A nucleosome is the basic repeating subunit of chromatin, which is the complex of DNA and proteins that make up chromosomes inside the nucleus of a eukaryotic cell. The structure of a nucleosome consists of a segment of DNA wound around eight histone proteins, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4. The DNA wrapped around the histone octamer is about 147 base pairs long. The resulting structure resembles thread wrapped around a spool.

Nucleosomes play a key role in organizing and packing DNA inside the nucleus. They help to keep chromosomes neatly organized and packed away, similar to how a tent is tightly folded up when not in use. When the DNA is needed for something, such as when the cell needs to read the DNA or make a copy of it, the nucleosomes help to unfold it.

Multiple nucleosomes form in a linear fashion along the DNA molecule, ultimately producing the 11 nm fiber, which is traditionally described, based on its appearance, as “beads on a string” . A chain of nucleosomes can be arranged in a 30 nm fiber, a compacted structure with a packing ratio of ~50 and whose formation is dependent on the presence of the H1 histone.

Nucleosomes are highly dynamic and can undergo spontaneous sliding, “splitting” or even complete dissociation. They also play a role in regulating gene expression, as the accessibility of DNA to transcription factors and other regulatory proteins is influenced by the positioning of nucleosomes along the DNA molecule.