Q.155 Zoology ⭐ 360° Option Elimination Active

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NEET 2021 Zoology Question 155 Solution

Topic: Molecular Basis of Inheritance · Official NTA Booklet Code M1

Question

If Adenine makes 30% of the DNA molecule, what will be the percentage of Thymine, Guanine and Cytosine in it?

(1)T: 20; G: 30; C: 20
(2)T: 20; G: 20; C: 30
(3)T: 30; G: 20; C: 20
(4)T: 20; G: 25; C: 25

✓

Verified Against NTA Final Answer Key

Option (3) : T : 30 ; G : 20 ; C : 20

📚 Molecular Basis of Inheritance Easy

📋 Given Parameters & Problem Setup

DNA Type Double-stranded DNA molecule

Adenine content (%A) 30%

Target Parameters % Thymine (%T), % Guanine (%G), % Cytosine (%C)

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The 30-Second Exam Hall Shortcut: ⚡ "Chargaff's Law: %A = %T, and %G = %C = (100 - 2A) / 2"

⏱️ Save ~30 seconds

In double-stranded DNA, Adenine pairs with Thymine (%T = %A = 30%). A + T = 60%, leaving 40% equally divided between Guanine and Cytosine (%G = %C = 20%). Hence: T: 30, G: 20, C: 20. Match Option (3) in 10 seconds!

1 Set %T = %A = 30% (base complementarity A = T).

2 Calculate %G + %C = 100% - (%A + %T) = 100% - 60% = 40%.

3 By base complementarity (G = C): %G = 20% and %C = 20%.

4 Match Option (3): T : 30 ; G : 20 ; C : 20.

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Molecular Basis of Inheritance (Erwin Chargaff's rule: for dsDNA, [A] = [T] and [G] = [C], [A+G] = [T+C]).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Formulation of Chargaff's Base Equivalence Rules

Erwin Chargaff established that in double-stranded DNA (dsDNA): 1. The molar ratio of Adenine (A) to Thymine (T) is always 1: $[A] = [T]$ . 2. The molar ratio of Guanine (G) to Cytosine (C) is always 1: $[G] = [C]$ . 3. The total percentage of all four nitrogenous bases equals 100%:

\%A + \%T + \%G + \%C = 100\%

Step 2

Step-by-Step Stoichiometric Calculation

Given that Adenine makes up $30\%$ of the DNA: - Because Adenine strictly pairs with Thymine via two hydrogen bonds ( $A = T$ ): $\%T = \%A = 30\%$ - The sum of A and T is: $\%A + \%T = 30\% + 30\% = 60\%$ - Therefore, the remaining fraction of the molecule comprises Guanine and Cytosine: $\%G + \%C = 100\% - 60\% = 40\%$ - Because Guanine pairs with Cytosine via three hydrogen bonds ( $G \equiv C$ ): $\%G = \%C = \frac{40\%}{2} = 20\%$

\begin{aligned} \%T &= 30\% \\[3pt] \%G &= 20\% \\[3pt] \%C &= 20\% \end{aligned}

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360° Option Elimination Engine Key Feature

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4-Option Deep Scan

1 1

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Sets T = 20%, which violates the fundamental base pairing requirement A = T (A was given as 30%).

⚠️

Student Fallacy (Why candidates pick this trap): Aspirants accidentally assign the 30% figure to Guanine instead of Thymine.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Again fails A = T and G = C parity.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing purine-to-pyrimidine pairing partners.

3 3

✓ 100% Correct

Verdict & Scientific Reason:

Correct. Obeys Chargaff's rule: %A = %T = 30% and %G = %C = 20%, summing to 100%.

💡

Why this is airtight: None. Perfect mathematical and biological execution.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Sums to 30 + 20 + 25 + 25 = 100%, but violates %A = %T.

⚠️

Student Fallacy (Why candidates pick this trap): Creating an arbitrary distribution of bases that totals 100% while ignoring base-pairing specificity.

⚠️

NTA Trap Alert: Applying Chargaff's Rules to Single-Stranded Nucleic Acids

Chargaff's equivalence rules apply strictly to double-stranded DNA. In single-stranded DNA (e.g. bacteriophage $\phi \times 174$ ) or RNA, %A does not necessarily equal %T or %U.

🛡️ How to avoid the -1 mark: Confirm the molecule is dsDNA before setting %A = %T and %G = %C.

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Core NEET Syllabus Standard

NEET / NCERT Biology · Molecular Basis of Inheritance Standard Benchmark

Core Scientific Principle Tested:

"In dsDNA, purines equal pyrimidines: %A = %T and %G = %C."

Syllabus Focus: Chargaff's Rules of DNA Base Composition

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Zoology Question 155 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Chargaff's Law: %A = %T, and %G = %C = (100 - 2A) / 2"

Step-by-Step Rigorous Workflow & Mathematical Proof

Formulation of Chargaff's Base Equivalence Rules

Step-by-Step Stoichiometric Calculation

360° Option Elimination Engine Key Feature

NTA Trap Alert: Applying Chargaff's Rules to Single-Stranded Nucleic Acids

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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