DNA Stretching Stress Strain 100 bp Young’s Modulus 1 GPa

7. If a 100 bp DNA molecule is stretched by 1 bp, what is the stress and strain acting on the molecule? Given: Young's modulus of elasticity of DNA is 1 Gigapascal a. Strain is 1% and stress is 106 Pa b. Strain is 1% and stress is 107 Pa c. Strain is 10% and stress is 107 Pa d. Strain is 10% and stress is 106 Pa

7. If a 100 bp DNA molecule is stretched by 1 bp, what is the stress and strain acting on the
molecule? Given: Young’s modulus of elasticity of DNA is 1 Gigapascal
a. Strain is 1% and stress is 106 Pa
b. Strain is 1% and stress is 107 Pa
c. Strain is 10% and stress is 107 Pa
d. Strain is 10% and stress is 106 Pa

Introduction
DNA stretching stress strain analysis reveals mechanical properties of double-stranded B-form DNA under tension, crucial for CSIR NET Life Sciences. When a 100 bp DNA molecule is stretched by 1 bp using Young’s modulus of 1 GPa, strain equals extension divided by original length. This MCQ tests understanding of elasticity where stress equals Young’s modulus times strain.[web:1][web:21][web:23]

Correct Answer

Option b. Strain is 1% and stress is 107 Pa provides the precise values.
B-form DNA has 0.34 nm rise per base pair, so 100 bp contour length measures 34 nm and 1 bp extension equals 0.34 nm.[web:21][web:23][web:24]
Strain calculates as ΔL/L = 1/100 = 0.01 or 1%.
Young’s modulus Y = 1 GPa = 109 Pa gives stress σ = Y × strain = 109 × 0.01 = 107 Pa.[web:1][web:2]

Option Analysis

a. Strain 1%, stress 106 Pa: Strain correctly 1% from ε = ΔL/L = 1/100 = 0.01, but stress underestimates by factor 10 since 109 × 0.01 = 107 Pa.[web:2]
b. Strain 1%, stress 107 Pa: Correct—matches σ = 109 × 0.01 = 107 Pa. DNA Young’s modulus ~1 GPa verified for double helix.[web:1][web:5]
c. Strain 10%, stress 107 Pa: Strain wrong (actual 1%, not 10/100); stress magnitude fits 1% but pairs incorrectly.[web:2]
d. Strain 10%, stress 106 Pa: Both wrong—overstated strain, understated stress.[web:2]

Key Concepts

Young’s modulus quantifies DNA stiffness: Y = stress/strain = (F/A)/(ΔL/L), units Pa.[web:2][web:4] For dsDNA, values 0.3-1 GPa align with hard plastics, measured via AFM/optical tweezers.[web:1][web:7]

Stretching 100 bp by 1 bp assumes linear elasticity below B-S transition (~65 pN force).[web:6]

This CSIR NET question emphasizes biophysics integration in molecular biology, testing precise numerical application over approximation.[web:1][web:23]

Optimized for CSIR NET Life Sciences | DNA Mechanical Properties & Biophysics

 

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