A nucleosome is a unit made of DNA and histones. It is found in the cell nucleus in eukaryotes. All chromatin (except for that packaged in the sperm nucleus) consists of nucleosomes. Nucleosomes can be seen with an electron microscope as bead-like structures along the DNA.

Structure

The histones make up the central core particle of the nucleosome. The DNA is wrapped around this protein complex in two turns, with each turn consisting of about 80 base pairs. Together with the DNA linking two nucleosomes, there are about 200 bps of DNA per nucleosome.

The main purpose of nucleosomes seems to be organization and packing of DNA. Stacked nucleosomes are called 10-nm-fiber and have a packing rate of ~6, compared to "free" DNA (per nm length). A chain of nucleosomes can be arranged in a 30-nm-fiber, a helical structure with a packing rate of ~40 called a solenoid. This structure needs the H1 histone. The solenoid itself is arranged into loops along a central protein scaffold forming transcriptionally active euchromatin. When the scaffold is folded into a helix once more and further compressed into an undefined structure, it becomes transcriptionally inactive as heterochromatin.

Formation

Formation of a nucleosome occurs when histones bind to TATA boxes.

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