A DNA fragment contains 20% Thymine (T). What would be the combined c…

The correct answer is Option (3) → 60%

In double-stranded DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). So the amount of A equals T, and the amount of G equals C (Chargaff’s rule). All four bases together make 100%.

• T = 20%, so A = 20% (A pairs with T).
• A + T = 20% + 20% = 40%
• C + G = 100% − 40% = 60%
• Each of C and G = 60% ÷ 2 = 30%

Option (1) 40% — Incorrect. 40% is the combined content of A and T, not of C and G.

Option (2) 20% — Incorrect. 20% is the content of thymine alone (and of adenine alone).

Option (3) 60% — Correct. The bases left after A and T make up 100% − 40% = 60%, shared equally by C and G.

Option (4) 30% — Incorrect. 30% is the content of cytosine alone (or guanine alone). The question asks for C and G together.

NCERT Extract:
Structure of Polynucleotide Chain – “… It was only in 1953 that James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but famous Double Helix model for the structure of 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. … (iii) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds.”