What is Nucleosome and chromatin? — Biology

Definition 1.1 University Biology — Year 3 · Chapter 1 — Chromatin and Epigenetics

Chromatin is the complex of DNA with proteins in which the genome exists in a eukaryotic nucleus. Its repeating unit is the nucleosome: $147\,\mathrm{bp}$ of DNA wound in $1.65$ left-handed turns around a histone octamer — two copies each of the small, very basic proteins H2A, H2B, H3 and H4 — followed by a linker of $20\text{ to }60\,\mathrm{bp}$ to the next nucleosome. A fifth histone, H1, binds the linker where it enters and leaves the core and helps the beads pack against one another. Each core histone carries an unstructured amino-terminal tail of some $20\text{ to }40$ residues that projects out of the particle. Chromatin that stains lightly, is loosely packed and is transcribed is euchromatin; densely packed, late-replicating and largely silent chromatin, at centromeres, telomeres and repeated sequences, is heterochromatin.

The 10\, nm fibre: nucleosome cores (blue) on a continuous DNA (red), each bead a histone octamer with its tails (orange) projecting out, linker histone H1 (green) at the entry–exit point. About six nucleosomes occupy the length that 1200\, bp of naked DNA would. — The $10\,\mathrm{nm}$ fibre: nucleosome cores (blue) on a continuous DNA (red), each bead a histone octamer with its tails (orange) projecting out, linker histone H1 (green) at the entry–exit point. About six nucleosomes occupy the length that $1200\,\mathrm{bp}$ of naked DNA would.

Chromatin at the molecular scale: the double helix wound around successive histone cores, the “beads on a string” that the nuclease ladder revealed.

Examples

Example 1.3 (Two metres in ten micrometres)

A human diploid nucleus holds $6.4\times 10^{9}$ base pairs. At $0.34\,\mathrm{nm}$ per base pair that is $2.2\,\mathrm{m}$ of double helix, in a nucleus about $10\,\text{µ}\mathrm{m}$ across. It is packed into $6.4\times 10^{9}/200 \approx 3.2\times 10^{7}$ nucleosomes, each carrying about $108\,\mathrm{kDa}$ of histone against $96\,\mathrm{kDa}$ of DNA ( $147\,\mathrm{bp}$ at $650\,\mathrm{Da}$ per pair): the nucleus contains roughly as much histone as DNA by mass. The DNA itself, treated as a cylinder $2\,\mathrm{nm}$ across, has a volume of only about $7\,\text{µ}\mathrm{m}^{3}$ , some $1\,\%$ of a spherical nucleus of radius $5\,\text{µ}\mathrm{m}$ ; the packing problem is one of entanglement and access, not of room.

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