Chargaff's Rule in IB Biology: Base Pairing Practice
Revise Chargaff's rule, base pairing and DNA percentage calculations with worked examples and targeted IB Biology practice.
Chargaff's rule is the shortcut that helps you turn DNA base pairing into exam-ready calculations. In double-stranded DNA, adenine pairs with thymine and guanine pairs with cytosine, so %A = %T and %G = %C.
The student trap is treating this as a slogan instead of a calculation rule. In IB Biology, Chargaff's rule usually appears inside DNA structure, complementary base pairing, replication or data-style questions where you need to use the percentages carefully.
Quick Answer
| Rule | Meaning | Exam use |
|---|---|---|
| A pairs with T | Adenine and thymine are complementary bases | If A is 24%, T is 24% |
| G pairs with C | Guanine and cytosine are complementary bases | The remaining percentage is split equally |
| Total bases = 100% | A + T + G + C must add up to 100% | Use this to check your answer |
What Chargaff's Rule Means
Chargaff's rule applies to double-stranded DNA because each base on one strand pairs with a complementary base on the other strand. That is why the amount of adenine matches thymine, and the amount of guanine matches cytosine.
The rule is about complementarity, not equal amounts of every base. A sample can be AT-rich or GC-rich, but A still matches T and G still matches C.
Exam-Style Worked Example
Question: In a double-stranded DNA sample, 18% of the bases are guanine. Calculate the percentages of cytosine, adenine and thymine.
Worked answer:
G pairs with C, so cytosine is also 18%. Together, G and C make 36%. That leaves 64% for A and T. Because A pairs with T, adenine is 32% and thymine is 32%.
How marks are awarded:
| Mark | What the examiner is looking for |
|---|---|
| 1 | Correct use of G = C |
| 1 | Correct subtraction from 100% |
| 1 | Correct split between A and T |
This is why you should not start by dividing 100 by 4. The given base percentage changes the whole calculation.
How Base Percentage Questions Work
If a question says A = 24%, start with the pair. T must also be 24%. Together, A and T make 48%, so G and C must share the remaining 52%. That gives G = 26% and C = 26%.
Use this three-step method:
- Pair the given base first.
- Subtract the paired total from 100%.
- Split the remainder equally between the other pair.
Non-Example: Why 25% Each Is Usually Wrong
The answer A = T = G = C = 25% is only true in a special case where the DNA sample happens to contain equal amounts of all four bases. Chargaff's rule does not guarantee that.
Better exam wording:
In double-stranded DNA, A equals T and G equals C because of complementary base pairing. The total percentage of all four bases must equal 100%.
That sentence is safer than writing that all bases are equal.
Mini Practice Set
Try these before opening the question bank:
| Question | Quick check |
|---|---|
| If A = 31%, what is T? | T = 31% |
| If C = 22%, what are G, A and T? | G = 22%, A = 28%, T = 28% |
| Why is A = T = G = C not always correct? | Chargaff only matches paired bases |
Practice This Topic
Try this exam-style question:
In a double-stranded DNA sample, 24% of the bases are adenine. Calculate the percentages of thymine, guanine and cytosine.
Answer guide:
- A pairs with T, so thymine is 24%.
- A + T = 48%, leaving 52% for G and C.
- G pairs with C, so guanine and cytosine are each 26%.
- Check that all four percentages add to 100%.
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FAQ
What is Chargaff's rule in IB Biology?
Chargaff's rule says that in double-stranded DNA, adenine pairs with thymine and guanine pairs with cytosine. Therefore, the percentage of A equals the percentage of T, and the percentage of G equals the percentage of C.
How do you solve Chargaff's rule percentage questions?
Start by matching the paired base. If A is given, T has the same percentage. If G is given, C has the same percentage. Then subtract that paired total from 100% and split the remainder equally between the other complementary pair.
Does Chargaff's rule mean all four bases are 25%?
No. Chargaff's rule only says paired bases are equal. A equals T, and G equals C. The total amount of A and T can be different from the total amount of G and C.
Does Chargaff's rule apply to single-stranded DNA?
Chargaff's simple A = T and G = C relationship is for double-stranded DNA, where bases are paired across two strands. Single-stranded DNA does not necessarily show the same paired percentages because there is no complementary strand paired along its full length.
Is Chargaff's rule tested in IB Biology?
Yes. It can appear in DNA structure, complementary base pairing, replication and data-style questions. The marks usually come from using the base-pairing rule accurately rather than only naming the rule.
Final Takeaway
Remember the exam-safe line: A = T and G = C, but all four bases do not have to be equal. Pair first, subtract second, split last.
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