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DNA serves as the cornerstone of genetic information in most organisms, but why is DNA a stable genetic material? This common Assertion-Reason question tests your understanding of molecular biology fundamentals. Let’s break down the exact question, reveal the correct answer, and explain every option step-by-step.

Correct Answer: Option (3) – (A) is correct but (R) is not correct

Why this answer?
DNA truly is a stable genetic material—making (A) correct. Its stability arises from multiple features like strong phosphodiester bonds, hydrophobic core, base stacking, and efficient repair mechanisms. However, Reason (R) fails because DNA is not “double-stranded only.” Some viruses (e.g., parvoviruses) have single-stranded DNA (ssDNA). Even in double-stranded DNA (dsDNA), the double helix provides stability via complementary base pairing, but that’s not the sole reason—other factors like deoxyribose sugar (lacking reactive 2′-OH) contribute hugely compared to unstable RNA.

Detailed Explanation of All Options

• Option (1): Both correct, (R) explains (A)
  Incorrect. While (A) holds, (R) doesn’t explain it fully. Double-strandedness aids stability by allowing repair (one strand templates the other), but ssDNA exists in nature, and stability also stems from chemical inertness, not “double-stranded only.”

• Option (2): Both correct, but (R) doesn’t explain (A)
  Incorrect. (R) is false—DNA isn’t exclusively double-stranded. Examples include M13 phage (ssDNA genome) and φX174 bacteriophage.

• Option (3): (A) correct, (R) not correct
  Correct. (A) is true: DNA’s stability suits long-term heredity (e.g., large genome storage without degradation). (R) is wrong due to ssDNA viruses, proving DNA isn’t “double-stranded only.”

• Option (4): (A) not correct, (R) correct
  Incorrect. (A) is undeniably true—evolution favors DNA over RNA for stability (RNA world hypothesis notes RNA’s fragility from 2′-OH).

Why DNA Excels in Stability

Think of DNA like a armored vault:

• Double helix (when present): Base pairs lock info redundantly.

• Deoxyribose sugar: No 2′-OH, preventing hydrolysis (unlike RNA).

• Large size & repair: Handles mutations via enzymes like DNA polymerase.

RNA, by contrast, degrades easily, suiting short-term roles like mRNA.

This question often appears in NEET, AIIMS, or Class 12 exams to trap students overlooking ssDNA exceptions. Master it for better scores!