Nucleosome Diagram: Structure, Function, and How to Draw It
By Dr. Zubair Khalid, DVM, MS, PhD ·
What Is a Nucleosome?
Every human cell contains roughly two meters of DNA packed into a nucleus that measures only about six micrometers across. This remarkable feat of compaction is achieved through a hierarchical system of DNA organization, and the first level of this system is the nucleosome. A nucleosome is the fundamental repeating unit of chromatin—the complex of DNA and proteins that makes up chromosomes. It consists of a segment of DNA wrapped around a core of eight histone proteins, forming a structure that resembles a bead on a string when viewed under an electron microscope.
The term "nucleosome" was coined in 1974 by Roger Kornberg, who proposed that chromatin is composed of repeating units each containing about 200 base pairs of DNA and a set of histone proteins. This proposal followed earlier observations that chromatin, when digested with certain enzymes, produced fragments of DNA with a consistent length, suggesting a regular repeating structure. The nucleosome is not merely a passive packaging device; it plays an active role in regulating gene expression, DNA replication, and DNA repair by controlling access to the genetic information encoded in DNA.
The biological importance of the nucleosome cannot be overstated. Without nucleosomes, the DNA in a single human cell would be far too long to fit inside the nucleus, and even if it could fit, the machinery that reads genes would be unable to navigate the tangled mass. Nucleosomes provide a solution to both problems: they compact DNA in a regular, organized manner, and they can be dynamically rearranged to expose specific regions of DNA when needed. Understanding the nucleosome is therefore essential for understanding how genomes function at the molecular level. For a deeper introduction, see the Nucleosome Definition.
Key Components of a Nucleosome
Histone Proteins
The protein core of a nucleosome is an octamer composed of eight histone proteins: two copies each of histone H2A, H2B, H3, and H4. These are small, highly basic proteins, rich in the amino acids lysine and arginine, which carry positive charges at physiological pH. This positive charge is crucial because it allows the histones to bind tightly to the negatively charged phosphate backbone of DNA through electrostatic interactions.
Each histone protein has a characteristic structure consisting of a central globular domain and flexible N-terminal and C-terminal tails. The globular domains form the core of the octamer and make extensive contacts with the DNA, while the tails extend outward and are subject to a wide variety of post-translational modifications, including acetylation, methylation, and phosphorylation. These modifications do not alter the basic structure of the nucleosome but instead serve as signals that recruit other proteins to the chromatin, influencing gene activity.
The histone fold domain, a conserved structural motif shared by all four core histones, consists of three alpha-helices connected by two loops. This domain mediates the dimerization of histones: H3 pairs with H4, and H2A pairs with H2B. The octamer is assembled as a tetramer of H3-H4, flanked by two H2A-H2B dimers. This assembly is not merely structural; it has functional consequences. During DNA replication, the H3-H4 tetramer is deposited onto new DNA first, followed by the H2A-H2B dimers, and this ordered assembly is important for maintaining the integrity of chromatin structure.
DNA Wrapping and the Superhelix
The DNA that wraps around the histone octamer is not random sequence; it is organized in a precise geometry. Approximately 147 base pairs of DNA make 1.65 turns around the histone core, forming a left-handed superhelix. This means that the DNA, which itself is a right-handed double helix, is bent around the histones in a left-handed coil. The term "superhelix" refers to the coiling of the DNA helix itself around another axis, in this case the axis defined by the histone octamer.
The DNA makes contact with the histone surface at 14 distinct binding sites, each separated by about 10 base pairs—roughly one turn of the DNA double helix. At each site, the minor groove of the DNA faces inward toward the histone core, and the major groove faces outward. This periodic arrangement is a consequence of the DNA bending, and it means that certain nucleotides are preferentially positioned at specific locations relative to the histone surface. The DNA sequence can influence nucleosome positioning because some sequences bend more easily than others; A-T-rich regions tend to be found where the minor groove faces inward, while G-C-rich regions are more common where the minor groove faces outward.
The histone octamer and the DNA wrapped around it together constitute the nucleosome core particle. The DNA that connects adjacent nucleosome core particles is called linker DNA. Linker DNA is not wrapped around histones and is therefore more accessible to proteins that need to read or copy the genetic information. The length of linker DNA varies between species and cell types, typically ranging from 20 to 80 base pairs. In many diagrams, the linker DNA is shown as a straight or slightly curved line connecting the bead-like cores. The complete unit—core particle plus linker DNA—is sometimes referred to as a chromatosome when it also includes a linker histone, such as histone H1, which binds to the entry and exit points of the DNA on the nucleosome.
For a more detailed treatment of the structural arrangement, consult the Nucleosome Structure resource.
How to Read a Nucleosome Diagram
A nucleosome diagram is a visual representation of the nucleosome structure, and learning to read one correctly is essential for understanding chromatin biology. Most diagrams depict the nucleosome from a side view or a top-down view, and each view conveys different information.
In a typical side-view diagram, the histone octamer is drawn as a flattened, disc-shaped object, often colored to distinguish the different histone proteins. The DNA is shown as a ribbon or tube that wraps around the disc, making approximately 1.65 turns. The entry and exit points of the DNA are usually visible at the top or bottom of the disc, and the linker DNA extends away from these points. The N-terminal tails of the histones are often shown as wavy lines protruding from the core, indicating that they are flexible and extend into the surrounding space.
A top-down view, looking along the axis of the DNA superhelix, shows the DNA as a circular or near-circular ring around the histone core. This view emphasizes the symmetry of the nucleosome and the periodic contacts between DNA and histones. The dyad axis, a line of pseudo-twofold symmetry that passes through the center of the nucleosome, is often indicated in such diagrams. The dyad is the point at which the two halves of the wrapped DNA are related by a 180-degree rotation, and it is an important reference point for describing nucleosome positioning.
When reading a nucleosome diagram, pay attention to the following elements:
- The histone core: Usually drawn as a colored disc or sphere. The eight histones are often shown in different colors, with H3 in one color, H4 in another, and so on. The core is not a solid mass; it has a defined shape with grooves that accommodate the DNA.
- The DNA superhelix: Drawn as a ribbon or tube that wraps around the core. The number of wraps is typically indicated, and the direction of wrapping (left-handed) may be shown with arrows.
- The linker DNA: The free DNA segments that extend from the core. These are often drawn as straight or slightly curved lines, and their length can vary.
- The histone tails: Flexible extensions from the core, often drawn as thin lines. These are sites of post-translational modification and are not visible in high-resolution crystal structures because they are disordered.
- The dyad axis: A dashed line or symbol indicating the symmetry axis of the nucleosome.
A common mistake when reading nucleosome diagrams is to assume that the DNA is wrapped around the outside of the histone core like a ribbon around a ball. In reality, the DNA sits in a shallow groove on the surface of the histone octamer, making extensive contacts with the protein. The DNA is not merely draped over the histones; it is bound tightly through a combination of electrostatic interactions, hydrogen bonds, and hydrophobic contacts.
Step-by-Step Guide to Drawing a Nucleosome Diagram
Drawing a nucleosome diagram is a useful exercise for consolidating your understanding of the structure, and it is a common requirement in biology examinations. The following method produces a clear, accurate diagram that conveys the essential features of the nucleosome.
Step 1: Draw the Histone Core
Begin by drawing a flattened ellipse or disc in the center of your page. This represents the histone octamer. The disc should be oriented with its long axis horizontal. Do not make it a perfect circle; the nucleosome core is slightly elliptical, and this shape better represents the actual structure. If you wish to indicate the individual histones, divide the disc into eight segments, with four on the top and four on the bottom. The H3-H4 tetramer occupies the central region, and the H2A-H2B dimers are positioned at the periphery.
Step 2: Add the DNA Superhelix
Draw a curved line that wraps around the disc, starting from the upper left, going behind the disc, and emerging at the lower right. This line represents the DNA. To indicate that the DNA makes 1.65 turns, the line should wrap around the disc about one and two-thirds times. The entry point of the DNA is at the top, and the exit point is at the bottom, or vice versa. The DNA should be drawn close to the surface of the disc, not floating far away from it.
Step 3: Indicate the Linker DNA
From the entry and exit points, draw two straight or slightly curved lines extending away from the disc. These are the linker DNA segments that connect to adjacent nucleosomes. The length of these lines is not critical, but they should be clearly distinct from the wrapped DNA. In a diagram of a single nucleosome, the linker DNA is often shown extending to the left and right, suggesting the "beads on a string" arrangement.
Step 4: Add the Histone Tails
From the surface of the disc, draw several thin, wavy lines extending outward. These represent the N-terminal tails of the histones. You do not need to draw all eight tails; four or five is sufficient to convey the idea. The tails should be drawn emerging from different points on the disc, not all from the same location.
Step 5: Label the Components
Label the histone core, the DNA, and the linker DNA. If you have drawn the individual histones, label them as H2A, H2B, H3, and H4. Indicate the dyad axis with a dashed vertical line through the center of the disc. You may also wish to label the entry and exit points of the DNA.
Step 6: Add a Scale or Dimension Note
For an exam diagram, it is often helpful to include approximate dimensions. The nucleosome core particle is about 11 nanometers in diameter and 6 nanometers in height. The wrapped DNA is about 147 base pairs. Including these values demonstrates that you understand the scale of the structure.
This method produces a diagram that is accurate enough for most educational purposes. For a more detailed visual reference, see the Nucleosome Model page, which provides annotated illustrations of the structure.
The Role of Nucleosomes in DNA Packaging
The nucleosome is the first level of DNA compaction, but it is far from the last. The "beads on a string" arrangement of nucleosomes, with linker DNA connecting them, forms a fiber that is about 10 nanometers in diameter. This is known as the 10-nm fiber, and it represents the most basic level of chromatin organization. However, this fiber is not the final structure; it is further compacted into higher-order structures.
The next level of organization is the 30-nm fiber, which is formed by the coiling of the 10-nm fiber into a helical or solenoid structure. The exact arrangement of nucleosomes in the 30-nm fiber has been debated, but it is clear that the linker histone H1 plays a critical role in stabilizing this structure. H1 binds to the entry and exit points of the DNA on the nucleosome, locking the DNA in place and promoting the compaction of the fiber. The 30-nm fiber is then further folded into loops and domains that are anchored to a protein scaffold, ultimately forming the metaphase chromosome, which is about 700 nanometers in diameter.
The degree of compaction achieved by this hierarchy is staggering. The 2 meters of DNA in a human cell are compacted about 10,000-fold to fit into the nucleus. However, this compaction is not uniform. Different regions of the genome are packaged to different extents, and this differential packaging is functionally significant. Regions that are actively transcribed are generally less compacted, allowing access to the transcriptional machinery, while regions that are silenced are more compacted.
The nucleosome is not just a static packaging unit; its position along the DNA can influence gene expression. Nucleosomes that are positioned over promoter regions can block the binding of transcription factors, thereby repressing gene expression. Conversely, the removal or repositioning of nucleosomes can activate gene expression. This dynamic behavior is mediated by chromatin remodeling complexes, which use the energy of ATP hydrolysis to move nucleosomes along the DNA or to eject them entirely. The relationship between nucleosome positioning and gene regulation is a central theme in modern molecular biology, and it is explored further in the context of Nucleosome Chromatin organization.
Nucleosome Dynamics and Gene Regulation
The nucleosome is not a static structure; it is subject to dynamic regulation that allows cells to control access to DNA. Two major mechanisms govern nucleosome dynamics: ATP-dependent chromatin remodeling and histone post-translational modifications.
ATP-Dependent Chromatin Remodeling
Chromatin remodeling complexes are multi-protein machines that use the energy from ATP hydrolysis to alter nucleosome structure. The best-studied family of remodelers is the SWI/SNF family, named after the yeast genes SWI2/SNF2 that encode its catalytic subunit. These complexes can perform several types of reactions:
- Nucleosome sliding: The histone octamer is moved along the DNA without being ejected, changing the position of the nucleosome relative to the underlying DNA sequence. This can expose or occlude regulatory elements.
- Nucleosome ejection: The histone octamer is removed from the DNA entirely, creating a stretch of naked DNA that is accessible to transcription factors.
- Histone dimer exchange: The H2A-H2B dimers are replaced with variant histones, such as H2A.Z, which can alter nucleosome stability and function.
The mechanism of nucleosome sliding involves the transient formation of a DNA bulge on the surface of the histone octamer, which then propagates around the core, effectively moving the histone relative to the DNA. This process is processive, meaning that a single remodeling complex can move a nucleosome many base pairs in one binding event. For a detailed discussion of the sliding mechanism, see Nucleosome Sliding.
Histone Modifications
Histone post-translational modifications are covalent additions to the N-terminal tails that extend from the nucleosome core. These modifications include acetylation, methylation, phosphorylation, ubiquitination, and sumoylation, among others. Each modification is added by a specific enzyme and removed by a specific enzyme, and the combination of modifications on a given nucleosome is sometimes referred to as the "histone code."
Acetylation is the best-characterized modification. Histone acetyltransferases (HATs) add acetyl groups to lysine residues on histone tails, neutralizing the positive charge of the lysine and weakening the interaction between the histone and DNA. This generally leads to a more open chromatin structure and increased gene expression. Histone deacetylases (HDACs) reverse this modification, restoring the positive charge and promoting chromatin compaction.
Methylation is more complex because it can have different effects depending on which lysine or arginine residue is modified and how many methyl groups are added. For example, methylation of lysine 4 on histone H3 (H3K4me3) is associated with active gene promoters, while methylation of lysine 27 on histone H3 (H3K27me3) is associated with gene silencing. These modifications are recognized by specific reader proteins that bind to the modified residues and recruit additional factors to the chromatin.
The interplay between chromatin remodeling and histone modifications is complex. Remodeling complexes can create access for histone-modifying enzymes, and histone modifications can recruit remodeling complexes. This crosstalk allows cells to fine-tune gene expression in response to developmental signals and environmental cues.
Methods Used to Study Nucleosomes
Our current understanding of nucleosome structure has been built on decades of experimental work using a variety of techniques. Each method has contributed unique insights, and together they have provided a comprehensive picture of the nucleosome.
Nuclease Digestion
The discovery of the nucleosome was made possible by nuclease digestion experiments. When chromatin is treated with micrococcal nuclease, an enzyme that cuts DNA preferentially in linker regions, the DNA is cleaved into fragments that are multiples of a unit length. This "ladder" pattern, visible on an agarose gel, revealed that the DNA is protected at regular intervals by proteins. The protected fragment is about 147 base pairs, corresponding to the DNA wrapped around the histone octamer. This technique is still used today to map nucleosome positions across the genome.
X-Ray Crystallography
The first high-resolution structure of the nucleosome core particle was determined by X-ray crystallography in 1997 by Karolin Luger and colleagues. The structure, solved at 2.8 Å resolution, revealed the detailed arrangement of the histone octamer and the path of the DNA superhelix. The structure showed that the histone fold domains form a characteristic "handshake" motif and that the DNA makes extensive contacts with the histone surface. Subsequent crystal structures at higher resolution have provided even more detail, including the positions of water molecules and the precise geometry of protein-DNA interactions.
Cryo-Electron Microscopy
Cryo-electron microscopy (cryo-EM) has emerged as a powerful complementary technique, particularly for studying larger chromatin structures that are difficult to crystallize. In cryo-EM, samples are rapidly frozen in a thin layer of vitreous ice, and images are collected with an electron microscope. Computational methods are then used to reconstruct a three-dimensional structure from the two-dimensional images. Recent advances in detector technology and image processing have made it possible to determine structures at near-atomic resolution, and cryo-EM has been used to visualize nucleosomes in complex with remodeling enzymes, histone chaperones, and transcription factors.
Chromatin Immunoprecipitation
Chromatin immunoprecipitation (ChIP) is a method for determining where specific proteins are bound to DNA in living cells. Cells are treated with a crosslinking agent, such as formaldehyde, to covalently link proteins to DNA. The chromatin is then sheared into fragments, and an antibody specific to the protein of interest is used to immunoprecipitate the protein-DNA complexes. The associated DNA is then purified and analyzed, either by PCR or by high-throughput sequencing (ChIP-seq). This technique has been used to map the genome-wide positions of nucleosomes, histone modifications, and chromatin remodeling complexes.
Common Mistakes When Interpreting Nucleosome Diagrams
Students frequently make several errors when drawing or interpreting nucleosome diagrams. Being aware of these mistakes can help you avoid them.
Mistake 1: Miscounting the DNA Wraps
The most common error is drawing the DNA as making two full turns around the histone core. In reality, the DNA makes 1.65 turns. This is not a trivial distinction; the 1.65 turns mean that the entry and exit points of the DNA are not directly opposite each other but are offset. When drawing the nucleosome, count the wraps carefully. The DNA should wrap around the core about one and two-thirds times, not two full times.
Mistake 2: Confusing Histone Types
Another frequent error is misidentifying the histones. The core octamer contains two copies each of H2A, H2B, H3, and H4. Histone H1 is not part of the core octamer; it is a linker histone that binds to the entry and exit points of the DNA. Students sometimes include H1 in the core, which is incorrect. Additionally, the arrangement of the histones is not random: the H3-H4 tetramer is in the center, and the H2A-H2B dimers are at the periphery.
Mistake 3: Misrepresenting the Symmetry
The nucleosome has a dyad axis of symmetry, meaning that the two halves of the structure are related by a 180-degree rotation around a central axis. Students sometimes draw the nucleosome as having a mirror symmetry, which is incorrect. The dyad axis is a pseudo-twofold axis, not a mirror plane. When drawing the nucleosome, the H3-H4 tetramer should be positioned such that the two H3 histones are related by the dyad, and the two H2A-H2B dimers are also related by the dyad.
Mistake 4: Drawing the DNA as a Straight Line Around the Core
The DNA in a nucleosome is not a simple circle; it is a superhelix that follows a helical path around the histone core. The DNA is also not uniformly spaced from the core; it makes closer contacts at some points and is slightly farther away at others. When drawing the nucleosome, the DNA should be shown as a curved ribbon that follows the surface of the core, not as a straight line or a perfect circle.
Mistake 5: Ignoring the Histone Tails
The histone tails are often omitted from diagrams, but they are functionally important. The tails are sites of post-translational modifications and are involved in interactions between nucleosomes. Including the tails in your diagram, even as simple wavy lines, makes the diagram more accurate and demonstrates an understanding of the structure.
Mistake 6: Confusing Nucleosome and Nucleotide
A nucleosome is a protein-DNA complex, while a nucleotide is the monomeric unit of DNA and RNA. These are fundamentally different structures, and confusing them is a serious error. If you need to review the difference, see the comparison of Nucleosome vs Nucleotide and the Nucleotide Diagram for reference.
Summary and Key Takeaways
The nucleosome is the fundamental repeating unit of chromatin, consisting of 147 base pairs of DNA wrapped around an octamer of histone proteins. The structure is highly conserved across eukaryotes, reflecting its essential role in DNA packaging and gene regulation. Understanding the nucleosome is essential for understanding how genomes are organized and how gene expression is controlled.
The key features of the nucleosome are the histone octamer, composed of two copies each of H2A, H2B, H3, and H4; the DNA superhelix, which makes 1.65 left-handed turns around the core; and the linker DNA, which connects adjacent nucleosomes. The histone tails extend from the core and are subject to a wide range of post-translational modifications that influence chromatin structure and gene activity.
Nucleosomes are not static structures; they are dynamically regulated by ATP-dependent remodeling complexes and histone-modifying enzymes. These mechanisms allow cells to control access to DNA for transcription, replication, and repair. The study of nucleosomes has been advanced by techniques such as nuclease digestion, X-ray crystallography, cryo-EM, and chromatin immunoprecipitation, each of which has provided unique insights into nucleosome structure and function.
Frequently Asked Questions
What is a simple nucleosome diagram?
A simple nucleosome diagram shows a disc-shaped histone core with DNA wrapped around it. The diagram typically includes the histone octamer, the DNA superhelix making 1.65 turns, and the linker DNA extending from the entry and exit points. Histone tails are often shown as wavy lines extending from the core. This type of diagram is commonly used in textbooks and exams to illustrate the basic structure of the nucleosome.
How many DNA wraps are in a nucleosome?
The DNA in a nucleosome makes 1.65 turns around the histone octamer. This is often approximated as "about one and two-thirds turns" in diagrams and descriptions. The 147 base pairs of DNA that wrap around the core are not quite two full turns, which is why the entry and exit points of the DNA are offset from each other rather than being directly opposite.
What are the parts of a nucleosome?
A nucleosome consists of two main parts: the histone core and the DNA. The histone core is an octamer containing two copies each of histones H2A, H2B, H3, and H4. The DNA is approximately 147 base pairs long and wraps around the core in a left-handed superhelix. The nucleosome also includes linker DNA, which connects adjacent nucleosomes, and may include linker histone H1, which binds to the entry and exit points of the DNA.
Why is a nucleosome diagram important in biology?
A nucleosome diagram is important because it provides a visual representation of the fundamental unit of chromatin structure. Understanding this structure is essential for understanding how DNA is packaged in the nucleus, how genes are regulated, and how DNA replication and repair occur. Diagrams help students and researchers visualize the spatial arrangement of DNA and histones, which is critical for interpreting experimental data and for designing new experiments.
What is the difference between a nucleosome and chromatin?
A nucleosome is a single unit consisting of DNA wrapped around a histone octamer. Chromatin is the entire complex of DNA and proteins that makes up chromosomes, including all the nucleosomes, linker DNA, and associated proteins. Chromatin is the higher-order structure that results from the packaging of many nucleosomes together. In other words, the nucleosome is the repeating subunit of chromatin, and chromatin is the complete DNA-protein complex.
How do you draw a nucleosome for an exam?
To draw a nucleosome for an exam, start by drawing a flattened ellipse for the histone core. Then draw a curved line that wraps around the core about one and two-thirds times to represent the DNA. Add straight lines extending from the entry and exit points for the linker DNA. Draw a few wavy lines from the core to represent the histone tails. Label the components: histone core, DNA, linker DNA, and histone tails. If you have time, indicate the dyad axis with a dashed line and include approximate dimensions.
What is the function of linker DNA in a nucleosome diagram?
In a nucleosome diagram, linker DNA is the DNA that connects adjacent nucleosome core particles. It is not wrapped around histones and is therefore more accessible to proteins that need to bind to DNA. The length of linker DNA varies between species and cell types, and it influences the packing of nucleosomes into higher-order structures. In a diagram, linker DNA is typically shown as straight or slightly curved lines extending from the entry and exit points of the wrapped DNA.
Key Takeaways
- The nucleosome is the basic repeating unit of chromatin, composed of 147 base pairs of DNA wrapped around an octamer of histone proteins (two each of H2A, H2B, H3, and H4).
- The DNA makes 1.65 left-handed turns around the histone core, forming a superhelix with 14 periodic contact points between DNA and histones.
- Linker DNA connects adjacent nucleosomes and is more accessible to DNA-binding proteins than the wrapped DNA.
- Nucleosomes compact DNA into the 10-nm fiber, which is further folded into higher-order structures to achieve the massive compaction required to fit the genome into the nucleus.
- Nucleosomes are dynamic structures that are remodeled by ATP-dependent complexes and modified by histone-modifying enzymes to regulate access to DNA.
- Key techniques for studying nucleosomes include nuclease digestion, X-ray crystallography, cryo-EM, and chromatin immunoprecipitation.
- When drawing or interpreting nucleosome diagrams, pay attention to the number of DNA wraps, the identity and arrangement of histones, and the dyad axis of symmetry.