According to Erwin Chargaff, for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and

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Q. According to Erwin Chargaff, for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equal one. Question: Based on Erwin Chargaff's rule.

(i) If a double stranded DNA has 25% of Adenine, calculate the percentage of Cytosine in the DNA.

(ii) The sequence of Nucleotides on one strand of DNA is written as follows:
5'-TATTGAACG-3'
Write down the sequence of Nucleotides in the complementary strand.

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Concept understanding — Chargaffs Rule

Imagine you are sorting a huge box of mixed beads, but you are only allowed to use two colours: red and blue. You notice that every time you pick a red bead, you also find a blue bead that seems to match it in some way. After counting the entire box, you realise the number of red beads equals the number of blue beads. That is the core intuition behind Chargaff’s Rule — but instead of beads, we are talking about the four chemical letters (called bases) that make up DNA.

DNA is a long, double-stranded molecule. Each strand is a chain of four kinds of bases: A (adenine), T (thymine), G (guanine), and C (cytosine). In the early 1950s, the biochemist Erwin Chargaff made a simple but powerful observation by analysing DNA from many different species. He found that, no matter which organism he looked at, the amount of A always equalled the amount of T, and the amount of G always equalled the amount of C. This is the essence of Chargaff’s Rule.

Note

Chargaff’s Rule is not a formula — it is a pattern of equality between specific base pairs. It says: A = T and G = C in any double-stranded DNA molecule. There is no calculation involved; it is a statement of fact about the structure of DNA.

Why does this happen? Because of how the two strands of DNA fit together. In the double helix, A on one strand always pairs with T on the opposite strand, and G always pairs with C. So if you count all the A’s on one strand, you are indirectly counting all the T’s on the other strand — they must be equal. The same logic applies to G and C. This pairing is not random; it is dictated by the shape and chemical properties of the bases.

Chargaff’s Rule was a crucial clue for Watson and Crick when they built the model of DNA. It told them that the two strands are complementary — each base on one strand has a fixed partner on the other. This complementarity is what allows DNA to copy itself accurately during cell division.

Here is what the NCERT textbook (Class 12 Biology, Chapter 6) states clearly:

The rule is based on the observation that in double-stranded DNA, the ratios between bases are constant: the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine.
This is often written as A = T and G = C, but again, no numbers are involved — it is a relationship.
The rule does not mean that A + T equals G + C. In fact, the proportion of A–T pairs versus G–C pairs varies from species to species. Chargaff’s Rule only says that within a species, A and T are equal to each other, and G and C are equal to each other. …

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By Chargaff's rule (%A = %T and %G = %C in double-stranded DNA), 25% Adenine implies 25% Cytosine; and the complementary strand is built by antiparallel, complementary base pairing (A↔T, G↔C).

Percentage of Cytosine: Chargaff's rule states that in double-stranded DNA, the amount of Adenine equals the amount of Thymine (%A = %T), and the amount of Guanine equals the amount of Cytosine (%G = %C). It also follows that %(A+T) + %(G+C) = 100%. Given %A = 25%, then by Chargaff's rule %T = 25% too (since A = T). So %(A+T) = 25% + 25% = 50%. Therefore %(G+C) = 100% − 50% = 50%. Since %G = %C, each must be 50% ÷ 2 = 25%. So the percentage of Cytosine = 25%.
Complementary strand: Given strand: 5'-T A T T G A A C G-3' …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set A1 markMCQ
$Q. Who stated that in double stranded DNA sum of purine nitrogenous bases is equal to the sum of pyrimidine nitrogenous bases?$
(a) Miescher
(b) Watson and Crick
(c) Wilkins
(d) Chargaff
›Reveal solutionSolution

Chargaff's equivalence rule: A = T and G = C, so total purines (A+G) = total pyrimidines (T+C).

Erwin Chargaff analysed base compositions of DNA and found that in double-stranded DNA the amount of adenine equals thymine and guanine equals cytosine. Consequently the sum of the purine bases (adenine + guanine) equals the sum of the pyrimid …

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CBSE 2025Set ANNUAL1 mark
$Q. How many Thymine bases will a double stranded DNA have, if it has 30 Adenine bases ?$
›Reveal solutionSolution

By Chargaff's base-pairing rule, Adenine always pairs with Thymine in double-stranded DNA, so the number of Thymine bases equals the number of Adenine bases — 30.

DNA is a double-stranded, antiparallel helical molecule in which the two strands are held together by hydrogen bonds between specifically paired nitrogenous bases: Adenine (A) always pairs with Thymine (T) (via two hydrogen bonds), and Guanine (G) always pairs with Cytosine (C) (via three hydrogen bonds). This is known as the base-pairing rule / Chargaff's rule, and it means that in any double-stranded DNA molecule, the total number of A bases equals the total number of …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. What will be the amount of Cytosine in a DNA, if the total amount of Thiamine and Adenine in the DNA is 55 % ?$
(i) 27.5 %
(ii) 22.5 %
(iii) 45 %
(iv) 55 %
›Reveal solutionSolution

By Chargaff's rule, %A = %T and %G = %C; since A + T = 55%, each is 27.5%, leaving G + C = 45%, so Cytosine = 22.5%.

In double-stranded DNA, base pairing is complementary: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This is Chargaff's rule, which means:

$\%A = \%T \quad \text{and} \quad \%G = \%C$

Given: $\%T + \%A = 55\%$ .

Since $\%A = \%T$ , each individually is:

$\%A = \%T = \dfrac{55}{2} = 27.5\%$ …

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CBSE 2024Set 57/3/11 markMCQ
$Q. A DNA fragment has 2000 nucleotides, out of which 140 are Adenine. How many bases does this DNA segment possess that have triple hydrogen bonds between them ? (A) 280 (B) 860 (C) 1720 (D) 1860$
›Reveal solutionSolution
This DNA segment contains 1720 bases that form triple hydrogen bonds, specifically Guanine and Cytosine.

To understand how we arrive at the number of bases with triple hydrogen bonds, we first need to recall the fundamental structure of DNA and the rules governing its base composition.

DNA, as you know, is a double-helical molecule. Its backbone is made of alternating sugar and phosphate groups, and extending inwards from the sugar are nitrogenous bases. There are four types of nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). In the double helix, these bases pair up in a very specific manner.

This specific pairing was elucidated by Erwin Chargaff, leading to what is known as Chargaff's Rule. His experiments showed that in any double-stranded DNA molecule:

The amount of Adenine (A) is always equal to the amount of Thymine (T).

The amount of Guanine (G) is always equal to the amount of Cytosine (C).

$Chargaff's Rule: A = T and G = C$
This rule is crucial because it tells us that the total number of purines (A and G) equals the total number of pyrimidines (T and C), and also that the ratio of (A+G) to (T+C) is 1.

Now, let's consider how these bases pair up and what holds them together. The two strands of the DNA double helix are held together by hydrogen bonds formed between the complementary base pairs.

Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.

Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.

Important

Remember the number of hydrogen bonds:

A-T pair: 2 hydrogen bonds

G-C pair: 3 hydrogen bonds

The question asks for the number of bases that possess triple hydrogen bonds between them. Based on our understanding, these are the Guanine (G) and Cytosine (C) bases. Therefore, we need to calculate the total number of G and C bases in the given DNA fragment.

Let's apply this knowledge to the problem:

We are given a DNA fragment with a total of 2000 nucleotides.

We are also told that 140 of these nucleotides are Adenine (A). …
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CBSE 2024Set ANNUAL1 markMCQ
$Q. A double stranded DNA has 15% cytosine. What will be the percentage of thiamine in it ?$
(a) 15%
(b) 25%
(c) 35%
(d) 45%
›Reveal solutionSolution

By Chargaff's rule, %C = %G and %A = %T, and all four bases sum to 100%.

According to Chargaff's rule for double-stranded DNA, the amount of cytosine equals the amount of guanine, and the amount of adenine equals the amount of thymine. Given %C = 15%, therefore %G = 15% as well, so %(G+C) = 30%. The re …

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CBSE 2022Set ANNUAL1 markMCQ
$Q. If a double stranded DNA has 20% cytosine, the percentage of adenine in DNA would be —$
(a) 30%
(b) 20%
(c) 40%
(d) 60%
›Reveal solutionSolution

By Chargaff's rule (A = T, G = C) in double-stranded DNA, 20% cytosine gives 30% adenine.

In double-stranded DNA, base pairing is complementary: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This means:

$\%A = \%T \quad \text{and} \quad \%G = \%C$

…

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CBSE 2021Set D1 markMCQ
$Q. In a DNA molecule, the cytosine is 18%, then the percentage of adenine will be$
(a) 64
(b) 36
(c) 85
(d) 32
›Reveal solutionSolution

Using Chargaff's base-pairing rule, C = G = 18%, so A + T = 100 - 36 = 64% and A = T = 32%.

Chargaff's rule for double-stranded DNA states that adenine pairs with thymine (A = T) and guanine pairs with cytosine (G = C).

Given cytosine = 18%, therefore guanine = 18%. …

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CBSE 2019Set ANNUAL1 markMCQ
$Q. A double stranded DNA has 20% cytosine. What will be the percentage of thiamine in it?$
(a) 20%
(b) 40%
(c) 30%
(d) 10%
›Reveal solutionSolution

By Chargaff's rule, %G = %C and %A = %T; so %C = 20% gives %G = 20%, leaving %A = %T = 30% each.

In a double-stranded DNA molecule, base pairing is complementary: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This means the percentage of A equals the percentage of T, and the percentage of G equals the percentage of C (Chargaff's rule).

Given %Cytosine = 20%, therefore %Guanine = 20% as well (since G = C). …

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CBSE 2019Set ANNUAL1 markMCQ
$Q. A DNA molecule has 200 Adenine and 400 Guanine, therefore the total number of nitrogenous bases in this DNA is ______$
(a) 600
(b) 800
(c) 1200
(d) 1600
›Reveal solutionSolution

By Chargaff's rule, in double-stranded DNA, Adenine = Thymine and Guanine = Cytosine, so total bases = 2×(A) + 2×(G).

Given: Adenine (A) = 200, Guanine (G) = 400.

Since DNA is double-stranded and base pairing is A=T and G≡C:

Thymine (T) = A = 200

Cytosine (C) = G = 400

…

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CBSE 2017Set ANNUAL1 markMCQ
$Q. The constant ratio for DNA is$
(a) A+G/T+C
(b) A+T/G+C
(c) A+C/U+G
(d) A+U/C+G
›Reveal solutionSolution

Chargaff's equivalence rule shows that purines and pyrimidines occur in a fixed 1:1 ratio in double-stranded DNA because of complementary base pairing.

According to Chargaff's rule, in any double-stranded DNA molecule the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C), because A pairs specifically with T (via 2 hydrogen bonds) and G pairs specifically with C (via 3 hydrogen bonds) in the double helix. Consequently, the ratio of total purines (A+G) to total pyrimidines (T+C) is constant, and always equal to 1, regardless of the species or …

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CBSE 2017Set ANNUAL1 markMCQ
$Q. According to Chargaff's rule, which one is correct?$
(a) A + T = G + C
(b) A + C = G + T
(c) A + G = T + C
(d) none of these
›Reveal solutionSolution

Chargaff's rule: purines (A+G) always equal pyrimidines (T+C) in double-stranded DNA, because of complementary base pairing.

Erwin Chargaff analysed the base composition of DNA from many species and found two consistent rules: (1) the amount of adenine (A) always equals the amount of thymine (T), and the amount of guanine (G) always equals the amount of cytosine (C) — i.e., A = T and G = C; and (2) consequently, the total amount of purines (A + G) always equals the total amount of pyrimidines (T + C). This became known as Chargaff's rule and was one of the crucial clues …

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Q.According to Erwin Chargaff, for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equal one. Question: Based on Erwin Chargaff's rule.

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