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One full turn of the DNA helix spans 10 base pairs and rises 3.4 nm, so each base pair adds 0.34 nm. A human chromosome with 100 million base pairs would stretch about 3.4 cm if unwound — yet it packs into a nucleus only micrometres wide, wound around histone proteins. A always pairs with T (2 hydrogen bonds) and G with C (3 hydrogen bonds) — which is why the two strands are complementary.