Q.If a double stranded DNA has 20 per cent of cytosine, calculate the per cent of adenine in the DNA.

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.

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.

To remember it easily: think of the two strands as a zipper. Every tooth on one side has a matching tooth on the other side. A always clicks with T, and G always clicks with C. That is why the counts match perfectly. This simple rule is the foundation of DNA replication, transcription, and all of molecular biology — and it requires no maths, only a clear understanding of pairing.

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