There are 5 nitrogen bases in nucleic acids. The purines are Adenine and Guanine, and they have a double (fused two-ring) structure. The pyrimidines are Cytosine, Uracil and Thymine, and they have a single ring. Memory hook: "PURe As Gold" = PURines are Adenine and Guanine; the rest are pyrimidines.
Purines (Adenine, Guanine) have a double ring; pyrimidines (Cytosine, Uracil, Thymine) have a single ring. Thymine is in DNA only, uracil in RNA only.
Your doubts, answered
Which bases are purines and which are pyrimidines?
Purines are Adenine (A) and Guanine (G). Pyrimidines are Cytosine (C), Uracil (U) and Thymine (T). NCERT says clearly: adenine and guanine are substituted purines, while the rest are substituted pyrimidines. So there are only 2 purines and 3 pyrimidines in total.
Are adenine and guanine purines or pyrimidines?
Adenine and guanine are both PURINES. This is a very common NEET question. A quick trick: the two purines both have the letter shape of a bigger, double ring. Both A and G are also present in both DNA and RNA.
How many nitrogen bases are there, and which are in DNA vs RNA?
There are 5 nitrogen bases total: adenine, guanine, cytosine, uracil and thymine. DNA uses A, G, C and Thymine. RNA uses A, G, C and Uracil. So DNA has thymine (not uracil) and RNA has uracil (not thymine). A, G and C are common to both.
Do purines have one ring or two rings?
Purines (A and G) have TWO fused rings (a double-ring structure), so they are the bigger bases. Pyrimidines (C, U, T) have only ONE ring, so they are smaller. NCERT calls all of them heterocyclic rings because the rings contain nitrogen atoms, not just carbon.
Which purines are found in both DNA and RNA?
Adenine and Guanine are the purines found in BOTH DNA and RNA. This is exactly what NEET 2019 asked. Cytosine is also in both, but cytosine is a pyrimidine, not a purine, so it is not the answer when the question asks only for purines.
⚠️ The NEET trap ✗ Adenine and guanine are substituted pyrimidines. ✓ Adenine and guanine are substituted PURINES. The pyrimidines are cytosine, uracil and thymine. 🧠 NTA flips this in a 'choose the correct/wrong statement' set (seen in NEET 2026). If a statement says A and G are pyrimidines, it is WRONG. A and G are always purines.
Real NEET questions
2019
Purines found both in DNA and RNA are:
A · Adenine and thymine
B · Adenine and guanine ✓
C · Guanine and cytosine
D · Cytosine and thymine
Solution: Purines are adenine and guanine, and both occur in DNA and RNA. Thymine and cytosine are pyrimidines, so options with them are wrong. NCERT (Ch 9): adenine and guanine are substituted purines while the rest are substituted pyrimidines.
2026
Identify the correct statements about biomolecules. A. Lipids are generally water soluble. B. Proteins are polypeptides. C. Polysaccharides are long chains of sugars. D. Adenine and guanine are substituted pyrimidines. E. Almost all enzymes are proteins.
A · B, D and E only
B · B, C and E only ✓
C · A, B and C only
D · C, D and E only
Solution: Statement D is wrong because adenine and guanine are substituted PURINES, not pyrimidines. Statement A is also wrong (lipids are generally water insoluble). B, C and E are correct, so the answer is B, C and E only.
2016
Which one of the following statements is wrong?
A · Sucrose is a disaccharide
B · Cellulose is a polysaccharide
C · Uracil is a pyrimidine
D · Glycine is a sulphur containing amino acid ✓
Solution: Glycine has only hydrogen as its R-group, so it has no sulphur (the wrong statement). Note that 'Uracil is a pyrimidine' is CORRECT here, confirming uracil is a pyrimidine, not a purine.
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the difference between a purine and a pyrimidine?
A purine has two fused rings (double ring) and includes adenine and guanine. A pyrimidine has a single ring and includes cytosine, uracil and thymine. So purines are bigger and pyrimidines are smaller.
Why are they called nitrogen bases?
They are heterocyclic rings that contain nitrogen atoms in the ring, and they act as bases (they can accept protons). NCERT lists them as adenine, guanine, cytosine, uracil and thymine.
Which base is only in DNA and which is only in RNA?
Thymine is found only in DNA. Uracil is found only in RNA. Both are pyrimidines. Adenine, guanine and cytosine are shared by both DNA and RNA.
Is cytosine a purine or a pyrimidine?
Cytosine is a pyrimidine (single ring). Only adenine and guanine are purines. A common trap is to include cytosine with the purines, which is wrong.
How do I remember purines for NEET?
Use 'PURe As Gold': PURines = Adenine and Guanine. Everything else (C, U, T) is a pyrimidine. Also remember purines have two rings, pyrimidines have one.
