According to Erwin Chargaff's rules for double-stranded DNA, which of the following quantitative relationships is universally valid?
Answer and explanation
Correct answer: B. A = T and G = C; total purines equal total pyrimidines (A + G = T + C, or [A+G]/[T+C] = 1)
- Step 1: Chargaff analyzed the base composition of native double-stranded DNA from diverse organisms.
- Step 2: Because adenine specifically pairs with thymine via two hydrogen bonds () and guanine pairs with cytosine via three hydrogen bonds (), the molar amount of adenine equals thymine () and guanine equals cytosine ().
- Step 3: Summing these yields , meaning the total purine content always equals the total pyrimidine content.
Answer
(Purines = Pyrimidines) and .
Why not the others: is false because the ratio varies widely among species; is incorrect; Chargaff's rules apply only to double-stranded DNA, not single-stranded RNA.
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