Nucleosome Structure: What a Nucleosome Consists Of
By Dr. Zubair Khalid, DVM, MS, PhD ·
Introduction to Nucleosome Structure
The nucleosome is the fundamental repeating unit of chromatin, the complex of DNA and proteins that packages the eukaryotic genome inside the nucleus. Every human cell contains approximately two meters of DNA compressed into a nucleus roughly 10 micrometers in diameter. This remarkable feat of compaction is achieved through a hierarchical organization in which the nucleosome represents the first and most essential level.
A nucleosome consists of a core histone octamer—two copies each of histones H2A, H2B, H3, and H4—around which 147 base pairs (bp) of DNA are wrapped in 1.65 left-handed superhelical turns. This core particle, together with a segment of linker DNA and, in most cases, a linker histone H1, constitutes the complete nucleosome. The term "nucleosome" is sometimes used interchangeably with "nucleosome core particle" (NCP), but strictly speaking, the NCP refers only to the octamer plus the 147 bp of wrapped DNA, whereas the nucleosome includes the linker DNA and H1.
The biological importance of the nucleosome extends far beyond simple compaction. Nucleosome positioning influences every DNA-templated process, including transcription, replication, repair, and recombination. The precise arrangement of nucleosomes along the genome determines which regulatory elements are accessible to transcription factors and which are occluded. Understanding what a nucleosome consists of is therefore foundational to understanding gene regulation, epigenetics, and chromosome biology. For a broader overview of how nucleosomes organize into higher-order structures, see the Nucleosome Model.
Core Histone Proteins: The Octamer
Histone Fold Domains
The histone octamer is a protein complex with a molecular weight of approximately 108 kDa. It is assembled from four histone proteins, each present in two copies: H2A, H2B, H3, and H4. These are small, highly basic proteins, with molecular weights ranging from 11 to 15 kDa. Their high content of lysine and arginine residues gives them a strong positive charge, which is essential for their interaction with the negatively charged phosphate backbone of DNA.
Each core histone contains a conserved structural motif known as the histone fold domain. This domain consists of three alpha-helices (α1, α2, and α3) connected by two loops (L1 and L2). The histone fold is remarkably similar across all four core histones, despite their divergent amino acid sequences. This structural conservation allows histones to dimerize in a specific, obligatory fashion: H3 pairs with H4, and H2A pairs with H2B. The dimerization interface, known as the "handshake" interaction, involves the antiparallel alignment of the α2 and α3 helices from each monomer, creating a stable four-helix bundle.
The assembly of the octamer proceeds in a defined order. Two H3-H4 dimers first associate through a four-helix bundle formed between their respective α2 and α3 helices, creating an H3₂-H4₂ tetramer. Two H2A-H2B dimers then dock onto this tetramer, one on each face, to form the complete octamer. The resulting structure has a two-fold axis of symmetry, meaning the octamer is a dimer of two (H3-H4)₂-(H2A-H2B) heterotetramers.
The histone fold domains form a crescent-shaped surface on the octamer that accommodates the wrapped DNA. The L1 and L2 loops, along with the N-terminal ends of the α1 helices, project into the minor groove of the DNA at 14 distinct contact points. These contacts are predominantly electrostatic, involving hydrogen bonds between histone backbone amides and DNA phosphate groups, as well as salt bridges between arginine side chains and phosphate oxygens. Notably, arginine residues are disproportionately represented at these interfaces—each minor groove contact typically involves an arginine side chain inserting into the groove and making direct contacts with the DNA backbone.
Histone Tails and Post-Translational Modifications
In addition to the globular histone fold domain, each core histone possesses an unstructured N-terminal tail that extends outward from the nucleosome core. These tails range from 15 to 35 amino acids in length and are rich in lysine and arginine residues. The H2A and H2B histones also have short C-terminal tails. Because these tails are not part of the globular domain, they are highly flexible and can protrude through the DNA superhelix to interact with neighboring nucleosomes, linker DNA, and a wide array of protein factors.
The histone tails are the primary sites of post-translational modifications (PTMs), which constitute a major mechanism of epigenetic regulation. Over 100 distinct modifications have been identified, including acetylation, methylation, phosphorylation, ubiquitination, and SUMOylation. These modifications can alter the charge of the histone tail, thereby affecting histone-DNA and histone-histone interactions, or they can serve as docking sites for reader proteins that recruit downstream effectors.
Key modifications include:
- Acetylation of lysine residues (e.g., H3K9ac, H3K27ac, H4K16ac) neutralizes the positive charge on the lysine side chain, weakening histone-DNA interactions and generally correlating with open, transcriptionally active chromatin.
- Methylation of lysine residues can be mono-, di-, or tri-methylated (e.g., H3K4me3 marks active promoters, H3K27me3 marks silenced promoters, H3K9me3 marks heterochromatin).
- Phosphorylation of serine and threonine residues (e.g., H3S10ph) is associated with chromosome condensation during mitosis and with immediate-early gene activation.
The enzymes that add these modifications are called writers (e.g., histone acetyltransferases such as p300/CBP, histone methyltransferases such as EZH2 and SUV39H1), those that remove them are erasers (e.g., histone deacetylases such as HDAC1, histone demethylases such as LSD1 and JmjC-domain proteins), and those that bind them are readers (e.g., bromodomains bind acetyl-lysine, chromodomains bind methyl-lysine). The combinatorial patterns of these modifications form the basis of the "histone code" hypothesis, which posits that specific modification patterns dictate particular chromatin states and functional outcomes.
For a deeper discussion of how histone modifications relate to nucleosome function, the Histone Nucleosome entry provides additional context.
DNA Wrapping Around the Histone Core
Superhelical Turns
The 147 bp of DNA that wrap around the histone octamer do so in 1.65 left-handed superhelical turns. This means the DNA makes slightly more than one and a half full rotations around the octamer. The path of the DNA is not uniform; it is bent sharply at several points, particularly where the minor groove faces inward toward the histone surface.
The DNA double helix itself has a periodicity of approximately 10.5 bp per turn. As it wraps around the octamer, the DNA is bent to a radius of curvature of about 4.2 nm, which is significantly tighter than the persistence length of DNA (approximately 50 nm). This extreme bending is energetically costly—it has been estimated that wrapping 147 bp of DNA around the octamer requires approximately 20–25 kcal/mol of free energy. This energetic cost is offset by the numerous electrostatic and hydrogen-bonding contacts between the histones and the DNA.
The structure of the nucleosome core particle was first solved at atomic resolution by X-ray crystallography in 1997. The structure revealed that the DNA is not uniformly bent but rather follows a path that is straight for short stretches and then sharply kinked at specific positions. These kinks occur where the minor groove faces the histone surface, allowing the DNA to achieve the required curvature without overstretching the backbone.
Minor Groove Interactions
The interaction between histones and DNA occurs primarily through the minor groove. At 14 discrete sites along the wrapped DNA, the minor groove faces inward toward the octamer. At each of these sites, an arginine side chain from one of the histone proteins inserts into the minor groove, making direct contacts with the DNA bases and the phosphate backbone.
These arginine residues are not randomly distributed; they are positioned at specific locations within the histone fold domains. For example, in histone H3, arginine 45 (R45) and arginine 63 (R63) insert into the minor groove at defined positions. In H4, arginine 45 plays a similar role. The insertion of arginine into the minor groove serves two purposes: it provides a stable electrostatic interaction with the negatively charged DNA, and it helps to set the rotational positioning of the DNA relative to the histone surface.
In addition to the arginine-mediated minor groove contacts, there are numerous hydrogen bonds between the protein backbone and the DNA phosphate groups. On average, each histone-DNA contact point involves 3–5 direct hydrogen bonds, plus additional water-mediated contacts. The total number of direct contacts between the octamer and the 147 bp of DNA is approximately 120, making the nucleosome an exceptionally stable complex. The dissociation constant (Kd) for the nucleosome core particle is in the picomolar range, reflecting this extensive interaction network.
The periodic nature of these contacts explains why nucleosome positioning is sequence-dependent. Certain dinucleotide sequences, particularly AA/TT dinucleotides, are preferentially found where the minor groove faces inward, because these sequences are more easily bent. Conversely, GC-rich sequences are more rigid and are often found where the minor groove faces outward. This sequence preference is the basis for computational models that predict nucleosome positioning from genomic DNA sequence.
The Linker DNA and H1 Histone
Linker DNA
Between adjacent nucleosome core particles lies a segment of DNA known as linker DNA. The length of linker DNA varies between species and cell types, ranging from as short as 10 bp to as long as 80 bp. In most somatic cells, the average linker length is approximately 20–60 bp. The variation in linker length affects the packing density of nucleosomes and contributes to the overall compaction of chromatin.
The linker DNA is not simply passive spacer; it is the site where the linker histone H1 binds and where many chromatin-associated proteins interact. The entry and exit points of the linker DNA from the nucleosome core are also sites of dynamic unwrapping, where the DNA transiently peels away from the octamer, allowing access to transcription factors and other DNA-binding proteins.
The repeat length of chromatin—the distance between the start of one nucleosome and the start of the next—is the sum of the 147 bp of core DNA plus the linker length. This value, known as the nucleosome repeat length (NRL), is typically 180–200 bp in most eukaryotes. The NRL is not uniform across the genome; it varies between regions and can change during development or in response to cellular signals.
Linker Histone H1
The linker histone H1 is a separate class of histone that binds to the nucleosome at the entry/exit point of the DNA. Unlike the core histones, H1 is not part of the octamer and does not contain a histone fold domain. Instead, H1 has a tripartite structure: a short N-terminal domain, a central globular winged-helix domain, and a long, intrinsically disordered C-terminal domain rich in lysine.
The globular domain of H1 binds to the nucleosome at the dyad axis, where it contacts both the DNA major groove and the H3 and H2A histones. The C-terminal domain interacts with the linker DNA, stabilizing the angle at which the DNA enters and exits the nucleosome. This binding effectively "locks" the DNA in place, reducing nucleosome mobility and promoting chromatin compaction.
H1 binding increases the stability of the nucleosome and promotes the formation of higher-order chromatin structures. The 30-nm fiber, a compacted chromatin structure that was long thought to be the next level of organization above the nucleosome, requires H1 for its formation. However, the existence and physiological relevance of the 30-nm fiber in vivo has been debated; recent evidence suggests that chromatin in the nucleus exists primarily as a disordered, dynamic array of nucleosomes rather than a regular helical fiber.
There are multiple H1 variants in mammals, including H1.1 through H1.5 (the somatic variants), H1t (testis-specific), H1oo (oocyte-specific), and H1.0 (a replacement variant associated with differentiated cells). These variants differ in their affinity for chromatin and their effects on nucleosome stability. The ratio of H1 to nucleosomes is approximately 0.5–1.0 in most cells, and this ratio is tightly regulated.
For a comparison of the nucleosome with other DNA-protein complexes, the Nucleosome vs Nucleotide entry clarifies a common point of confusion.
Nucleosome Dynamics and Remodeling
Nucleosome Sliding and Unwrapping
Nucleosomes are not static structures. They undergo spontaneous, thermally driven fluctuations that are essential for DNA accessibility. Two major modes of dynamics are:
- Site exposure (unwrapping): The DNA at the entry/exit sites of the nucleosome transiently peels away from the octamer, exposing 10–50 bp of DNA. This unwrapping occurs on a timescale of milliseconds to seconds and allows transcription factors to access their binding sites without complete nucleosome disassembly. The rate of unwrapping is influenced by the DNA sequence, histone modifications, and the presence of chromatin-binding proteins.
- Nucleosome sliding: The entire histone octamer can move along the DNA, repositioning itself by 1–10 bp at a time. This process, called nucleosome sliding, occurs spontaneously at a slow rate but is greatly accelerated by ATP-dependent chromatin remodelers. Sliding changes the translational position of the nucleosome, thereby altering which DNA sequences are exposed.
The spontaneous mobility of nucleosomes is limited by the high energetic barrier to breaking the numerous histone-DNA contacts. The barrier is estimated to be approximately 12–15 kcal/mol, which corresponds to a slow spontaneous rate. This is why cells have evolved dedicated enzymes to actively reposition nucleosomes.
ATP-Dependent Chromatin Remodeling Complexes
Chromatin remodeling complexes are ATP-dependent enzymes that use the energy of ATP hydrolysis to alter nucleosome structure, position, or composition. All remodelers share a conserved ATPase domain of the Snf2 family, but they differ in their accessory subunits, which target them to specific genomic locations and regulate their activity.
The four major families of chromatin remodelers are:
- SWI/SNF family: These complexes (including BAF and PBAF in mammals) slide or eject nucleosomes, creating nucleosome-free regions at promoters and enhancers. They are important for transcriptional activation.
- ISWI family: These complexes (including ACF, CHRAC, and NURF) slide nucleosomes to regular spacing, promoting chromatin assembly and transcriptional repression.
- CHD family: These complexes (including NuRD and CHD1) slide or evict nucleosomes and are involved in both activation and repression.
- INO80/SWR1 family: These complexes exchange histone variants (e.g., replacing H2A with H2A.Z) and are involved in DNA repair and transcriptional regulation.
The mechanism of remodeling involves several steps:
- The remodeler binds to the nucleosome, typically at the entry site of the DNA.
- ATP hydrolysis drives the translocation of the ATPase domain along the DNA, creating a DNA loop.
- The loop propagates around the octamer, causing the histone octamer to shift relative to the DNA.
- The remodeler dissociates, leaving the nucleosome at its new position.
This process is processive; a single remodeler can move a nucleosome by tens of base pairs before dissociating. The direction of movement is determined by the geometry of the remodeler-nucleosome interaction and by the DNA sequence.
Histone Variants
In addition to the canonical histones, cells express histone variants that differ in amino acid sequence and confer specialized functions. These variants are incorporated into nucleosomes at specific genomic locations by dedicated chaperones and remodeling complexes.
Major histone variants include:
- H3.3: Differs from canonical H3 by only four amino acids but is incorporated into nucleosomes throughout the cell cycle (not just during S phase). H3.3 marks actively transcribed genes and regulatory elements.
- CENP-A: A centromere-specific H3 variant that defines the site of kinetochore assembly. CENP-A-containing nucleosomes are structurally distinct and may have a different wrapping geometry.
- H2A.Z: An H2A variant that is enriched at promoters and regulatory elements. H2A.Z-containing nucleosomes are less stable and are often found flanking nucleosome-free regions.
- H2A.X: An H2A variant that becomes phosphorylated at serine 139 (γ-H2A.X) in response to DNA double-strand breaks. This modification serves as a signal for the recruitment of DNA repair factors.
- macroH2A: A large H2A variant with a C-terminal macrodomain that is enriched on the inactive X chromosome and at silenced loci.
The incorporation of histone variants changes the biophysical properties of the nucleosome, affecting its stability, mobility, and interactions with other proteins. For example, H2A.Z-containing nucleosomes are more easily remodeled and are less stable than canonical nucleosomes, which is thought to facilitate transcription factor binding at promoters.
The dynamic behavior of nucleosomes, including sliding and remodeling, is covered in more detail in the Nucleosome Sliding entry.
Methods to Study Nucleosome Structure
X-ray Crystallography and Cryo-Electron Microscopy
The atomic structure of the nucleosome core particle was determined by X-ray crystallography in 1997, using crystals of the NCP derived from Xenopus laevis histones and a 146 bp palindromic DNA sequence. The structure, solved to 2.8 Å resolution, revealed the detailed architecture of the octamer and the path of the DNA. Subsequent structures at higher resolution (1.9 Å) and with different DNA sequences and histone variants have refined our understanding.
X-ray crystallography requires the formation of well-ordered crystals, which is challenging for nucleosomes due to their size and flexibility. The histone tails, in particular, are often disordered in crystal structures and may be invisible in the electron density map. To overcome this limitation, researchers often use truncated histones lacking the tails, or they add stabilizing mutations.
Cryo-electron microscopy (cryo-EM) has emerged as a powerful complementary technique. Unlike crystallography, cryo-EM does not require crystallization and can capture nucleosomes in multiple conformational states. Recent advances in detector technology and image processing have enabled near-atomic resolution structures of nucleosomes, nucleosome arrays, and nucleosome-remodeler complexes. Cryo-EM has been particularly valuable for visualizing the structure of the nucleosome in complex with chromatin remodelers, transcription factors, and other chromatin-associated proteins.
Nuclease Digestion and MNase-Seq
The classic method for studying nucleosome positioning is digestion with micrococcal nuclease (MNase). MNase cleaves DNA preferentially in linker regions, which are more accessible than the DNA wrapped around the histone octamer. When chromatin is digested with MNase and the DNA is analyzed by gel electrophoresis, a characteristic ladder of bands is observed, corresponding to mono-, di-, tri-, and higher-order nucleosome fragments. The mononucleosome band is approximately 147 bp, confirming the length of DNA protected by the octamer.
In modern genomics, MNase digestion is combined with high-throughput sequencing in a technique called MNase-seq. After digestion, the protected DNA fragments are size-selected (typically 140–180 bp), sequenced, and mapped to the genome. The resulting coverage profile reveals the positions of nucleosomes genome-wide. Peaks in MNase-seq signal correspond to well-positioned nucleosomes, while valleys correspond to nucleosome-free regions.
The conditions for MNase digestion must be carefully optimized. Typical reactions use 0.1–1.0 units of MNase per microgram of chromatin, incubated at 37°C for 5–15 minutes in a buffer containing 1–5 mM CaCl₂ (MNase requires calcium for activity). Over-digestion leads to nucleosome sliding and loss of positioning information, while under-digestion leaves linker DNA intact.
Chromatin Immunoprecipitation (ChIP)
Chromatin immunoprecipitation (ChIP) is used to determine the genomic locations of specific histone modifications or histone variants. The basic protocol involves:
- Crosslinking cells with formaldehyde (typically 1% for 10 minutes at room temperature) to covalently link proteins to DNA.
- Shearing the chromatin by sonication or MNase digestion to generate fragments of 200–600 bp.
- Immunoprecipitating the chromatin with an antibody specific to the histone modification of interest.
- Reversing the crosslinks and purifying the DNA.
- Analyzing the DNA by quantitative PCR (ChIP-qPCR) or high-throughput sequencing (ChIP-seq).
ChIP-seq has been used to map the genome-wide distribution of dozens of histone modifications and variants across many cell types. For example, H3K4me3 is enriched at active promoters, H3K27ac marks active enhancers and promoters, and H3K27me3 marks Polycomb-repressed regions. These maps have been instrumental in defining the chromatin states that regulate gene expression.
The quality of ChIP data depends critically on antibody specificity. Many commercially available antibodies cross-react with related modifications or with unmodified histones, leading to spurious results. Validation by peptide arrays or knockout cell lines is essential.
Common Misconceptions and Pitfalls
Students frequently encounter several conceptual errors when learning about nucleosome structure. Being aware of these pitfalls will help you avoid them in exams and in your own reasoning.
Pitfall 1: Confusing nucleosome with chromatin. The nucleosome is a single unit consisting of the octamer, 147 bp of DNA, and (optionally) H1 and linker DNA. Chromatin is the entire complex of DNA and proteins that makes up the chromosome. A single nucleosome is not chromatin; chromatin is an array of many nucleosomes plus associated proteins.
Pitfall 2: Thinking histones are only structural proteins. While histones do provide structural packaging, they are also dynamic regulatory proteins. Their tails are extensively modified, and these modifications are read by other proteins to control gene expression. Histones are also subject to exchange and replacement by variants, and they participate in DNA repair signaling.
Pitfall 3: Misremembering the DNA length. The core particle contains 147 bp of DNA, not 146, 150, or 200 bp. The 146 bp figure appears in some older literature because the first crystal structure used a 146 bp DNA construct, but the biologically relevant length is 147 bp. The total nucleosome repeat length (including linker) is typically 180–200 bp.
Pitfall 4: Believing the octamer contains one copy of each histone. The octamer contains two copies each of H2A, H2B, H3, and H4—eight proteins total. A common error is to think there are four different histones in a single copy.
Pitfall 5: Assuming nucleosomes are static. Nucleosomes are highly dynamic. They unwrap, slide, and are actively remodeled. The "beads on a string" image is a useful simplification, but it fails to convey the constant motion and remodeling that occurs in vivo.
Pitfall 6: Confusing the direction of the superhelical turn. The DNA wraps around the histone octamer in a left-handed superhelix. This is opposite to the right-handed twist of the DNA double helix itself. The left-handed wrapping is important for the formation of higher-order chromatin structures.
Pitfall 7: Thinking H1 is part of the core particle. H1 is a linker histone that binds outside the core particle. It is not part of the octamer and is not required for the formation of the core particle. Some organisms and cell types have very low levels of H1.
Pitfall 8: Assuming all nucleosomes are identical. Nucleosomes can contain histone variants (H3.3, H2A.Z, CENP-A, etc.) and carry different post-translational modifications. These variations confer distinct functional properties.
Summary and Key Takeaways
The nucleosome is the fundamental unit of chromatin, consisting of a histone octamer (two copies each of H2A, H2B, H3, and H4) around which 147 bp of DNA are wrapped in 1.65 left-handed superhelical turns. The structure is stabilized by extensive histone-DNA contacts, particularly arginine side chains inserting into the minor groove at 14 sites. Linker DNA connects adjacent nucleosomes, and linker histone H1 binds at the entry/exit point to stabilize higher-order structure. Nucleosomes are dynamic entities that slide, unwrap, and are remodeled by ATP-dependent enzymes. Histone variants and post-translational modifications add a layer of regulatory complexity. The Nucleosome Definition and Nucleosome Concept entries provide additional framing for these ideas.
Frequently Asked Questions
What does a nucleosome consist of?
A nucleosome consists of a core histone octamer—two copies each of H2A, H2B, H3, and H4—around which 147 base pairs of DNA are wrapped in 1.65 left-handed superhelical turns. The complete nucleosome also includes linker DNA (10–80 bp) connecting adjacent core particles and, in most cells, a linker histone H1 bound at the DNA entry/exit point.
How many histone proteins are in a nucleosome?
The core particle contains eight histone proteins: two copies each of H2A, H2B, H3, and H4. If linker histone H1 is included, the total is nine proteins. The eight core histones are organized as an H3₂-H4₂ tetramer flanked by two H2A-H2B dimers.
What is the role of linker DNA in nucleosomes?
Linker DNA connects adjacent nucleosome core particles. Its length varies from 10 to 80 bp depending on cell type and genomic region. Linker DNA is the site of H1 binding and is important for chromatin compaction. The linker length determines the nucleosome repeat length and influences the packing density of nucleosomes.
What is the difference between a nucleosome and chromatin?
A nucleosome is a single repeating unit consisting of the histone octamer, wrapped DNA, and associated linker DNA and H1. Chromatin is the entire complex of DNA and proteins that constitutes the chromosome, including all nucleosomes, histone modifications, chromatin-associated proteins, and higher-order structures. Chromatin is composed of many nucleosomes arranged along the DNA.
Are histones only structural proteins?
No. While histones provide structural packaging of DNA, they are also dynamic regulatory proteins. Histone tails undergo extensive post-translational modifications (acetylation, methylation, phosphorylation, ubiquitination) that regulate gene expression. Histone variants (H3.3, H2A.Z, CENP-A) confer specialized functions. Histones are also involved in DNA repair signaling (γ-H2A.X) and chromosome segregation.
How many base pairs of DNA wrap around a nucleosome?
The nucleosome core particle contains 147 base pairs of DNA. This DNA makes 1.65 left-handed superhelical turns around the octamer. The total nucleosome repeat length, including linker DNA, is typically 180–200 base pairs.
Key Takeaways
- The nucleosome core particle consists of 147 bp of DNA wrapped around a histone octamer (two copies each of H2A, H2B, H3, H4) in 1.65 left-handed superhelical turns.
- The histone fold domain is the conserved structural motif that mediates histone dimerization and DNA binding; arginine side chains insert into the minor groove at 14 contact sites.
- Histone tails are unstructured, extend outward from the core, and are the primary sites of post-translational modifications that regulate chromatin function.
- Linker DNA (10–80 bp) connects adjacent nucleosomes, and linker histone H1 binds at the entry/exit point to stabilize chromatin compaction.
- Nucleosomes are dynamic: they spontaneously unwrap, slide along DNA, and are actively repositioned by ATP-dependent chromatin remodelers (SWI/SNF, ISWI, CHD, INO80 families).
- Histone variants (H3.3, H2A.Z, CENP-A, H2A.X) and post-translational modifications create functional diversity among nucleosomes.
- Key experimental methods include X-ray crystallography, cryo-EM, MNase-seq for nucleosome positioning, and ChIP-seq for mapping histone modifications.