If a double-stranded DNA has 15% of adenine, find out the percent of cytosine in the DNA?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 35%
By Chargaff’s rule, in double-stranded DNA adenine pairs with thymine and guanine pairs with cytosine. So A = T and G = C, and all four together make 100%.
Adenine (A) = 15%, so thymine (T) = 15%
A + T = 30%
G + C = 100% − 30% = 70%
Cytosine (C) = 70% ÷ 2 = 35%
Option (1) 15% — Incorrect. 15% is the share of thymine, the base that pairs with adenine.
Option (2) 30% — Incorrect. 30% is A + T together, not cytosine.
Option (3) 35% — Correct. In double-stranded DNA, A = T and G = C. With 15% adenine there is 15% thymine, so A + T = 30%. The rest, 70%, is G + C, shared equally: cytosine = 35%.
Option (4) 85% — Incorrect. 85% is 100% minus adenine only. It ignores thymine and does not split the rest between G and C.
NCERT Extract:
Structure of Polynucleotide Chain – …
