# Nucleosome Model: Structure, Function, and DNA Packaging

## What Is the Nucleosome Model?

The nucleosome model describes how eukaryotic DNA is packaged into chromatin, the protein–DNA complex that fills the nucleus. Every human cell contains roughly two meters of DNA, yet the nucleus is only about six micrometers in diameter. The nucleosome is the fundamental repeating unit that solves this packaging problem: it compacts DNA approximately sevenfold into a repeating array of protein–DNA particles.

A nucleosome consists of two components: a **core particle** and a **linker DNA** segment. The core particle is a disc-shaped complex of eight histone proteins—two copies each of H2A, H2B, H3, and H4—around which 147 base pairs (bp) of DNA are wrapped in 1.65 left-handed superhelical turns. Linker DNA is the stretch of 20–80 bp that connects one core particle to the next. When viewed under an electron microscope, this arrangement produces the classic "beads on a string" appearance, with each bead being a [nucleosome core particle](/knowledge/molecular-biology/nucleosome-core-particle) and the string being linker DNA.

The term "nucleosome model" refers not merely to the structure of a single particle but to the entire framework describing how these particles array along the genome, how they fold into higher-order chromatin structures, and how they regulate access to the underlying DNA sequence. This model is central to understanding gene expression, DNA replication, and DNA repair, because the packaging of DNA into nucleosomes profoundly affects every process that requires reading the genetic code.

### Core Particle and Linker DNA

The core particle is the stable, repeating structural unit. Its 147 bp of DNA make approximately 1.65 turns around the histone octamer, with the DNA entering and exiting on the same side of the disc. The length of linker DNA varies between cell types and genomic regions. In most somatic cells, the average nucleosome repeat length—the distance from the start of one core particle to the start of the next—is about 200 bp, meaning roughly 50 bp of linker DNA between cores. In yeast, the repeat length is shorter, around 165 bp, while in sea urchin sperm it can exceed 240 bp. This variability is not random; it influences how tightly chromatin is packed and how accessible the DNA is to regulatory proteins.

### Histone Octamer

The histone octamer is a protein complex with a precise stoichiometry: two molecules each of H2A, H2B, H3, and H4. These are small, highly basic proteins rich in lysine and arginine residues, which give them a strong positive charge that neutralizes the negatively charged phosphate backbone of DNA. The octamer is assembled as a dimer of H3–H4 heterodimers flanked by two H2A–H2B heterodimers. This arrangement is not arbitrary; it dictates how the octamer is assembled and disassembled during DNA replication and transcription, as discussed later.

## Historical Background and Key Discoveries

### Early Chromatin Studies

Before the nucleosome model emerged, chromatin was known to be a complex of DNA and proteins, but its organization was unclear. In the 1960s, biochemists observed that chromatin digested with enzymes yielded DNA fragments of regular sizes, suggesting a repeating structure. However, the precise nature of this repeat was unknown.

The breakthrough came in the early 1970s. In 1973, Ada and Donald Oliins used electron microscopy to visualize chromatin fibers and observed repeating spherical particles along the DNA. They called these particles "nu bodies." Independently, Roger Kornberg, working with Jean-Paul Thomas, used X-ray diffraction and biochemical crosslinking to show that chromatin is composed of repeating units containing approximately 200 bp of DNA and a set of four histone proteins in equal amounts. Kornberg proposed that the fundamental unit consists of an octamer of histones (two each of H2A, H2B, H3, and H4) with DNA wrapped around it. This proposal, published in 1974, became the foundation of the nucleosome model.

### The Beads-on-a-String Image

The "beads on a string" image was cemented by electron microscopy studies in the mid-1970s. When chromatin was gently spread on an electron microscope grid at low ionic strength, it appeared as a series of 10-nanometer (nm) particles connected by thin DNA filaments. Each bead corresponded to a [nucleosome core particle](/knowledge/molecular-biology/nucleosome-core-particle), and the string was linker DNA. This image was transformative because it provided direct visual evidence for a repeating subunit structure. It also explained the earlier nuclease digestion results: enzymes cut preferentially in the linker regions, producing DNA fragments that were multiples of the nucleosome repeat length.

The nucleosome model was further refined in 1997 when the first high-resolution crystal structure of the nucleosome core particle was solved by Karolin Luger and Timothy Richmond. This structure, at 2.8 Å resolution, revealed the atomic details of how DNA wraps around the histone octamer, including the specific contacts between histone residues and the DNA backbone. It confirmed the left-handed superhelical path of DNA and identified the histone fold domains that mediate protein–protein and protein–DNA interactions.

## Structural Details of the Nucleosome

The atomic structure of the nucleosome core particle is a masterpiece of molecular engineering. The 147 bp of DNA are wrapped around the histone octamer in a left-handed superhelix, with the DNA minor groove facing inward toward the histone surface. The overall shape is a flattened disc, approximately 11 nm in diameter and 5.7 nm in height.

### Histone Fold Domains

Each of the four core histones contains a conserved structural motif called the **histone fold domain**. This domain consists of three alpha helices connected by two loops, arranged in a specific topology. The histone fold domains mediate two critical interactions. First, they drive heterodimerization: H3 pairs with H4, and H2A pairs with H2B, forming stable heterodimers through extensive hydrophobic contacts between their fold domains. This "handshake" interaction is so stable that H3–H4 and H2A–H2B dimers exist as stable entities even in the absence of DNA.

Second, the histone fold domains organize the structure of the octamer. The octamer is assembled as a central (H3–H4)₂ tetramer, with two H2A–H2B dimers docking on either side. The H3–H4 tetramer forms the core of the particle, and the H2A–H2B dimers are positioned at the periphery, where they contact the DNA at the entry and exit points. The histone fold domains also present the histone N-terminal tails, which protrude from the nucleosome surface. These tails are flexible and extend outward, where they can be post-translationally modified (acetylated, methylated, phosphorylated) to regulate chromatin structure and gene expression.

### DNA Superhelix and Minor Groove Contacts

The DNA in the nucleosome follows a path that can be described as a superhelix with a radius of about 4.2 nm and a pitch of about 2.6 nm. The DNA is bent sharply, with an average bend angle of about 60 degrees per helical turn, which is far more than free DNA would tolerate. This bending is stabilized by contacts between the DNA backbone and the histone surface.

The key contacts occur at 14 distinct sites where the DNA minor groove faces the histone octamer. At each site, an arginine residue from a histone protein inserts into the minor groove, making contacts with the DNA backbone. These arginine residues are highly conserved across eukaryotes, underscoring their functional importance. The insertion of arginine into the minor groove neutralizes the negative charge of the phosphate backbone and stabilizes the sharp bending of the DNA. The DNA is also contacted along its phosphate backbone by numerous hydrogen bonds and salt bridges with histone residues, particularly lysines and arginines.

The path of DNA around the nucleosome is not uniform. The DNA is more tightly wrapped at the center of the particle (the dyad axis) and more loosely at the entry and exit points. This asymmetry has functional consequences: the central region of the nucleosomal DNA is the most difficult to access, while the ends are more dynamic and can spontaneously unwrap and rewrap on timescales of milliseconds to seconds.

## How Nucleosomes Form: Assembly and Disassembly

Nucleosome assembly and disassembly are highly regulated processes that occur during DNA replication, transcription, and repair. These processes are not spontaneous; they require dedicated protein factors that control the ordered association and dissociation of histone proteins with DNA.

### Histone Chaperones

Histone chaperones are proteins that bind histones and prevent their non-specific aggregation with DNA. They are essential because free histones are highly basic and will bind any negatively charged DNA or RNA with little sequence specificity. Chaperones ensure that histones are delivered to the correct locations and assembled in the correct order.

The assembly of a nucleosome proceeds in a defined order. First, an H3–H4 dimer binds to a chaperone such as ASF1 (anti-silencing function 1). Two H3–H4 dimers are then combined to form a tetramer, which is deposited onto DNA by chaperones such as CAF-1 (chromatin assembly factor 1) or HIRA. The (H3–H4)₂ tetramer binds to DNA and defines the central 60–70 bp of the nucleosome. Next, two H2A–H2B dimers are added, each assisted by chaperones such as NAP1 (nucleosome assembly protein 1) or FACT (facilitates chromatin transcription). The addition of the H2A–H2B dimers completes the octamer and wraps the full 147 bp of DNA.

During DNA replication, nucleosomes must be disassembled ahead of the replication fork and reassembled behind it. The parental histones are transferred to the newly synthesized DNA strands in a process that is not fully understood but involves the chaperone FACT and the replicative helicase. New histones are synthesized in S phase and assembled by CAF-1, which is recruited to the replication fork by the proliferating cell nuclear antigen (PCNA). This ensures that both daughter DNA molecules receive a full complement of nucleosomes.

### Nucleosome Remodeling

Nucleosomes are not static structures; they can be moved, ejected, or restructured by ATP-dependent [chromatin remodeling](/knowledge/molecular-biology/chromatin-remodeling) complexes. These complexes use the energy of ATP hydrolysis to alter histone–DNA contacts. The best-studied family is the SWI/SNF family, named after the yeast genes SWI2/SNF2. These complexes contain an ATPase subunit that translocates along the DNA, creating torsional stress that causes the histone octamer to slide along the DNA or be transferred to a different DNA molecule.

The process of **nucleosome sliding** is particularly important for gene regulation. By moving a nucleosome along the DNA, a remodeling complex can expose or occlude transcription factor binding sites. For example, the yeast SWI/SNF complex can reposition nucleosomes at the PHO5 promoter, exposing binding sites for the Pho4 transcription factor and activating gene expression. The direction and extent of sliding are determined by the ATPase motor and by the sequence of the underlying DNA, which affects the energetics of nucleosome positioning.

Nucleosome disassembly is also catalyzed by remodeling complexes. The FACT complex, for instance, can remove an H2A–H2B dimer from a nucleosome, creating a hexasome (a nucleosome lacking one H2A–H2B dimer) that is less stable and more permissive to transcription. This is a key step during RNA polymerase II elongation, as the polymerase must traverse nucleosomal DNA.

## Functions of the Nucleosome Model

### Chromatin Compaction

The primary function of nucleosomes is to compact DNA. The 147 bp wrapped around each octamer reduces the contour length of DNA by a factor of about 1.65 per nucleosome, and the array of nucleosomes along the DNA produces a 10-nm fiber. This fiber is then folded into higher-order structures: the 30-nm fiber (though its existence in vivo is debated), and ultimately into the densely packed metaphase chromosome, achieving a total compaction of about 10,000-fold.

The degree of compaction is not uniform across the genome. Regions of active transcription, called euchromatin, are less compact and more accessible. Regions of silenced transcription, called heterochromatin, are more compact. The nucleosome model explains this difference: heterochromatin contains nucleosomes with specific histone modifications (such as methylation of H3 at lysine 9) that recruit proteins that promote compaction, while euchromatin contains nucleosomes with modifications (such as acetylation of H3 and H4 lysines) that promote a more open structure.

### Regulation of Gene Expression

Nucleosomes are not merely passive packaging; they are active participants in gene regulation. The position of a nucleosome relative to a promoter or enhancer determines whether [transcription factors](/knowledge/molecular-biology/transcription-factor) can access their binding sites. A nucleosome positioned over a promoter generally represses transcription by blocking the assembly of the pre-initiation complex. Conversely, a nucleosome-free region at a promoter is associated with active transcription.

This regulatory role is exemplified by the PHO5 gene in yeast. In the repressed state, the PHO5 promoter is occupied by four positioned nucleosomes. When phosphate is limiting, the Pho4 transcription factor binds to its sites, recruits the SWI/SNF remodeling complex, and the nucleosomes are evicted or repositioned, exposing the TATA box and activating transcription. This system demonstrates that nucleosome positioning is a dynamic, regulated process that directly controls gene expression.

Nucleosomes also regulate transcription elongation. RNA polymerase II must traverse nucleosomal DNA, and this process is slow and inefficient. The polymerase pauses at nucleosome barriers, and passage requires the action of remodeling complexes and histone chaperones that transiently disassemble the nucleosome ahead of the polymerase and reassemble it behind. The rate of transcription through a nucleosome is influenced by histone modifications; for example, acetylation of histone tails weakens histone–DNA contacts and facilitates polymerase passage.

### Protection of DNA from Damage

The wrapping of DNA around the histone octamer protects the DNA from certain types of damage. The DNA in a nucleosome is less accessible to nucleases, reactive oxygen species, and UV radiation. Studies have shown that UV-induced cyclobutane pyrimidine dimers form preferentially in linker DNA and in the regions of nucleosomal DNA that face outward, away from the histone surface. This protective effect is not absolute—damage still occurs in nucleosomal DNA—but the rate is reduced, and the damage is repaired more slowly in nucleosomal DNA, likely because repair enzymes have limited access.

The nucleosome also plays a role in DNA repair by serving as a platform for repair proteins. For example, the DNA damage checkpoint protein γH2AX, a phosphorylated form of histone H2A variant H2AX, is deposited at sites of double-strand breaks and recruits repair factors. This illustrates that nucleosomes are not just barriers to repair but also active signaling platforms.

## Methods Used to Study Nucleosomes

### Micrococcal Nuclease Digestion

Micrococcal nuclease (MNase) is an enzyme that cleaves DNA preferentially in linker regions, which are less protected than nucleosomal DNA. When chromatin is digested with MNase and the DNA is analyzed by gel electrophoresis, a ladder of fragments is observed, with each rung corresponding to a multiple of the nucleosome repeat length (e.g., 200 bp, 400 bp, 600 bp). This technique was used in the original discovery of nucleosomes and remains a standard method for mapping nucleosome positions.

The protocol is straightforward. Cells are lysed, and the chromatin is digested with MNase at 37°C for 5–15 minutes. The reaction is stopped by adding EDTA (which chelates the calcium required for MNase activity) and EGTA. The DNA is then purified and analyzed. The extent of digestion is controlled by varying the enzyme concentration or digestion time. Limited digestion produces mostly mononucleosomes (200 bp), while more extensive digestion trims the linker DNA to produce core particles (147 bp).

### Cryo-Electron Microscopy

Cryo-electron microscopy (cryo-EM) has revolutionized the study of nucleosomes and chromatin. In cryo-EM, samples are frozen in vitreous ice and imaged in a transmission electron microscope. Thousands of particle images are averaged to produce a three-dimensional reconstruction at near-atomic resolution. This technique has been used to determine the structures of nucleosomes in complex with remodeling complexes, transcription factors, and histone chaperones, revealing the conformational changes that occur during nucleosome dynamics.

Cryo-EM has a key advantage over [X-ray crystallography](/knowledge/molecular-biology/x-ray-crystallography): it does not require the formation of crystals, which is often difficult for large, flexible complexes. This has allowed structural biologists to visualize nucleosomes in various states, such as partially unwrapped or with histone tails in different conformations. For example, cryo-EM structures of the SWI/SNF family remodeler RSC bound to a nucleosome have shown how the remodeler pumps DNA around the octamer, providing a mechanistic basis for nucleosome sliding.

## Common Misconceptions and Pitfalls

### Nucleosome vs. Chromatin

A common error is to use "nucleosome" and "chromatin" interchangeably. Chromatin is the entire complex of DNA and proteins in the nucleus, including nucleosomes, linker histones, and non-histone proteins. A nucleosome is a single repeating unit of chromatin. Chromatin is to nucleosomes what a brick wall is to individual bricks. The distinction matters because chromatin includes higher-order structures and regulatory proteins that are not part of the nucleosome itself.

### Dynamic Nature of Nucleosomes

Another misconception is that nucleosomes are static, rigid structures. In reality, nucleosomes are highly dynamic. The DNA at the entry and exit sites spontaneously unwraps and rewraps on timescales of milliseconds to seconds. Nucleosomes can slide along DNA, be evicted entirely, or exchange their histone components. The histone octamer is not a permanent assembly; H2A–H2B dimers exchange more rapidly than H3–H4 tetramers, and histone variants (such as H2A.Z or H3.3) can replace canonical histones in a regulated manner. This dynamic behavior is essential for gene regulation, DNA replication, and repair.

### Misunderstanding the Role of Linker Histones

Linker histones, such as histone H1, are often confused with core histones. H1 binds to the linker DNA at the entry and exit points of the nucleosome and stabilizes the wrapping of DNA around the octamer. It is not part of the core particle and is not required for [nucleosome formation](/knowledge/molecular-biology/nucleosome-formation). H1 binding promotes the compaction of nucleosome arrays into higher-order structures. The presence of H1 increases the nucleosome repeat length by protecting additional linker DNA from nuclease digestion. Misunderstanding the role of H1 leads to errors in interpreting nuclease digestion patterns and chromatin compaction assays.

## Summary and Practical Takeaways

### Key Points to Remember

- The nucleosome is the fundamental repeating unit of eukaryotic chromatin, consisting of 147 bp of DNA wrapped around an octamer of histones H2A, H2B, H3, and H4.
- The nucleosome model was established in the early 1970s through the work of Kornberg and the Oliins, and the atomic structure was solved in 1997.
- Nucleosomes compact DNA approximately sevenfold and are the first level of chromatin organization.
- Nucleosome assembly is ordered: H3–H4 tetramers bind DNA first, followed by two H2A–H2B dimers, with histone chaperones controlling each step.
- Nucleosomes are dynamic; they slide, unwrap, and are remodeled by ATP-dependent complexes.
- Nucleosomes regulate gene expression by controlling access of transcription factors to DNA and by modulating RNA polymerase progression.
- Nucleosomes protect DNA from damage and serve as signaling platforms for DNA repair.

### Analogy: Beads on a String

The classic analogy for the nucleosome model is "beads on a string." Imagine a long piece of thread (the DNA) with beads (nucleosome core particles) threaded along it. The thread between beads is the linker DNA. This simple image captures the repeating nature of the nucleosome array. However, the analogy has limits: the beads are not fixed in place—they can slide along the string—and the string can be wound into tighter coils, just as nucleosome arrays fold into higher-order chromatin structures. The beads-on-a-string model is the first level of packaging, but it is not the final structure.

## Frequently Asked Questions

### What is the nucleosome model?

The nucleosome model is the scientific framework describing how eukaryotic DNA is packaged into chromatin. It states that DNA is wrapped around histone octamers to form repeating units called nucleosomes, which are connected by linker DNA. This model explains the first level of DNA compaction and provides the basis for understanding higher-order chromatin structure and gene regulation.

### Can you show a nucleosome model diagram?

A nucleosome model diagram typically shows a disc-shaped histone octamer with DNA wrapped around it in a left-handed superhelix. The diagram usually labels the four core histones (H2A, H2B, H3, H4), the 147 bp of DNA, and the linker DNA extending from the entry and exit points. Many textbooks and online resources provide such diagrams; the [nucleosome diagram](/knowledge/molecular-biology/nucleosome-diagram) is a useful reference.

### What are the types of nucleosome model?

There is essentially one nucleosome model, but it has been refined over time. The original model proposed by Kornberg described the basic repeating unit. Subsequent refinements include the atomic structure of the core particle, the identification of histone variants, and the recognition that nucleosomes are dynamic rather than static. Some researchers distinguish between the "core particle" model (147 bp of DNA and the octamer) and the "chromatosome" model (core particle plus linker histone H1 and additional linker DNA).

### What is the definition of nucleosome model?

The nucleosome model defines the nucleosome as the basic repeating unit of chromatin, consisting of a histone octamer (two each of H2A, H2B, H3, and H4) with approximately 147 bp of DNA wrapped around it, connected to adjacent nucleosomes by linker DNA. The model describes both the structure of individual nucleosomes and their arrangement along the genome.

### What is the function of the nucleosome model?

The nucleosome model serves multiple functions: it explains how DNA is compacted to fit inside the nucleus, it provides a framework for understanding how DNA accessibility is regulated, and it explains how chromatin structure influences gene expression, DNA replication, and DNA repair. The model is also a practical tool for interpreting experimental data, such as nuclease digestion patterns and chromatin immunoprecipitation results.

### How many DNA base pairs are in a nucleosome?

A nucleosome core particle contains 147 base pairs of DNA. Including the linker DNA, the total DNA associated with one nucleosome is typically about 200 bp, though this varies by organism and cell type. The [nucleosome definition](/knowledge/molecular-biology/nucleosome-definition) provides further detail on this distinction.

### Who discovered the nucleosome?

The nucleosome was discovered through the combined work of several researchers in the early 1970s. Roger Kornberg proposed the histone octamer model based on biochemical and X-ray diffraction data. Ada and Donald Oliins independently observed the repeating particles by electron microscopy, which they called "nu bodies." The high-resolution structure was later solved by Karolin Luger and Timothy Richmond in 1997.

## Key Takeaways

- The nucleosome is the fundamental repeating unit of chromatin, composed of 147 bp of DNA wrapped around a histone octamer (H2A, H2B, H3, H4)₂.
- The nucleosome model was established in the 1970s and refined to atomic resolution in 1997, providing a structural basis for DNA packaging.
- Nucleosomes compact DNA approximately sevenfold and are the first level of chromatin organization, with higher-order folding achieving up to 10,000-fold compaction.
- Nucleosome assembly is a regulated, stepwise process involving histone chaperones, and disassembly is catalyzed by ATP-dependent remodeling complexes.
- Nucleosomes are dynamic structures that slide, unwrap, and exchange histones, enabling regulated access to DNA for transcription, replication, and repair.
- Nucleosomes protect DNA from damage and serve as platforms for recruiting repair and regulatory proteins.
- The "beads on a string" analogy captures the repeating nucleosome array but understates the dynamic and regulatory complexity of chromatin.

## Further Reading

- Wu J, Grunstein M. *25 years after the nucleosome model: chromatin modifications*. Trends in biochemical sciences. 2000. [PubMed 11116189](https://doi.org/10.1016/s0968-0004(00)01718-7)
- Mavrich TN et al. *A barrier nucleosome model for statistical positioning of nucleosomes throughout the yeast genome*. Genome research. 2008. [PubMed 18550805](https://doi.org/10.1101/gr.078261.108)
- Sun T et al. *CG modeling of nucleosome arrays reveals the salt-dependent chromatin fiber conformational variability*. The Journal of chemical physics. 2025. [PubMed 39774881](https://doi.org/10.1063/5.0242509)
- Möbius W, Gerland U. *Quantitative test of the barrier nucleosome model for statistical positioning of nucleosomes up- and downstream of transcription start sites*. PLoS [computational biology](/knowledge/bioinformatics/computational-approaches-to-understanding-antimicrobial-resistance-amr). 2010. [PubMed 20808881](https://doi.org/10.1371/journal.pcbi.1000891)
- Higuchi Y, Sakaue T, Yoshikawa K. *Torsional effect on the wrapping transition of a semiflexible polymer around a core as a model of nucleosome*. Physical review. E, Statistical, nonlinear, and soft matter physics. 2010. [PubMed 21230110](https://doi.org/10.1103/PhysRevE.82.031909)
- Forties RA et al. *A quantitative model of nucleosome dynamics*. [Nucleic acids research](/blog/news/nucleic-acids-research). 2011. [PubMed 21764779](https://doi.org/10.1093/nar/gkr422)

## Related Topics

- [Nucleosome Structure](/knowledge/molecular-biology/nucleosome-structure)
- [Histone Nucleosome](/knowledge/molecular-biology/histone-nucleosome)
- [Nucleosome Chromatin](/knowledge/molecular-biology/nucleosome-chromatin)
- [Nucleosome Sliding](/knowledge/molecular-biology/nucleosome-sliding)

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* [Mammalian Cell Culture Bioreactors: A Practical Guide](/knowledge/molecular-biology/mammalian-cell-culture-bioreactor)
* [Nucleotide Formation: Biosynthesis and Assembly of DNA/RNA Building Blocks](/knowledge/molecular-biology/nucleotide-formation)