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Most students can do the one-line Chargaff sum: 20% adenine, so 20% thymine and 30% each of G and C. The harder questions give you the bases on one strand, ask about the other strand, or ask for hydrogen bonds and length. This post works through those types with numbers you can check, including real genome data.
Is this in the NEET syllabus?
Yes. The NEET (UG) 2026 syllabus lists "Structure of DNA and RNA" under Molecular basis of Inheritance, and the CBSE Class XII Biology curriculum for 2026-27 has the same topic. The source is Chapter 5 of the NCERT Class 12 Biology book (Reprint 2026-27). It credits Chargaff's observation that, for double-stranded DNA, the ratios of A to T and G to C are constant and equal one. It also states that A pairs with T through two hydrogen bonds, G pairs with C through three, and base pairs are 0.34 nm apart.
What does Chargaff's rule actually say about one strand?
Nothing directly. The rule comes from pairing: every A on one strand faces a T on the other. So:
A on strand 1 = T on strand 2, and T on strand 1 = A on strand 2.
G on strand 1 = C on strand 2, and C on strand 1 = G on strand 2.
Inside a single strand, A and T can differ widely. Only when you add both strands do A and T become equal.
Worked example 1: from one strand to the duplex
One strand of a DNA molecule has A = 25%, T = 30%, G = 28% and C = 17% (sample numbers). Find the composition of the other strand and of the whole molecule.
Other strand: swap partners. A = 30%, T = 25%, G = 17%, C = 28%.
Whole molecule: average the two strands, since they have the same number of bases. A = (25 + 30)/2 = 27.5%, T = 27.5%, G = (28 + 17)/2 = 22.5%, C = 22.5%.
Check: 27.5 + 27.5 + 22.5 + 22.5 = 100%. A = T and G = C, as Chargaff's rule demands for the duplex.
Worked example 2: which ratios flip and which stay the same?
Use the same strand.
(A + G)/(T + C) on strand 1 = 53/47 = 1.13.
On strand 2 it is 47/53 = 0.89, the reciprocal.
In the duplex it is exactly 1, because purines always face pyrimidines.
(A + T)/(G + C) on strand 1 = 55/45 = 1.22. On strand 2 it is also 55/45 = 1.22, and the duplex has the same value.
Worked example 3: counting bases, bonds and length
A double-stranded DNA of 5,000 base pairs contains 1,200 cytosines. Find the adenines, the hydrogen bonds and the length.
Nucleotides = 2 × 5,000 = 10,000.
G = C = 1,200, so A + T = 10,000 − 2,400 = 7,600, and A = T = 3,800.
Hydrogen bonds = 2 × 3,800 + 3 × 1,200 = 7,600 + 3,600 = 11,200.
Length = 5,000 × 0.34 nm = 1,700 nm = 1.7 µm.
The common slip is to take 5,000 as the number of nucleotides. Base pairs count each rung once; nucleotides count both ends of each rung.
What does a real genome look like?
NCERT mentions bacteriophage lambda, with 48,502 base pairs. Its reference sequence, NCBI record NC_001416.1, lists one strand. Counting that strand gives A = 12,334, T = 11,986, G = 12,820 and C = 11,362. So on one strand, A and T are not equal.
Treat the molecule as fully double-stranded. Total A = 12,334 + 11,986 = 24,320, and total T is the same, because the other strand's T count equals this strand's A count. Likewise G = C = 12,820 + 11,362 = 24,182. The GC content is 24,182/48,502 = 49.9%, and the molecule holds 2 × 24,320 + 3 × 24,182 = 121,186 hydrogen bonds.
When does Chargaff's rule fail?
When the nucleic acid is single-stranded. NCERT notes that bacteriophage φX174 has 5,386 nucleotides. Its genome is a single-stranded DNA, as Robert Sinsheimer reported in 1959, and counting its reference sequence, NCBI record NC_001422.1, gives A = 24.0%, T = 31.3%, G = 23.3% and C = 21.5%. Neither A = T nor G = C holds. The same goes for RNA, which has uracil in place of thymine and is usually single-stranded.
Which mistakes cost marks?
Averaging the wrong way. Duplex percentages are the average of the two strands, not the sum.
Assuming A + T = G + C. Lambda happens to be close, but the ratio differs between species. Only A + G = T + C follows from pairing.
Forgetting the reciprocal. If (A + G)/(T + C) is 0.8 on one strand, it is 1.25 on the other, not 0.8.
Mixing base pairs and nucleotides in length and bond counts.
Applying the rule to φX174 or RNA. Read the question for "double-stranded".
Our profile of Erwin Chargaff and his base counts shows how the original measurements were made, and the molecular basis of inheritance guide covers the rest of the chapter.
A practice routine
Write three single strands with made-up percentages. For each, find the other strand, the duplex and both ratios.
Do two counting problems a day for a week: given base pairs and one base, find all bases, hydrogen bonds and length.
Once a week, take a question where you got the ratio wrong and say aloud which strand each number belongs to.
When a sum goes wrong and you cannot see where, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer, so you can find the step where you switched strands.
Try this now: one strand has 15% G and 35% C. Find (G + C)% in the duplex, then check that it matches the strand's own G + C.
