A DNA fragment contains 18% Thymine(T). The combined content of Cytosine (C) and Guanine (G) in it would be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 64%
By Chargaff's rule, in double-stranded DNA A = T and G = C, because A always pairs with T and G with C.
T = 18%, so A = 18%. A + T = 36%.
G + C = 100% − 36% = 64%.
Option (3) 64% — Correct. In DNA, A = T, so 18% thymine means 18% adenine as well; together A + T = 36%. The rest is G + C: 100 − 36 = 64%.
NCERT Extract:
The DNA – “… 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.”