters & Problem Setup

Molecular Structure Double-stranded B-DNA

Nitrogenous Base Pair Adenine (A) and Thymine (T)

Question Objective Identify the correct hydrogen bonding statement

⚡

The 30-Second Exam Hall Shortcut: ⚡ "A = T (Two H-Bonds); G ≡ C (Three H-Bonds)!"

⏱️ 20 seconds

According to Watson and Crick base-pairing rules, Adenine pairs with Thymine from the opposite strand through TWO hydrogen bonds ( $A = T$ ). Guanine pairs with Cytosine through THREE hydrogen bonds ( $G \equiv C$ ). Statement (1) is 100% correct! Mark Option (1) in 5 seconds.

1 Watson-Crick double helix: Purine always pairs with pyrimidine.

2 Adenine (A) forms 2 hydrogen bonds with Thymine (T) (

A = T

3 Guanine (G) forms 3 hydrogen bonds with Cytosine (C) (

G \equiv C

4 Statement 1 states: "Adenine pairs with thymine through two H-bonds."

5 Select Option (1).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Molecular Basis of Inheritance (Structure of Polynucleotide Chain and Double Helix)

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Watson-Crick Base Pairing Principle

In 1953, James Watson and Francis Crick proposed the double helix model for the structure of DNA based on X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins and Chargaff's base equivalence rules.

Step 2

Complementary Hydrogen Bonding Details

The two strands are held together by hydrogen bonds between nitrogenous bases: - Adenine forms TWO hydrogen bonds with Thymine ( $A = T$ ). - Guanine forms THREE hydrogen bonds with Cytosine ( $G \equiv C$ ).

Step 3

Uniform Diameter of the Double Helix

Because a purine (two rings) always pairs with a pyrimidine (one ring), the distance between the two polynucleotide backbones remains constant at 2.0 nm throughout the length of the duplex.

Step 4

Evaluate the Four Statements

Statement (1) correctly states two H-bonds for A-T. Statements (2), (3), and (4) propose one, three, or no bonds, all of which are chemically erroneous.

⭐

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

✓ 100% Correct

Verdict & Scientific Reason:

Factually and chemically exact; Adenine and Thymine form two complementary hydrogen bonds in B-DNA.

💡

Why this is airtight: None; universally known cornerstone of molecular biology.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; one hydrogen bond is insufficient to stabilize base pairing.

⚠️

Student Fallacy (Why candidates pick this trap): Misremembering bond count.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; THREE hydrogen bonds exist between Guanine and Cytosine ( $G \equiv C$ ), not Adenine and Thymine.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing A-T pairing with G-C pairing.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; Adenine canonically pairs with Thymine in DNA.

⚠️

Student Fallacy (Why candidates pick this trap): Absurd distractor.

⚠️

NTA Trap Alert: A-T (2 bonds) vs G-C (3 bonds)

Never mix up the bond numbers: A-T has 2 H-bonds (lower melting temperature); G-C has 3 H-bonds (higher melting temperature).

🛡️ How to avoid the -1 mark: Mnemonic: "A-T = 2 letters = 2 bonds; G-C = 3 bonds"!

📖

Core NEET Syllabus Standard

Zoology · Molecular Basis of Inheritance Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Watson-Crick Base Pairing Rules: Adenine Pairs with Thymine via Two H-Bonds

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Zoology Question 77 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "A = T (Two H-Bonds); G ≡ C (Three H-Bonds)!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Watson-Crick Base Pairing Principle

Complementary Hydrogen Bonding Details

Uniform Diameter of the Double Helix

Evaluate the Four Statements

360° Option Elimination Engine Key Feature

NTA Trap Alert: A-T (2 bonds) vs G-C (3 bonds)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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