The double helix structure
DNA is a polymer of nucleotides. Each nucleotide has three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogen-containing bases. The sugar and phosphate form the two outer "rails" of the helix; the bases form the "rungs", sticking inward and pairing across the two strands.
Base pairing follows strict rules (Chargaff's rules): adenine pairs with thymine (A–T, two hydrogen bonds); guanine pairs with cytosine (G–C, three hydrogen bonds). This complementarity means the two strands carry the same information — which makes faithful copying possible.
The two strands run antiparallel: one 5′→3′, the other 3′→5′. The helix makes a complete turn every 10 base pairs, with a diameter of about 2 nm.
DNA is stored in the nucleus, wound tightly around histone proteins into nucleosomes and further compacted into chromosomes. See our cells and organelles guide for how the nucleus is structured. The hydrogen bonds holding the bases together are explained in our covalent bond glossary entry.
Nucleotide components
Semi-conservative replication
Before a cell divides, its DNA must be copied so each daughter cell receives a complete genome. Replication is semi-conservative: the double helix unwinds, and each strand acts as a template. The result is two DNA molecules, each containing one original and one new strand.
- Helicase unwinds the helix at the replication fork.
- Primase lays a short RNA primer to start synthesis.
- DNA polymerase III adds complementary nucleotides 5′→3′.
- DNA ligase joins Okazaki fragments on the lagging strand.
Replication occurs during S phase of the cell cycle — before mitosis. See our mitosis glossary entry for how the cell cycle is controlled.
DNA and modern biotechnology
Understanding DNA structure enabled a revolution in biotechnology. PCR amplifies specific sequences for diagnosis, forensics and research. DNA sequencing (now costing under £100 for a human genome) underpins personalised medicine.
CRISPR-Cas9 targets specific DNA sequences for precise editing — see our CRISPR gene editing article for how the Cas9 protein uses a guide RNA to find and cut specific sequences. mRNA vaccines exploit the transcription step of the central dogma to instruct cells to produce target antigens — see our mRNA vaccines article.