A nucleosome is the fundamental repeating subunit of eukayotic DNA, and is made up of DNA and histone proteins. 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.
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.
Nucleosomes are involved in processes ranging from compaction of DNA to transcriptional control, and appear to be major carriers of epigenetically inherited information as well. Repeating nucleosomes with intervening "linker" DNA form the 10-nm-fiber, known descriptively as "beads on a string", and have a packing ratio of ~6, compared to "free" DNA (per nm length). A chain of nucleosomes can be arranged in a 30-nm-fiber, a compacted structure (once thought to be a helical solenoid, but now considered likely to exist as a zig-zag structure in vivo) with a packing ratio of ~40. The H1 histone stabilizes this structure. Beyond 30 nm fiber the structure of chromatin is poorly understood, but it is classically suggested that the 30 nm fiber is arranged into loops along a central protein scaffold to form transcriptionally active euchromatin. Further compaction leads to transcriptionally inactive heterochromatin.