Q.74 Zoology ⭐ 360° Option Elimination Active

NEET 2020 Zoology Question 74 Solution

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

Question

If the distance between two consecutive base pairs is 0.34 nm and the total number of base pairs of a DNA double helix in a typical mammalian cell is 6.6×109 bp, then the length of the DNA is approximately:

(1)2.0 meters
(2)2.5 meters
(3)2.2 meters
(4)2.7 meters

✓

Verified Against NTA Final Answer Key

Option (3) : 2.2 meters

📚 Molecular Basis of Inheritance Easy

📋 Given Parameters & Problem Setup

Distance between consecutive bp 0.34 nm = 0.34 × 10^-9 m

Total bp in mammalian diploid genome 6.6 × 10^9 bp

Calculation Formula Length = Total bp × distance per bp

⚡

The 30-Second Exam Hall Shortcut: ⚡ "Length = 6.6 × 10^9 × 0.34 × 10^-9 = 2.244 m ≈ 2.2 meters!"

⏱️ 20 seconds

Multiply total base pairs ( $6.6 \times 10^9$ ) by the distance between consecutive base pairs ( $0.34 \times 10^{-9}\text{ m}$ ): the powers of 10 cancel ( $10^9 \times 10^{-9} = 1$ ), leaving $6.6 \times 0.34 = 2.244\text{ m} \approx 2.2\text{ meters}$ . Mark Option (3) in 5 seconds!

1 Formula: Length of DNA double helix = (Total number of bp) × (Distance between two consecutive bp).

2 Substitute: Length = 6.6 × 10^9 bp × 0.34 × 10^-9 m/bp.

3 Simplify exponents: 10^9 × 10^-9 = 1.

4 Calculate: 6.6 × 0.34 = 2.244 m ≈ 2.2 meters.

5 Select Option (3) 2.2 meters.

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Molecular Basis of Inheritance (Packaging of DNA Helix)

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Formula for DNA Double Helix Length

The physical length of a double-stranded B-DNA molecule is simply the product of the total number of base pairs and the distance between adjacent base pairs along the helical axis: $\text{Length} = N_{\text{bp}} \times d_{\text{bp}}$

Step 2

Identify Given Numerical Values

Total base pairs in diploid mammalian cell ( $N_{\text{bp}}$ ) = $6.6 \times 10^9\text{ bp}$ . Distance between adjacent base pairs ( $d_{\text{bp}}$ ) = $0.34\text{ nm} = 0.34 \times 10^{-9}\text{ meters}$ .

Step 3

Perform Dimensional Multiplication

$\text{Length} = (6.6 \times 10^9) \times (0.34 \times 10^{-9}\text{ m})$ $\text{Length} = 6.6 \times 0.34 = 2.244\text{ meters} \approx 2.2\text{ meters}$

Step 4

Biological Significance in NCERT

NCERT emphasizes that this ~2.2 meter long DNA thread is packaged into a typical microscopic nucleus measuring only approximately $10^{-6}\text{ m}$ in diameter, necessitating elaborate supercoiling around histone octamers into nucleosomes and chromatin fibers.

⭐

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; mathematical calculation yields 2.244 meters, not 2.0 meters.

⚠️

Student Fallacy (Why candidates pick this trap): Rough rounding error.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; overestimation of the product $6.6 \times 0.34$ .

⚠️

Student Fallacy (Why candidates pick this trap): Calculation arithmetic error.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

Factually and mathematically exact; $6.6 \times 10^9 \times 0.34 \times 10^{-9} = 2.244\text{ m} \approx 2.2\text{ meters}$ .

💡

Why this is airtight: None; textbook quantitative constant.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; significant overestimation.

⚠️

Student Fallacy (Why candidates pick this trap): Mistakenly multiplying by pitch (3.4 nm) instead of distance between base pairs.

⚠️

NTA Trap Alert: Haploid vs Diploid Base Pair Count

Human haploid genome = $3.3 \times 10^9\text{ bp}$ (length ~1.1 m). Human diploid mammalian cell = $6.6 \times 10^9\text{ bp}$ (length ~2.2 m).

🛡️ How to avoid the -1 mark: Double check whether question specifies haploid (1.1 m) or typical diploid mammalian cell (2.2 m)!

📖

Core NEET Syllabus Standard

Zoology · Molecular Basis of Inheritance Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Length of Human Diploid DNA Calculation: Total Base Pairs × Base-to-Base Distance

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Zoology Question 74 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Length = 6.6 × 10^9 × 0.34 × 10^-9 = 2.244 m ≈ 2.2 meters!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Formula for DNA Double Helix Length

Identify Given Numerical Values

Perform Dimensional Multiplication

Biological Significance in NCERT

360° Option Elimination Engine Key Feature

NTA Trap Alert: Haploid vs Diploid Base Pair Count

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Molecular Basis of Inheritance

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