Purine-to-pyrimidine pairing keeps the DNA helix a consistent width while hydrogen bonds stabilize complementary base pairs in the molecule in IB Biology exam contexts.
In DNA, each base pair combines one two-ring purine with one one-ring pyrimidine, so A-T and C-G pairs have equal length and the helix keeps a consistent width.
Adenine and guanine are purines; cytosine and thymine are pyrimidines. Complementary hydrogen bonding selects A-T and C-G and helps stabilize the two strands.
Classify first, then pair: A or G (purine) must face T or C (pyrimidine); the complementary pairs are A-T and C-G.
Replacing an A-T pair with a C-G pair changes the sequence but not the helix width, because both pairs contain one purine and one pyrimidine.
A purine-purine pair would be too wide and a pyrimidine-pyrimidine pair too narrow; equal pair length does not mean any purine can form the correct hydrogen bonds with any pyrimidine.
HL adds directionality, helix geometry, DNA packaging, and classic evidence. These ideas stay separate: 5′/3′ explains strand direction; purine-pyrimidine pairing explains width; nucleosomes explain packaging; Hershey-Chase and Chargaff explain evidence for DNA and base pairing.