Short answerModerate2 marks
Mention the two pairs of nitrogenous bases which pair with each other with 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-ringed: ADENINE and GUANINE Pyrimidines, single-ringed: THYMINE and CYTOSINE This keeps the two strands an even distance apart all along the helix — two double rings would bulge, two single rings would pinch.
A memory aid: the two CURVED letters C and G pair together; the two ANGULAR letters A and T pair together.
It is this complementary pairing that allows each strand to serve as a template for rebuilding the other, which is how DNA replicates.
More from Structure of Chromosomes, Cell Cycle and Cell Division
A sequence of DNA has 300 nitrogenous base pairs, of which 75 are Guanine. What is the number of Thymine in this sequence?2026Which is the resting but metabolically active stage of the cell cycle?2026Mention the number of Autosomes and Allosomes in a human body cell.2026Draw a neat, labelled diagram of an animal cell showing the Prophase stage of Mitosis with four chromosomes.2026Differentiate between Mitosis in a plant cell and an animal cell based on Cytokinesis.2026A phase of cell division where the spindle fibres disappear is:2025