Q.21 The most stable structure for a random-sequence DNA molecule under physiological conditions is
The most stable structure for a random-sequence DNA molecule under physiological conditions is B form of DNA. B-DNA predominates in aqueous solutions with moderate salt (~150 mM NaCl, 37°C) typical of cells, featuring 10.5 bp/turn, smooth right-handed helix, and optimal base stacking/hydrogen bonding for random AT/GC sequences.
Option Analysis
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(1) C form of DNA: Rare, 9.3 bp/turn, seen in low humidity; less stable than B due to suboptimal stacking geometry.
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(2) A form of DNA: Short/wide, 11 bp/turn, stable in 75% ethanol/low water; physiological conditions favor B over dehydrated A.
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(3) Z form of DNA: Left-handed, zig-zag backbone, requires alternating purine-pyrimidine (CGCGCG) and high salt; unstable for random sequences.
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(4) B form of DNA: Correct—Physiological standard (1-2 Å hydration shell), antiparallel strands with major/minor grooves enabling protein binding.
Answer: (4) B form of DNA.
Introduction to DNA Structural Stability
The most stable structure for a random-sequence DNA molecule under physiological conditions is B form DNA. This right-handed helix with 10.5 bp/turn optimizes base stacking (enthalpy-driven) and hydration, prevailing in vivo aqueous environments.
B-DNA Physiological Dominance
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Parameters: 23 Å pitch, 20 Å diameter, deep major (12 Å)/minor (6 Å) grooves
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Stability factors: Hydrophobic stacking > H-bonding; A-T destabilizing, G-C neutral
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Random sequence: AT/GC mix favors B over sequence-specific Z.
Comparative Helix Stability
| Form | bp/turn | Helix | Conditions | Random seq stability |
|---|---|---|---|---|
| A | 11 | Wide/short | Low water | Low |
| B | 10.5 | Medium | Physiological | High |
| C | 9.3 | Elongated | Low humidity | Medium |
| Z | 12 | Left/slim | CG repeats + salt | Very low |
GATE Biochemistry Application
Tests helix physiology: B-DNA default for random sequences. Distinguish environmental triggers (alcohol=A, salt+CG=Z) vs. cellular norm (B).
