If a dsDNA has 20% cytosine, calculate the percent of adenine in DNA?

The correct answer is Option (3) → 30% adenine

Chargaff’s rule: in a double-stranded DNA, the ratio of adenine to thymine and of guanine to cytosine is constant and equal to one. This is because of base pairing: A always pairs with T, and G always pairs with C. So the amount of A equals the amount of T, and the amount of G equals the amount of C.

    C = 20%, so G = 20%
    G + C = 20% + 20% = 40%
    A + T = 100% − 40% = 60%
    A = T = 60% ÷ 2 = 30%

(1) 20% adenine — Incorrect. 20% is the amount of guanine, which equals cytosine. Adenine pairs with thymine, not with cytosine.

(2) 40% adenine — Incorrect. 40% is the total of G and C together.

(3) 30% adenine — Correct. A and T share the remaining 60% equally.

(4) 60% adenine — Incorrect. 60% is the total of A and T together, not of adenine alone.

NCERT Extract:
Structure of Polynucleotide Chain – “… One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chains. However, this proposition was also based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.”