Among the following bases, which base is not present in RNA?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Thymine
The nitrogenous bases of nucleic acids are purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil). DNA and RNA share adenine, guanine and cytosine and differ in the fourth base: thymine in DNA, uracil in RNA.
Option (1) Adenine — Present in RNA. A purine with an –NH2 group, found in both DNA and RNA.
Option (2) Uracil — Present in RNA. A pyrimidine with two C=O groups; it is the fourth base of RNA, in place of thymine.
Option (3) Cytosine — Present in RNA. A pyrimidine with an –NH2 group, found in both DNA and RNA.
Option (4) Thymine — Not present in RNA. A pyrimidine with two C=O groups and a methyl group on the ring (5-methyluracil); it occurs in DNA, while RNA has uracil in its place.
NCERT (2026-27 edition):
Chemical Composition of Nucleic Acids – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 29-05 S1 20-05 S2 | – | – | – | – |