DNA forms a double helix because two antiparallel nucleotide strands are held together by complementary base pairing, while the geometry and interactions of the stacked bases cause the strands to twist into a stable helical structure. This explanation applies to IB Biology SL and HL, with additional structural detail required at HL.
Each strand has a strong sugartophosphate backbone formed by covalent bonds. The nitrogenous bases project inward, allowing hydrogen bonds to form between complementary bases: adenine pairs with thymine, while guanine pairs with cytosine.
| Structural feature | Contribution to the double helix |
|---|---|
| Antiparallel strands | One strand runs 5′ to 3′ and the other runs in the opposite direction, positioning complementary bases correctly. Directionality is AHL content. |
| Hydrogen bonding | AtoT and GtoC hydrogen bonds hold the two strands together and provide pairing specificity. |
| Base stacking | Adjacent, relatively hydrophobic bases stack inside the molecule. Their interactions increase stability and favour twisting. |
| Sugartophosphate backbones | The charged, hydrophilic backbones remain exposed to the aqueous environment on the outside. |
| Purinetopyrimidine pairing | Each pair contains one larger purine and one smaller pyrimidine, maintaining a uniform helix width. This is AHL detail. |
The helix is therefore not produced by one bond alone. Its shape results from the combined effects of backbone geometry, complementary hydrogen bonding, base stacking and interactions with water.
A common misconception is that hydrogen bonds form within each DNA strand. They do not: covalent bonds form each strand’s backbone, whereas hydrogen bonds connect complementary bases across the two strands.
Exam technique: For an SL response, state that DNA contains two antiparallel strands linked by hydrogen bonds between complementary base pairs. At HL, add directionality and explain how purinetopyrimidine pairing maintains a consistent three-dimensional structure. In a diagram, show AtoT and GtoC pairing and label the strands as antiparallel.