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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# DNA structure and chromatin

DNA is a double helix of two antiparallel strands held by base pairs; it wraps around histones to fit inside the nucleus.

Updated 2026-11-01

## Key facts

- Two antiparallel strands (5'→3' against 3'→5') joined by hydrogen bonds: A=T (2) and G≡C (3).
- The physiological form is B-DNA: a right-handed helix, about 10.5 base pairs per turn, with a major and a minor groove.
- Chargaff's rule: in double-stranded DNA, A = T and G = C, so purines = pyrimidines.
- Nucleosome: about 147 bp of DNA wrapped around a histone octamer (H2A, H2B, H3, H4 ×2); H1 seals the entry and exit.
- Euchromatin is loose and active; heterochromatin is compact and silent.

## In detail

- The backbone is sugar–phosphate: each deoxyribose is linked to the next by a 3'→5' phosphodiester bond. The bases point inwards.
- Phosphates make DNA negatively charged; histones are rich in lysine and arginine (positive charge), so they bind tightly.
- Regulatory proteins read the sequence mainly in the major groove, where the edges of the bases are exposed.
- Heat or extreme pH pull the strands apart (denaturation); the more G–C a DNA has, the higher its melting temperature (Tm).
- Levels of packing: double helix → 'beads on a string' of nucleosomes (10 nm fibre) → 30 nm fibre → loops → metaphase chromosome.
- Mitochondrial DNA is circular, about 16.5 kb, inherited from the mother; it encodes 13 respiratory-chain proteins, 22 tRNAs and 2 rRNAs.

**Also called**
Double helix, Chromatin

## Clinical notes

- Anti-double-stranded DNA and anti-histone antibodies are markers of lupus (and of drug-induced lupus, respectively).
- Many anticancer drugs damage DNA or its packing: intercalators such as doxorubicin, alkylating agents such as cisplatin.

## Related

- [Nucleotide structure](https://durgical.com/en/biochemistry/topics/nucleotide-structure/)
- [DNA replication: the replication fork](https://durgical.com/en/biochemistry/topics/dna-replication-the-replication-fork/)
- [Epigenetics: DNA and histone methylation](https://durgical.com/en/biochemistry/topics/epigenetics-dna-and-histone-methylation/)
- [5-Methylcytosine](https://durgical.com/en/biochemistry/molecules/5-methylcytosine/)
- [Central dogma of molecular biology](https://durgical.com/en/biochemistry/topics/central-dogma-of-molecular-biology/)

## Test yourself

Think of the answer, then tap the question to see it.

## Why does a G–C-rich DNA melt at a higher temperature?

Each G–C pair has three hydrogen bonds (A–T only two) and the bases also stack more strongly.

## Which histones make up the nucleosome octamer?

Two copies each of H2A, H2B, H3 and H4; H1 is the linker histone outside the octamer.

## Why do histones bind DNA so tightly?

They are rich in lysine and arginine, which are positively charged and attract the negative phosphates of the DNA backbone.

## If a double-stranded DNA is 30% adenine, how much cytosine does it have?

20%: A = T = 30% add up to 60%; the remaining 40% is split equally between G and C.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [What is DNA?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/basics/dna/) (MedlinePlus, US National Library of Medicine, 2026); [What is a chromosome?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/basics/chromosome/) (MedlinePlus, US National Library of Medicine, 2026); [Chromatin organization](https://reactome.org/PathwayBrowser/#/R-HSA-4839726) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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