Human DNA contains 18% C on a molar basis. What is the mole percentage
of A, G, and T respectively?
32, 18, 32
18, 32, 32
32, 32, 18
Insufficient information to calculate
In double-stranded DNA, Chargaff’s rules state adenine (A) equals thymine (T), and guanine (G) equals cytosine (C) on a molar basis. With 18% C given, G is also 18%, leaving 64% for A and T, so each is 32%. The correct answer is 32, 18, 32 for A, G, T respectively.
Chargaff’s Rules Basics
DNA’s double helix pairs A with T via two hydrogen bonds and G with C via three. Total bases sum to 100%, so %A = %T and %G = %C. For 18% C: %G = 18%, %A + %T = 82% – 18% = 64%, thus %A = %T = 32%.
Correct Option Breakdown
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32, 18, 32 (A=32%, G=18%, T=32%): Matches calculation; sufficient data from C via base pairing.
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Incorrect Options Explained
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18, 32, 32 (A=18%, G=32%, T=32%): Swaps A/T with G/C; ignores A=T pairing.
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32, 32, 18 (A=32%, G=32%, T=18%): Breaks A=T equality.
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Insufficient information: False; Chargaff’s rules enable exact computation from one base percentage.
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This holds for idealized double-stranded DNA models, though real human DNA varies slightly (e.g., ~20% C).
