Thymine and uracil are different in their structures due to the presence of
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → –CH3
Thymine and uracil have the same pyrimidine skeleton with two C=O groups at C-2 and C-4. The only difference is at C-5: thymine carries a methyl group, uracil a hydrogen, so thymine is 5-methyluracil.
So thymine and uracil differ due to the presence of the –CH3 group.
Why the other options are incorrect
Option (1) –NH2 group — Incorrect. Neither thymine nor uracil has an –\( \ce{NH2} \) group; it is found in cytosine (and in adenine and guanine).
Option (3) Purine ring — Incorrect. Both are pyrimidines; the purine ring is present in adenine and guanine.
Option (4) Pyrimidine ring — Incorrect. Both thymine and uracil contain the pyrimidine ring, so it is common to them, not a difference.
NCERT (2026-27 edition):
Chemical Composition of Nucleic Acids – …
