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.

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.

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.

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.

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

Sources: What is DNA?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); What is a chromosome?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Chromatin organization (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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Representative model, for educational purposes only.