Short answerModerate2 marks
Mention the two pairs of nitrogenous bases that are present in a DNA strand and pair with each other by hydrogen bonds.
Topic: Base pairing in DNA
Previous-year question practice database
Answer
Exam answer
- ADENINE pairs with THYMINE (A–T) by TWO hydrogen bonds.
- GUANINE pairs with CYTOSINE (G–C) by THREE hydrogen bonds.
Explanation
One mark per pair. Adding the NUMBER of hydrogen bonds costs three words and shows precise knowledge.
THE RULE (Chargaff’s rule): a PURINE always pairs with a PYRIMIDINE. Purines (double ring): Adenine, Guanine Pyrimidines (single ring): Thymine, Cytosine This keeps the two strands an even distance apart all along the helix — a purine-purine pair would bulge and a pyrimidine-pyrimidine pair would pinch.
A way to remember which goes with which: the two letters with CURVES, C and G, pair together; the two ANGULAR letters, A and T, pair together.
Because G–C has three hydrogen bonds and A–T only two, DNA rich in G and C is harder to separate — which is why it takes more heat to melt it.
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