Purines vs Pyrimidines: Adenine, Guanine, Cytosine, Thymine, Uracil
Biology · Molecular Basis of Inheritance · NEET
There are 5 nitrogenous bases in nucleic acids. Purines have TWO rings and are Adenine (A) and Guanine (G). Pyrimidines have ONE ring and are Cytosine (C), Thymine (T), and Uracil (U). Memory hook: "PURe As Gold" = PURines are A and G. And "CUT" = the pyrimidines Cytosine, Uracil, Thymine.
Purines (Adenine, Guanine) have two fused rings and are bigger; pyrimidines (Cytosine, Thymine, Uracil) have one ring and are smaller. A and G are in both DNA and RNA; Cytosine is in both, while Thymine is DNA-only and Uracil is RNA-only.
Your doubts, answered
Is adenine a purine or a pyrimidine?
Adenine is a PURINE. So is Guanine. Purines are the bigger bases with two fused rings. The two purines are Adenine (A) and Guanine (G) - both are found in DNA AND RNA. Tip: PURine = PURe As Gold, where A = Adenine and G = Guanine.
How many rings do purines and pyrimidines have?
A purine has TWO rings joined together (a six-membered ring fused to a five-membered ring). A pyrimidine has only ONE six-membered ring. That is why purines are larger. This size difference matters in base pairing: a big purine always pairs with a small pyrimidine, so the DNA helix stays a uniform width.
Which bases are in DNA and which are in RNA?
DNA has four bases: A, G, C, and Thymine (T). RNA also has A, G, C, but it uses Uracil (U) in the place of Thymine. So A, G, and C are common to both. Thymine is only in DNA. Uracil is only in RNA. This is a very common NEET one-mark trap.
What is the difference between a purine and a pyrimidine? (simple)
Purine = two rings, bigger, and includes A and G. Pyrimidine = one ring, smaller, and includes C, T, and U. That is the whole difference for NEET. Remember: the word 'pyrimidine' is longer but the molecule is smaller (one ring); purines are shorter word but bigger molecule (two rings).
Why does RNA have uracil instead of thymine?
Thymine is basically Uracil with an extra methyl (-CH3) group. RNA uses Uracil, the simpler base. DNA uses Thymine because the methyl group makes DNA more stable, and it helps repair enzymes tell real DNA bases apart from damaged cytosine. For NEET, just remember: RNA has U in place of T; both U and T are pyrimidines.
Is cytosine in both DNA and RNA?
Yes. Cytosine (a pyrimidine) is present in BOTH DNA and RNA. NCERT says clearly: 'Cytosine is common for both DNA and RNA.' The one that changes is Thymine (DNA only) versus Uracil (RNA only). So of the pyrimidines, C is shared, while T and U swap between DNA and RNA.
How do I remember which are purines and which are pyrimidines?
Two easy tricks. (1) PURe As Gold -> PURines are Adenine and Guanine. (2) The word 'PYRIMIDINE' and the words 'CYTOSINE, THYMINE, URACIL' all contain the letter 'Y' - so the pYrimidines are CYtosine, thYmine, and the odd one Uracil (CUT). Whatever is left (A and G) must be the purines.
⚠️ The NEET trap ✗ Adenine and Thymine are the two purines, because A always pairs with T. ✓ The two purines are Adenine and Guanine. A pairs with T, but pairing is not the same as being the same type. Thymine is a PYRIMIDINE. The purines found in both DNA and RNA are Adenine and Guanine (NEET 2019, answer B). 🧠 Pairing partners are always ONE purine + ONE pyrimidine, never two purines. So A (purine) pairs with T (pyrimidine). If a question asks for the purines, the answer is always A and 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 as well as RNA. Thymine and Cytosine are pyrimidines, and Thymine occurs only in DNA. NCERT: 'There are two types of nitrogenous bases - Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine).' So the correct pair is A and G.
NEET 2020
Which of the following statements is correct?
A · Adenine pairs with thymine through three H-bonds
B · Adenine does not pair with thymine
C · Adenine pairs with thymine through two H-bonds ✓
D · Adenine pairs with thymine through one H-bond
Solution: Adenine (a purine) pairs with Thymine (a pyrimidine) using TWO hydrogen bonds. Guanine pairs with Cytosine using THREE hydrogen bonds. This purine-pyrimidine pairing keeps the helix a constant width. So the correct statement is: Adenine pairs with Thymine through two H-bonds.
Solved Molecular Basis of Inheritance NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Adenine (A), Guanine (G), Cytosine (C), Thymine (T), and Uracil (U). A and G are purines; C, T, and U are pyrimidines.
Which two purines are present in both DNA and RNA?
Adenine and Guanine. Both purines are found in DNA and in RNA. This is a repeated NEET question (2019).
Which base is only in DNA and which is only in RNA?
Thymine is present only in DNA. Uracil is present only in RNA and takes the place of Thymine. Both are pyrimidines.
Which base is common to both DNA and RNA among pyrimidines?
Cytosine. It is a pyrimidine present in both DNA and RNA. Only Thymine (DNA) and Uracil (RNA) swap places.
Do purines pair with purines?
No. A purine always pairs with a pyrimidine (A with T, G with C). Two purines would be too wide and two pyrimidines too narrow, so the helix stays uniform only with purine-pyrimidine pairs.
