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Chargaff's Rule and Complementary Base Pairing (A=T, G=C)

Biology · Molecular Basis of Inheritance · NEET

Chargaff's rule says that in a double-stranded DNA the amount of Adenine equals Thymine (A = T) and Guanine equals Cytosine (G = C). This happens because A always pairs with T, and G always pairs with C, on the two strands. Memory hook: "A-T sit at a Table for 2 (2 H-bonds), G-C are a Group of 3 (3 H-bonds)."
Complementary Base Pairing (Chargaff's Rule)Strand 1 (5'->3')Strand 2 (3'->5')AT2 H-bondsGC3 H-bondsCGA = TG = CA+G = T+C
In double-stranded DNA, A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). This makes A = T and G = C, so total purines equal total pyrimidines (A + G = T + C = 50%).

Your doubts, answered

Does Chargaff's rule (A=T, G=C) work for single-stranded DNA?

No. Chargaff's rule holds only for double-stranded DNA, because it depends on base pairing between two strands. In single-stranded DNA (or RNA), the two strands are not present, so A need not equal T and G need not equal C. NEET often traps students by mentioning ssDNA.

Is A + G = T + C always true in DNA?

Yes, for double-stranded DNA. Since A = T and G = C, purines (A + G) equal pyrimidines (T + C). So total purines = total pyrimidines = 50% of the bases. This is a favourite NEET shortcut.

Why does A pair with T and not with G?

A pairs with T because their shapes and hydrogen-bond positions match: A-T forms exactly 2 hydrogen bonds. A purine (A) must pair with a pyrimidine (T) to keep the helix width constant (about 2 nm). A-G would be purine-purine, too wide, and the H-bonds would not fit.

If A = T, does that mean DNA is 50% adenine?

No. A = T means the percentage of A equals the percentage of T, not 50%. If A = 30%, then T = 30%, and the remaining 40% is shared by G and C (G = 20%, C = 20%). Students wrongly read A = T as 50% A.

Does Chargaff's rule apply to RNA?

Not in general. RNA is usually single-stranded, so A does not have to equal U and G does not have to equal C. The rule is meant for double-stranded DNA only.

⚠️ The NEET trap
✗ If Adenine is 30%, students split the rest wrongly, e.g. T = 20% or G = 30%.
✓ A = 30% means T = 30% (A=T). The leftover 40% splits equally: G = 20% and C = 20% (G=C).
🧠 First set T equal to A. Then subtract A+T from 100 and split the leftover equally between G and C.

Real NEET questions

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 for double-stranded DNA, A = T and G = C. Given A = 30%, so T = 30%. The remaining 40% (100 - 30 - 30) is split equally between G and C, giving G = 20% and C = 20%. Correct option: A.
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 (thymine occurs only in DNA). In base pairing, purine A pairs with pyrimidine T, and purine G pairs with pyrimidine C. Correct option: B.

Solved Molecular Basis of Inheritance NEET PYQs

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Frequently asked

What is Chargaff's rule in simple words?

In double-stranded DNA, the amount of Adenine equals Thymine (A = T) and Guanine equals Cytosine (G = C). Erwin Chargaff found the A/T and G/C ratios are constant and equal to one.

How many hydrogen bonds are in A-T and G-C pairs?

A-T pair has 2 hydrogen bonds. G-C pair has 3 hydrogen bonds. This is why DNA with more G-C is more stable and needs more heat to separate the strands.

Why does a purine always pair with a pyrimidine?

A purine (double-ring) pairs with a pyrimidine (single-ring) to keep the distance between the two strands uniform, about 2 nm. Purine-purine would be too wide and pyrimidine-pyrimidine too narrow.

What does complementary base pairing mean?

It means each base on one strand has a fixed partner on the other strand: A with T, G with C. So if you know one strand's sequence, you can write the other strand. This is the basis of DNA replication.

Why is Chargaff's rule important for NEET?

NEET regularly asks base-percentage sums (like the 2021 A=30% question). Knowing A=T, G=C, and A+G=T+C=50% lets you solve them in seconds without confusion.

Related concepts:
Purines vs Pyrimidines: A, G, C, T, UWatson and Crick Double Helix Model of DNADNA Dimensions: 0.34 nm, 3.4 nm Pitch, 10 bp/turnHow to Calculate DNA Length and Base PairsWhat is DNA? Molecular Basis Explained Simply
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Molecular Basis of Inheritance PYQs