Purines vs Pyrimidines: Adenine, Guanine, Cytosine, Thymine, Uracil
Biology · Molecular Basis of Inheritance · NEET
There are two types of nitrogenous bases. Purines have a double ring and they are Adenine (A) and Guanine (G). Pyrimidines have a single ring and they are Cytosine (C), Thymine (T) and Uracil (U). Memory hook: "PURe As Gold" = PURines are A and G; the pyrimidines (C, T, U) are the "CUT" bases.
Two purines (A, G) have a double ring and are in both DNA and RNA. Three pyrimidines (C, T, U) have a single ring; C is in both, T only in DNA, U only in RNA.
Your doubts, answered
Is adenine a purine or a pyrimidine?
Adenine is a purine. Purines are the two double-ring bases: Adenine (A) and Guanine (G). So both A and G are purines. This is directly asked in NEET, so memorise it firmly.
How do I remember which bases are purines and which are pyrimidines?
Purines = A and G. Trick: 'PURe As Gold' (PURines = Adenine, Guanine). The word pyrimidine is long, and the pyrimidines are the leftover bases C, T and U. Also, pyrimidines are single-ring, and the word 'pyrimidine' has a 'y' like 'thymine' and 'cytosine'.
Which nitrogenous bases are found in DNA and which in RNA?
DNA has A, G, C and T. RNA has A, G, C and U. Cytosine, Adenine and Guanine are common to both. Thymine is only in DNA. Uracil replaces Thymine in RNA. NCERT says: 'Uracil is present in RNA at the place of Thymine.'
Why is uracil not present in DNA?
DNA is the permanent store of genetic information, so it must be stable. Thymine (which is uracil with an extra methyl group) makes DNA more stable and helps repair enzymes tell apart correct bases from damaged cytosine. RNA is short-lived, so it uses the simpler uracil.
Are purines bigger than pyrimidines?
Yes. Purines (A, G) have two fused rings, so they are larger. Pyrimidines (C, T, U) have only one ring, so they are smaller. In the DNA double helix, a purine always pairs with a pyrimidine (A=T, G=C), which keeps the width of the helix constant at 2 nm.
⚠️ The NEET trap ✗ Guanine and Cytosine are both purines. ✓ The purines are Adenine and Guanine. Cytosine is a pyrimidine. The purines found in BOTH DNA and RNA are Adenine and Guanine. 🧠 NEET 2019 asked 'Purines found both in DNA and RNA are'. The trap options mix one purine with one pyrimidine (like Guanine + Cytosine). Only 'Adenine and Guanine' is fully purine. Remember: purines = A, G.
Real NEET questions
NEET 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: The two purines are Adenine and Guanine, and both are present in DNA and RNA. Thymine and Cytosine are pyrimidines. Options mixing a pyrimidine (thymine or cytosine) are wrong. NCERT Ch 6: 'Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine).'
NEET 2021
If Adenine makes 30% of the DNA molecule, what will be the percentage of Thymine, Guanine and Cytosine in it?
A · T : 30; G : 20; C : 20 ✓
B · T : 20; G : 25; C : 25
C · T : 20; G : 30; C : 20
D · T : 20; G : 20; C : 30
Solution: By Chargaff's rule in double-stranded DNA, A = T and G = C. If A = 30%, then T = 30%. The remaining 40% is shared equally by G and C, so G = 20% and C = 20%. This works because a purine (A) always pairs with its partner pyrimidine (T), and G pairs with C.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
There are two purines: Adenine and Guanine. There are three pyrimidines: Cytosine, Thymine and Uracil. But in one type of nucleic acid you see only two pyrimidines (DNA has C and T; RNA has C and U).
Which pyrimidine is common to both DNA and RNA?
Cytosine is the pyrimidine common to both DNA and RNA. Thymine is only in DNA and Uracil is only in RNA.
What is the difference between a purine and a pyrimidine ring?
A purine has two fused rings: a six-membered ring joined to a five-membered ring. A pyrimidine has a single six-membered ring. That is why purines are bigger than pyrimidines.
Does a purine pair with a purine in DNA?
No. A purine always pairs with a pyrimidine. Adenine pairs with Thymine (2 H-bonds) and Guanine pairs with Cytosine (3 H-bonds). This keeps the DNA helix a constant width.
Is uracil a purine or pyrimidine?
Uracil is a pyrimidine. It has a single ring, like cytosine and thymine. Uracil takes the place of thymine in RNA.
