# Nucleosome
A nucleosome is the basic repeating subunit of chromatin, consisting of double-stranded DNA wrapped around a core of eight histone proteins in the nucleus of eukaryotic cells[1](https://www.genome.gov/genetics-glossary/Nucleosome) • [2](https://www.ncbi.nlm.nih.gov/books/NBK534207/). Nucleosomes package genomic DNA into the tight confines of the cell nucleus while leaving it accessible for transcription, replication, and repair, and they serve as a platform for epigenetic regulation[3](https://arxiv.org/abs/0912.3954) • [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/).
| Key fact | Detail |
|---|---|
| Composition | Two molecules each of histones H2A, H2B, H3, and H4 form the histone octamer, with 146 base pairs of DNA wrapped around it[5](https://www.ncbi.nlm.nih.gov/books/NBK26834/) |
| DNA geometry | DNA winds 1.65 turns in a left-handed coil around a disc-shaped histone core[5](https://www.ncbi.nlm.nih.gov/books/NBK26834/) |
| Repeat length | Each nucleosome carries about 200 bp of DNA; 145 to 147 bp contact the core particle and roughly 50 bp of linker DNA connect neighboring particles[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/) |
| Linker histone | H1 binds near the DNA entry-exit point, forming a chromatosome that is more stable and allows tighter chromatin packing[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/) |
| Packing effect | Nucleosome formation converts a DNA molecule into a chromatin thread about one-third of its initial length, the first level of DNA packing[5](https://www.ncbi.nlm.nih.gov/books/NBK26834/) |
| Structural record | The first atomic structure was solved by X-ray diffraction in 1997; as of 20 September 2022 the Protein Data Bank held 441 nucleosome-containing structures[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/) |
## Structure
The nucleosome core particle consists of a histone octamer, built from two H3-H4 dimers and two H2A-H2B dimers, around which double-stranded DNA is wound[2](https://www.ncbi.nlm.nih.gov/books/NBK534207/) • [5](https://www.ncbi.nlm.nih.gov/books/NBK26834/). The high-resolution structure, solved in 1997, revealed a disc-shaped histone core with the DNA tightly wrapped 1.65 turns in a left-handed coil[5](https://www.ncbi.nlm.nih.gov/books/NBK26834/). That landmark 2.8 Å structure was made possible in part by using entirely recombinant histones and strong-positioning DNA, reassembled to create a more homogeneous population of core particles than those isolated from cells[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5458496/).
**Electrostatics drive the association.** Histone proteins have high lysine and arginine content, which makes them positively charged, while DNA is largely negatively charged; this favors the interactions that stabilize the nucleosome, along with protein-protein contacts, hydrogen bonding, and electrostatic forces[2](https://www.ncbi.nlm.nih.gov/books/NBK534207/). The octamer presents positively charged surfaces formed by basic amino acid side chains that interact with the negatively charged phosphate groups of the DNA backbone[7](https://www.ncbi.nlm.nih.gov/books/NBK585710/?report=printable).
A fifth histone, H1, is not part of the core octamer. One H1 molecule binds the DNA as it enters each nucleosome core particle, acting as a linker protein that stabilizes the nucleosome[2](https://www.ncbi.nlm.nih.gov/books/NBK534207/) • [8](https://www.ncbi.nlm.nih.gov/books/NBK9863/). The resulting particle, the chromatosome, is more stable than the nucleosome and allows chromatin to be packed more tightly[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/).
## DNA packing and higher-order structure
Nucleosomes are the basic unit of chromatin, and their formation provides the first level of DNA packing, shortening the DNA thread to about one-third of its initial length[2](https://www.ncbi.nlm.nih.gov/books/NBK534207/) • [5](https://www.ncbi.nlm.nih.gov/books/NBK26834/). Linker DNA of roughly 50 bp separates neighboring core particles, so the repeating unit spans about 200 bp[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/). Non-histone DNA-binding proteins, a large heterogeneous group, organize and compact the chromosome into higher-order structures beyond the nucleosome[2](https://www.ncbi.nlm.nih.gov/books/NBK534207/). [In vitro](https://www.edgepedia.org/in-vitro) reconstituted nucleosome arrays incubated with linker histone can form 30 nm-like fibers, a model of chromatin compaction[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5458496/).
## Positioning and accessibility
The compact structure of the nucleosome limits DNA accessibility and inhibits the binding of most sequence-specific proteins[9](https://www.sciencedirect.com/science/article/pii/S0006349518303813). Nucleosome positioning, the placement of nucleosomes along the DNA, is governed by four major determinants: DNA sequence, nonhistone DNA-binding proteins, chromatin-remodeling enzymes, and transcription[9](https://www.sciencedirect.com/science/article/pii/S0006349518303813). ATP-dependent chromatin remodelers modify the composition or location of nucleosomes using energy from ATP hydrolysis[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/).
**Transcription requires remodeling.** Before gene activation, transcription factors need access to their target sequences, which requires remodeling of the protective nucleosomal structure at sites such as enhancers; during elongation, the [RNA polymerase](https://www.edgepedia.org/rna-polymerase) complex encounters nucleosomes that must be removed and then reassembled after passage[10](https://www.ncbi.nlm.nih.gov/books/NBK585713/pdf/Bookshelf_NBK585713.pdf). Structural work shows that tight binding of nucleosomal DNA to the histone tetramer downstream of the dyad blocks [RNA polymerase II](https://www.edgepedia.org/rna-polymerase-ii), so the polymerase's ability to unwrap DNA from the tetramer on the downstream side is a critical aspect of how it passes through a nucleosome[11](https://pubs.acs.org/doi/10.1021/cr500373h).
## Epigenetic regulation
Histone post-translational modifications form one layer of epigenetic regulation; they can alter nucleosome structural dynamics and regulate DNA accessibility[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/). Core particles become decorated with a wide range of such modifications, which directly control DNA accessibility and binding to specific interactors[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5458496/). Among the best-studied marks are methylations of H3K4, H3K9, H3K27, H3K36, H3K79, and H4K20; H3K4 methylation, deposited predominantly by the COMPASS complex, indicates active gene promoters[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/). Histone variants, encoded by replication-independent genes, participate in specific nuclear processes including gene expression, [DNA replication](https://www.edgepedia.org/dna-replication), and DNA damage repair, and even a few amino acid substitutions in a variant can alter nucleosome dynamics and stability[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/).
## Structural biology of the nucleosome
The first atomic nucleosome core particle structure was obtained in 1997 by [X-ray diffraction](https://www.edgepedia.org/x-ray-diffraction) and deposited in the [Protein Data Bank](https://www.edgepedia.org/protein-data-bank) in 1998[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/). Until 2015, all 113 nucleosome structures in the PDB came from [X-ray crystallography](https://www.edgepedia.org/x-ray-crystallography), with a yearly median resolution better than 3 Å[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/). In 2016 the first cryo-electron microscopy structure of a nucleosome was published at 4.5 Å resolution, and cryoEM has since produced 253 nucleosome structures against 188 by X-ray diffraction[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/). More than 400 nucleosome-containing structures now exist in the PDB, including complexes with chromatin remodelers, transcription factors, RNA polymerases, and enzymes that write histone modifications[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/).
Most solved structures use a limited set of DNA sequences, chiefly the artificial high-affinity Widom 601 positioning sequence (246 structures) and human α-satellite centromeric DNA (166 structures), and systematic study of how DNA sequence influences nucleosome structure and positioning remains an open need[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/).
## References
1. [Nucleosome, National Human Genome Research Institute Genetics Glossary](https://www.genome.gov/genetics-glossary/Nucleosome)
2. [Genetics, DNA Packaging, StatPearls, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK534207/)
3. [Nucleosome positioning and energetics: recent advances in genomic and computational studies, arXiv](https://arxiv.org/abs/0912.3954)
4. [Nucleosomes and their complexes in the cryoEM era: trends and limitations, Frontiers in Molecular Biosciences (2022)](https://pmc.ncbi.nlm.nih.gov/articles/PMC9730872/)
5. [Chromosomal DNA and Its Packaging in the Chromatin Fiber, Molecular Biology of the Cell, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK26834/)
6. [Cryo-electron microscopy of chromatin biology, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5458496/)
7. [Biology of Chromatin: Introduction to Epigenetics, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK585710/?report=printable)
8. [Chromosomes and Chromatin, The Cell, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK9863/)
9. [Major Determinants of Nucleosome Positioning, Biophysical Journal](https://www.sciencedirect.com/science/article/pii/S0006349518303813)
10. [Chromatin Dynamics, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK585713/pdf/Bookshelf_NBK585713.pdf)
11. [Nucleosome Structure and Function, Chemical Reviews](https://pubs.acs.org/doi/10.1021/cr500373h)
---
*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Protein families and complexes › Biomolecular complexes and assemblies › Transcription and chromatin complexes*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license