The following structure of nucleic acid base is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Uracil
The bases of nucleic acids are of two kinds. Purines (adenine, guanine) have two fused rings; pyrimidines (cytosine, thymine, uracil) have a single six-membered ring with nitrogen atoms at positions 1 and 3. The pyrimidines are told apart by the groups on the ring:
Uracil: two C=O groups (at C2 and C4), N–H at N1 and N3, and only H atoms on C5 and C6.
Thymine: the same as uracil but with a methyl group at C5 (5-methyluracil).
Cytosine: one C=O group (at C2) and an –NH2 group at C4.
The given structure is a single pyrimidine ring with two C=O groups, two N–H groups and no methyl or amino group, so it is uracil.
Why the other options are incorrect
Option (2) Thymine — Incorrect. It would carry a –\( \ce{CH3} \) group at C5; the given ring has H there.
Option (3) Cytosine — Incorrect. It has only one C=O group and an –\( \ce{NH2} \) group on the ring.
Option (4) Guanine — Incorrect. It is a purine, with a five-membered ring fused to the six-membered one.
NCERT (2026-27 edition):
Chemical Composition of Nucleic Acids – “… DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U).”
