AtoT and CtoG base pairs have the same length because each contains one double-ring purine paired with one single-ring pyrimidine. This gives both complementary base pairs approximately the same width across the DNA double helix.
In DNA, adenine and guanine are purines, which have two fused rings. Thymine and cytosine are pyrimidines, which have one ring.
Base | Type | Complementary partner |
|---|---|---|
Adenine (A) | Purine: two rings | Thymine (T), a pyrimidine |
Guanine (G) | Purine: two rings | Cytosine (C), a pyrimidine |
The bases are attached to the sugartophosphate backbones on opposite sides of the DNA molecule. Pairing one larger purine with one smaller pyrimidine keeps the distance between the two backbones nearly constant, regardless of whether the pair is AtoT or CtoG. This uniform geometry contributes to the stable, regular structure of the DNA double helix.
The bases pair through hydrogen bonds: A to T forms two hydrogen bonds, whereas C to G forms three. These different numbers affect base-pair stability; C to G pairing is generally harder to separate, but they do not make C to G longer than A to T. The positions and orientations of the hydrogen-bonding groups allow both pairs to fit across the helix with similar dimensions.
A purine / purine pair would be too wide and would distort the helix. A pyrimidine / pyrimidine pair would be too narrow. Complementary base pairing therefore preserves both the base sequence information and the consistent three-dimensional structure of DNA.
For an IB Biology AHL explanation, state that AtoT and CtoG each contain one purine and one pyrimidine, then link this to a constant distance between the sugartophosphate backbones. Avoid the common misconception that equal length results from equal numbers of hydrogen bonds: AtoT and CtoG have different numbers of hydrogen bonds.
