Which rule formulated by Erwin Chargaff governs base pairing proportions in double-stranded DNA?

Answer and explanation

Correct answer: A. (Purines equal Pyrimidines)

  1. Step 1: Erwin Chargaff systematically quantified the molar ratios of nitrogenous bases in natural double-stranded DNA across diverse organisms.
  2. Step 2: Chargaff's rules establish that the concentration of adenine equals thymine () and the concentration of guanine equals cytosine ().
  3. Step 3: Summing these equalities confirms that total purines equal total pyrimidines: .
Answer
(Purines equal Pyrimidines).
Why not the others: across all species is false because the ratio varies widely among different organisms and is species-specific; and is incorrect because adenine does not pair with guanine, so their ratio is not fixed to unity; Purines always exceed pyrimidines by 2:1 contradicts the fundamental equimolar parity of complementary base pairs ().

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