6.1.4—DNA double helix structure

DNA is a double helix made from two polynucleotide strands. Each strand has a deoxyribose–phosphate backbone on the outside, while the nitrogenous bases face inward and pair by complementarity.

• Backbone and direction: Covalent phosphodiester bonds link deoxyribose sugars and phosphate groups along each strand. The two strands run antiparallel: one is 5′→3′ while the other is 3′→5′.
• A–T comparison: Adenine (A), a purine, pairs with thymine (T), a pyrimidine, using two hydrogen bonds.
• C–G comparison: Cytosine (C), a pyrimidine, pairs with guanine (G), a purine, using three hydrogen bonds.
• Shared consequence: Each pair combines one purine with one pyrimidine, helping maintain a regular distance between the backbones. Hydrogen bonds hold the strands together but can be separated during replication; phosphodiester bonds keep each individual backbone intact.

The outward-facing sugar–phosphate backbones provide continuous covalent support, while inward-facing complementary bases hold the two strands together through hydrogen bonds. Antiparallel direction and specific pairing mean that the sequence on one strand determines the complementary sequence on the other, giving DNA both stability and a usable template for copying.

Hydrogen bonds join complementary bases across the two strands; phosphodiester bonds join adjacent nucleotides within one backbone. The strands are antiparallel, not parallel. The A–T and C–G comparison is a bond-and-pairing aid, not a complete replication mechanism; replication details belong to card 4574. Staff-only visual brief retained in the Topic Blueprint: show two antiparallel backbones, inward bases, bond types and 2-vs-3 hydrogen-bond contrast; no image is generated here.