# Nucleosome vs Nucleotide: Key Differences in DNA Structure

DNA is a deceptively simple molecule that performs two fundamentally different jobs inside the cell. At the chemical level, it is a linear polymer of nucleotides—small molecular units that encode genetic information through their sequence. At the structural level, it is a highly condensed chromatin fiber, organized into repeating protein–DNA complexes called nucleosomes. Undergraduate students often conflate these two terms because both relate to DNA, but they operate at entirely different scales and serve entirely different purposes. A nucleotide is a single molecular building block; a nucleosome is a macromolecular assembly containing roughly 150 base pairs of DNA wrapped around a core of eight histone proteins. This article dissects the chemical composition, structural organization, size, and function of each, and clarifies the relationship between the two.

## Introduction to DNA Building Blocks and Packaging

[The central dogma of molecular biology](/blog/news/the-central-dogma-of-molecular-biology)—DNA to RNA to protein—depends on the information encoded in the nucleotide sequence of DNA. But the genome of a eukaryotic cell is enormous: a single human cell contains approximately 6.4 billion base pairs of DNA, which, if stretched end to end, would measure about 2 meters. This DNA must fit inside a nucleus that is typically 5 to 10 micrometers in diameter. The solution to this packaging problem is a hierarchical compaction system, at the heart of which lies the nucleosome.

It is essential to recognize that nucleotides and nucleosomes are not competing concepts. They are complementary components of a single system. Nucleotides are the monomers that polymerize to form DNA and RNA. Nucleosomes are the repeating structural units of eukaryotic chromatin, formed when DNA—itself a chain of nucleotides—wraps around histone proteins. Understanding the distinction requires grasping both the chemistry of nucleic acids and the biophysics of chromatin.

### What is a Nucleotide?

A nucleotide is an organic molecule composed of three covalently linked components: a nitrogenous base, a five-carbon sugar, and one or more phosphate groups. Nucleotides are the monomeric units of nucleic acids. When nucleotides are linked together via phosphodiester bonds, they form DNA or RNA polymers. The sequence of nucleotides along a DNA strand constitutes the genetic code. Each nucleotide is roughly 0.34 nanometers in length along the DNA axis, and its molecular weight is approximately 300 to 500 daltons, depending on the base and the number of phosphate groups.

### What is a Nucleosome?

A nucleosome is the fundamental repeating unit of chromatin in eukaryotic cells. It consists of a segment of DNA, approximately 147 base pairs long, wrapped around a protein core made of eight histone proteins—two copies each of H2A, H2B, H3, and H4. The [nucleosome core particle](/knowledge/molecular-biology/nucleosome-core-particle), together with a short segment of linker DNA connecting adjacent nucleosomes, forms a structure that resembles "beads on a string" under the electron microscope. The nucleosome is not a sequence-specific structure; it can form on virtually any DNA sequence, although certain sequences and histone variants influence its positioning and stability. Its primary function is to compact DNA and to regulate access to the genetic information encoded within it.

## Nucleotide: The Monomeric Unit of Nucleic Acids

Nucleotides are the fundamental chemical units of nucleic acids. Without nucleotides, there is no DNA, no RNA, and no genetic information. Their structure is highly conserved across all domains of life, reflecting their ancient and essential role in biology.

### Chemical Composition

Each nucleotide consists of three distinct components linked by specific covalent bonds:

1. **Nitrogenous base**: A nitrogen-containing aromatic heterocycle. In DNA, the bases are adenine (A), guanine (G), cytosine (C), and thymine (T). In RNA, uracil (U) replaces thymine. The bases are classified into two groups: purines (adenine and guanine), which have a double-ring structure, and pyrimidines (cytosine, thymine, and uracil), which have a single-ring structure.

2. **Pentose sugar**: A five-carbon monosaccharide. DNA contains 2'-deoxyribose, which lacks a hydroxyl group at the 2' carbon. RNA contains ribose, which has a hydroxyl group at the 2' position. This single chemical difference has profound consequences: the 2'-hydroxyl group in RNA makes it more chemically reactive and less stable than DNA, and it also affects the three-dimensional structure of the molecule.

3. **Phosphate group**: One or more phosphate groups attached to the 5' carbon of the sugar. The phosphate group is negatively charged at physiological pH, giving nucleic acids their overall acidic character. Nucleotides can exist as nucleoside monophosphates (NMPs), nucleoside diphosphates (NDPs), or nucleoside triphosphates (NTPs), depending on the number of phosphate groups.

The base is attached to the 1' carbon of the sugar via a β-N-glycosidic bond, forming a nucleoside. When a phosphate group is added to the 5' carbon, the nucleoside becomes a nucleotide. For example, adenine plus ribose is adenosine; adenosine plus one phosphate is adenosine monophosphate (AMP). The triphosphate forms—such as ATP, GTP, CTP, and TTP—are the substrates for nucleic acid synthesis.

### Types of Nucleotides

Nucleotides are categorized based on the sugar they contain and the base they carry:

- **Deoxyribonucleotides**: The building blocks of DNA. They contain 2'-deoxyribose and one of four bases: adenine, guanine, cytosine, or thymine. The corresponding deoxynucleoside triphosphates (dATP, dGTP, dCTP, dTTP) are used by DNA polymerases during replication.

- **Ribonucleotides**: The building blocks of RNA. They contain ribose and one of four bases: adenine, guanine, cytosine, or uracil. The ribonucleoside triphosphates (ATP, GTP, CTP, UTP) are used by RNA polymerases during transcription.

- **Cyclic nucleotides**: Specialized signaling molecules such as cyclic AMP (cAMP) and cyclic GMP (cGMP), in which the phosphate group forms a ring with the sugar. These function as second messengers in signal transduction pathways.

- **Nucleotide coenzymes**: Molecules such as NAD⁺, FAD, and coenzyme A, which contain nucleotide moieties and participate in redox reactions and acyl transfer reactions.

### Functions Beyond Genetic Material

Nucleotides are not merely passive building blocks. They serve multiple essential functions in cellular metabolism:

- **Energy currency**: ATP is the primary energy carrier in cells. Hydrolysis of the terminal phosphoanhydride bond releases approximately 30.5 kJ/mol under standard conditions, driving endergonic reactions.

- **Signaling**: cAMP and cGMP act as second messengers. GTP-bound G proteins are molecular switches in signal transduction cascades.

- **Coenzymes**: NAD⁺ and FAD carry electrons in metabolic pathways such as glycolysis, the citric acid cycle, and oxidative phosphorylation. Coenzyme A carries acyl groups in fatty acid metabolism.

- **Allosteric regulators**: ATP, ADP, and AMP regulate key metabolic enzymes. For example, phosphofructokinase-1, the rate-limiting enzyme of glycolysis, is activated by AMP and inhibited by ATP.

- **Activated intermediates**: UDP-glucose and CDP-diacylglycerol are activated precursors for glycogen and phospholipid synthesis, respectively.

The dual role of nucleotides—as genetic building blocks and as metabolic regulators—underscores their centrality in cellular biochemistry. For a deeper treatment of how nucleotide damage is repaired, see the [Nucleotide Excision Repair](/knowledge/molecular-biology/nucleotide-excision-repair) pathway, which removes bulky DNA lesions such as pyrimidine dimers.

## Nucleosome: The Fundamental Packaging Unit of Chromatin

While nucleotides are the chemical units of DNA, nucleosomes are the structural units of chromatin. In eukaryotic cells, DNA does not exist as a naked double helix; it is always associated with proteins, primarily histones, to form chromatin. The nucleosome is the first and most fundamental level of this packaging.

### Histone Proteins

Histones are small, highly basic proteins rich in lysine and arginine residues. These positively charged amino acids interact electrostatically with the negatively charged phosphate backbone of DNA. There are five major classes of histones:

- **Core histones**: H2A, H2B, H3, and H4. These form the octameric core of the nucleosome. Each core histone has a structured histone-fold domain, consisting of three α-helices connected by two loops, and a flexible N-terminal tail that extends outward from the nucleosome.

- **Linker histone**: H1. This histone binds to the linker DNA between nucleosomes and to the entry/exit site of DNA on the nucleosome core, stabilizing higher-order chromatin folding.

The histone octamer is a tripartite assembly: an H3–H4 tetramer forms the central core, flanked by two H2A–H2B dimers. The assembly of the octamer is a stepwise process. During DNA replication, histone chaperones such as CAF-1 and ASF1 deposit new histones onto nascent DNA. The H3–H4 tetramer is deposited first, followed by the two H2A–H2B dimers.

### DNA Wrapping and Linker DNA

The [nucleosome core particle](/knowledge/molecular-biology/nucleosome-core-particle) consists of 147 base pairs of DNA wrapped in 1.65 left-handed superhelical turns around the histone octamer. The DNA is bent sharply, with a radius of curvature of approximately 4.2 nanometers, which is much tighter than the persistence length of free DNA (about 50 nanometers). This extreme bending is stabilized by multiple contacts between the DNA phosphate backbone and the histone surface. Approximately 14 direct contacts are made between the histone octamer and the DNA, each involving hydrogen bonds and salt bridges with the phosphate groups.

The DNA sequence does not determine nucleosome positioning absolutely, but certain sequences are more favorable. For example, the dinucleotide repeat AA/TT/TA occurs with a periodicity of about 10 base pairs in nucleosomal DNA, reflecting the helical repeat of DNA. This sequence preference arises because certain dinucleotides bend more easily in the direction required for wrapping.

Adjacent nucleosomes are connected by linker DNA, which varies in length from 10 to 80 base pairs depending on the organism and cell type. The linker histone H1 binds to the linker DNA at the entry and exit points of the nucleosome, locking the DNA in place and promoting compaction into higher-order structures. The resulting "beads on a string" fiber is approximately 10 nanometers in diameter. Further folding, mediated by histone tails and linker histones, produces a 30-nanometer fiber, although the exact structure of this fiber in vivo remains debated. For a detailed structural model, see the [Nucleosome Model](/knowledge/molecular-biology/nucleosome-model).

### Role in Gene Regulation

Nucleosomes are not merely passive packaging material; they are dynamic regulators of gene expression. The positioning of nucleosomes along the genome determines which DNA sequences are accessible to [transcription factors](/knowledge/molecular-biology/transcription-factor) and RNA polymerase. Nucleosomes generally repress transcription by occluding promoter and enhancer elements. However, cells have evolved mechanisms to remodel or evict nucleosomes in a regulated manner.

Three major classes of [chromatin remodelers](/knowledge/molecular-biology/chromatin-remodelers) use the energy of ATP hydrolysis to alter nucleosome position or composition:

1. **SWI/SNF family**: Slides or evicts nucleosomes, creating nucleosome-free regions at promoters. The human SWI/SNF complex, also known as BAF, is frequently mutated in cancer.

2. **ISWI family**: Slides nucleosomes to regular spacing, promoting chromatin assembly and transcriptional repression.

3. **CHD family**: Contains chromodomains that recognize methylated histone tails. CHD remodelers can slide or evict nucleosomes depending on context.

In addition to ATP-dependent remodeling, histones are subject to a wide array of post-translational modifications, including acetylation, methylation, phosphorylation, and ubiquitination. These modifications occur predominantly on the N-terminal tails and are catalyzed by enzymes such as histone acetyltransferases (HATs), histone deacetylases (HDACs), histone methyltransferases (HMTs), and demethylases. Acetylation of lysine residues neutralizes the positive charge on histone tails, weakening their interaction with DNA and promoting an open chromatin state. Methylation can either activate or repress transcription depending on the specific lysine residue and the degree of methylation. For example, trimethylation of lysine 4 on histone H3 (H3K4me3) is associated with active promoters, while trimethylation of lysine 27 on histone H3 (H3K27me3) is associated with Polycomb-mediated repression.

The combination of histone modifications and nucleosome positioning constitutes the epigenetic landscape, which modulates gene expression without altering the underlying DNA sequence. This topic is explored further in the context of [Nucleosome Chromatin](/knowledge/molecular-biology/nucleosome-chromatin) organization.

## Size and Scale: Comparing Nucleotide and Nucleosome

The difference in scale between a nucleotide and a nucleosome is staggering. Comparing their dimensions and molecular weights clarifies why they occupy such different positions in the hierarchy of DNA organization.

### Molecular Dimensions

| Feature | Nucleotide | Nucleosome Core Particle |
|---|---|---|
| Molecular weight | ~300–500 Da | ~206 kDa (DNA + histones) |
| Diameter | ~0.34 nm (along DNA axis) | ~11 nm |
| Height | ~1 nm | ~5.7 nm |
| DNA content | 1 base | 147 base pairs |
| Number of protein components | 0 | 8 histone proteins |
| Charge | Negative (phosphate) | Positive (histone lysine/arginine) |

A single nucleotide is a small organic molecule, invisible even under an electron microscope. A nucleosome, by contrast, is a macromolecular complex with a mass of approximately 206 kilodaltons—about 400 to 600 times heavier than a single nucleotide. The nucleosome core particle is a disk-shaped structure, 11 nanometers in diameter and 5.7 nanometers in height, with the DNA wrapped around its circumference.

### Hierarchy from Nucleotide to Chromosome

The relationship between nucleotides and nucleosomes is hierarchical. The organization of DNA in a eukaryotic chromosome proceeds through several distinct levels:

1. **Nucleotide level**: DNA is a double helix, 2 nanometers in diameter, with a helical repeat of 10.5 base pairs per turn. The rise per base pair is 0.34 nanometers, giving a total length of approximately 0.34 micrometers per kilobase pair.

2. **Nucleosome level**: 147 base pairs of DNA wrap around the histone octamer, producing a 10-nanometer fiber. This compacts the DNA by a factor of approximately 6-fold.

3. **30-nanometer fiber**: Nucleosomes are folded into a higher-order structure, often depicted as a solenoid or zigzag helix. This compacts the DNA by an additional 7-fold, for a total compaction of about 40-fold.

4. **Loop domains**: The 30-nanometer fiber is organized into loops of 50 to 200 kilobase pairs, anchored to the nuclear matrix or to CTCF/cohesin complexes. This compacts the DNA by another 20-fold.

5. **Metaphase chromosome**: During mitosis, the loop domains are further condensed into the characteristic X-shaped chromosome, achieving a total compaction of approximately 10,000-fold.

At each level, the fundamental unit is the nucleosome. Without nucleosomes, the 2-meter human genome could not fit into a nucleus, and the regulated access to genetic information would be impossible.

## Functional Roles: Genetic Information vs Chromatin Dynamics

Nucleotides and nucleosomes serve fundamentally different functions. Nucleotides encode information; nucleosomes regulate access to that information. Understanding this distinction is critical for interpreting experiments and for understanding disease mechanisms.

### Information Storage

The sequence of nucleotides along a DNA strand is the genetic code. This sequence is read by RNA polymerase during transcription and by ribosomes during translation. The information content is determined by the order of the four bases, which can be thought of as a four-letter alphabet. A gene encoding a protein of 300 amino acids requires a coding sequence of 900 nucleotides (plus regulatory elements). The human genome contains approximately 3.1 billion base pairs, of which about 1.5% codes for proteins. The rest includes regulatory elements, introns, noncoding RNAs, and repetitive sequences.

The fidelity of nucleotide sequence is maintained by DNA polymerases, which have error rates of approximately 10⁻⁵ to 10⁻⁶ per base pair per replication cycle. Proofreading activity reduces this to about 10⁻⁸, and mismatch repair further reduces it to approximately 10⁻¹⁰. When these systems fail, mutations accumulate, potentially leading to cancer or genetic disease. The [Nucleotide Sequence](/knowledge/molecular-biology/nucleotide-sequence) is thus the primary determinant of phenotype.

### Epigenetic Regulation

Nucleosomes provide a layer of regulation above the nucleotide sequence. The position and modification state of nucleosomes determine whether a given DNA sequence is accessible to the transcriptional machinery. This is the basis of epigenetics: heritable changes in gene expression that do not involve changes to the DNA sequence itself.

Histone modifications are written by "writer" enzymes, read by "reader" proteins, and removed by "eraser" enzymes. For example, the bromodomain is a reader module that binds acetylated lysine residues. Proteins containing bromodomains, such as BRD4, recruit transcriptional activators to acetylated promoters. Conversely, the chromodomain of HP1 binds methylated lysine 9 on histone H3 (H3K9me3), promoting heterochromatin formation and gene silencing.

Nucleosome positioning also affects DNA replication. The replication machinery must disassemble nucleosomes ahead of the replication fork and reassemble them behind it. Histone chaperones such as FACT (facilitates chromatin transcription) and CAF-1 mediate this process. Failure to properly reassemble nucleosomes after replication can lead to loss of epigenetic marks and aberrant gene expression.

### DNA Replication and Transcription

Both DNA replication and transcription require access to the nucleotide sequence, which is blocked by nucleosomes. The cell has evolved multiple strategies to overcome this barrier:

- **Nucleosome remodeling**: ATP-dependent remodelers such as SWI/SNF slide or evict nucleosomes at promoters and origins of replication. For example, the SWI/SNF complex is recruited to the yeast HO promoter to create a nucleosome-free region that allows binding of the transcription factor Swi5p.

- **Histone chaperones**: FACT promotes transcription elongation by destabilizing nucleosomes ahead of RNA polymerase II and reassembling them behind it. FACT is particularly important for transcription through long genes, such as the human dystrophin gene, which spans 2.4 million base pairs.

- **Histone modifications**: Acetylation of histone tails by HATs such as Gcn5 and p300 neutralizes the positive charge on lysine residues, weakening histone–DNA interactions and promoting chromatin decondensation. This is why histone deacetylase inhibitors, such as trichostatin A, are used experimentally to reactivate silenced genes.

- **Replication-coupled nucleosome assembly**: During DNA replication, the parental histones are distributed to the two daughter strands, and new histones are deposited by CAF-1. The pattern of histone modifications is copied to the new histones, maintaining the epigenetic state.

The interplay between nucleotide sequence and nucleosome dynamics is particularly evident at promoters. Many promoters contain poly(dA:dT) tracts, which are intrinsically stiff and resist [nucleosome formation](/knowledge/molecular-biology/nucleosome-formation). These sequences create nucleosome-free regions that allow constitutive access to transcription factors. In contrast, inducible genes often have nucleosomes positioned over their promoters, requiring active remodeling for activation.

## Methods to Study Nucleotides and Nucleosomes

The study of nucleotides and nucleosomes requires different experimental approaches, reflecting their different scales and properties.

### DNA Sequencing

DNA sequencing determines the order of nucleotides in a DNA molecule. The Sanger method, developed in 1977, uses chain-terminating dideoxynucleotides to generate a series of fragments that differ in length by one nucleotide. These fragments are separated by capillary electrophoresis, and the sequence is read from the resulting electropherogram. Modern high-throughput sequencing platforms, such as Illumina sequencing, use sequencing-by-synthesis with fluorescently labeled reversible terminators. A typical Illumina run can generate hundreds of millions of reads, each 150 base pairs long, enabling whole-genome sequencing at a cost of less than $1,000 per human genome.

### Chromatin Immunoprecipitation (ChIP)

ChIP is used to determine where specific proteins, such as histones or transcription factors, bind to DNA in vivo. The procedure involves the following steps:

1. **Crosslinking**: Cells are treated with formaldehyde, which crosslinks proteins to DNA. Typical conditions are 1% formaldehyde for 10 minutes at room temperature.

2. **Cell lysis and sonication**: Cells are lysed, and chromatin is sheared by sonication to fragments of 200 to 600 base pairs.

3. **Immunoprecipitation**: An antibody specific to the protein of interest (e.g., anti-H3K4me3) is added, along with protein A/G beads. The antibody–protein–DNA complexes are pulled down.

4. **Reverse crosslinking**: The crosslinks are reversed by heating at 65°C for 4 to 6 hours, and the DNA is purified.

5. **Analysis**: The purified DNA is analyzed by quantitative PCR (qPCR), microarray (ChIP-chip), or high-throughput sequencing (ChIP-seq).

ChIP-seq is widely used to map nucleosome positions and histone modifications genome-wide. For example, a typical ChIP-seq experiment for H3K4me3 will show sharp peaks at active promoters, while H3K27me3 will show broad domains at repressed genes.

### [X-ray Crystallography](/knowledge/molecular-biology/x-ray-crystallography) and Cryo-EM

The high-resolution structure of the nucleosome was determined by X-ray crystallography. The first crystal structure of the nucleosome core particle, published in 1997 by Karolin Luger and colleagues, was solved at 2.8 Å resolution. The structure revealed the detailed interactions between the histone octamer and the 147 base pairs of DNA, including the 14 direct contacts between the histone fold domains and the DNA phosphate backbone.

Cryo-electron microscopy (cryo-EM) has become the method of choice for studying larger chromatin structures, such as the 30-nanometer fiber and nucleosome arrays. Cryo-EM allows the determination of structures at near-atomic resolution without the need for crystallization. Recent cryo-EM structures of the nucleosome in complex with [chromatin remodelers](/knowledge/molecular-biology/chromatin-remodelers), such as SWI/SNF and RSC, have revealed the conformational changes that occur during nucleosome sliding and ejection.

## Common Misconceptions and Pitfalls

Students frequently confuse nucleotides and nucleosomes. The following are the most common errors and their clarifications.

### Confusing Nucleotide with Nucleoside

A nucleotide is a nucleoside plus one or more phosphate groups. A nucleoside consists only of a nitrogenous base and a sugar. For example, adenosine is a nucleoside; adenosine monophosphate (AMP) is a nucleotide. The distinction matters because only nucleotides can be polymerized into nucleic acids. Nucleosides are not incorporated into DNA or RNA.

### Thinking Nucleosome is a DNA Sequence

A nucleosome is not a sequence of DNA; it is a protein–DNA complex. The DNA within a nucleosome is a sequence of nucleotides, but the nucleosome itself is defined by its structure, not its sequence. Nucleosomes can form on almost any DNA sequence, although some sequences are preferred. The term "nucleosome" refers to the entire complex of DNA plus histones.

### Overlooking the Role of Histones

Some students think of the nucleosome as simply "DNA wrapped around a protein." This understates the complexity of the histone octamer. The eight histone proteins are not a generic protein blob; they are a precisely organized assembly with specific contacts to the DNA. The histone tails, which extend outward, are the targets of numerous post-translational modifications that regulate chromatin function. Without histones, there is no nucleosome, and without nucleosomes, there is no chromatin.

### Assuming Nucleosomes Exist in Prokaryotes

Nucleosomes are a eukaryotic feature. Prokaryotes do not have histones or nucleosomes. Instead, their DNA is organized by histone-like proteins such as HU and H-NS, which bind DNA without forming regular repeating structures. This difference is important for understanding the distinct regulatory mechanisms in prokaryotes and eukaryotes.

### Confusing Nucleosome with Chromatosome

A chromatosome is a nucleosome plus one molecule of linker histone H1 bound to the entry/exit site of the DNA. The chromatosome protects approximately 166 base pairs of DNA from nuclease digestion, compared to 147 base pairs for the core particle alone. The H1 histone is not part of the nucleosome core particle.

## Practical Summary: Key Takeaways for Exams

The following table and memory aids summarize the most important distinctions between nucleotides and nucleosomes.

### Comparison Table

| Feature | Nucleotide | Nucleosome |
|---|---|---|
| Definition | Monomeric unit of nucleic acids | Fundamental packaging unit of chromatin |
| Composition | Base + sugar + phosphate | 147 bp DNA + histone octamer (H2A, H2B, H3, H4)₂ |
| Molecular weight | ~300–500 Da | ~206 kDa |
| Size | ~0.34 nm | ~11 nm diameter |
| Location | Throughout the cell | Nucleus (eukaryotes only) |
| Function | Encodes genetic information; energy metabolism; signaling | Compacts DNA; regulates gene expression |
| Number per human genome | ~6.4 billion | ~30 million |
| Charge | Negative | Positive (histones) |
| Synthesized by | DNA/RNA polymerases | Histone chaperones + chromatin remodelers |

### Memory Aids

- **"Nucleotide = ONE letter"**: A nucleotide is a single unit, like a letter in a word. A nucleosome is a whole word or sentence, containing many letters.

- **"Nucleosome = DNA + Histones"**: If you see a protein component, it is a nucleosome. Nucleotides contain no proteins.

- **"147 base pairs"**: The number of base pairs in a nucleosome core particle is a frequently tested fact. Remember it as "one-four-seven."

- **"Nucleotides build; nucleosomes package"**: Nucleotides are the building blocks; nucleosomes are the packaging units.

- **"Histones are basic, DNA is acidic"**: The positive charge of histones (due to lysine and arginine) interacts with the negative phosphate backbone of DNA.

## Frequently Asked Questions

### What is the difference between a nucleotide and a nucleosome?

A nucleotide is a small organic molecule consisting of a nitrogenous base, a five-carbon sugar, and one or more phosphate groups. It is the monomeric unit of DNA and RNA. A nucleosome is a large protein–DNA complex consisting of 147 base pairs of DNA wrapped around an octamer of histone proteins (two each of H2A, H2B, H3, and H4). Nucleotides encode genetic information; nucleosomes package and regulate access to that information.

### Is a nucleosome made of nucleotides?

Yes, but only indirectly. The DNA component of a nucleosome is a polymer of nucleotides. The nucleosome also contains histone proteins, which are not made of nucleotides. So a nucleosome contains nucleotides (in its DNA) but is not itself a nucleotide.

### How many nucleotides are in a nucleosome?

A nucleosome core particle contains 147 base pairs of DNA, which corresponds to 147 nucleotides on each strand, or 294 nucleotides total. If you include the linker DNA between nucleosomes, the total is higher, typically 160 to 200 base pairs per nucleosome repeat unit.

### Which is larger: a nucleotide or a nucleosome?

A nucleosome is vastly larger. A single nucleotide has a molecular weight of approximately 300 to 500 daltons and a length of about 0.34 nanometers. A nucleosome core particle has a molecular weight of approximately 206 kilodaltons and a diameter of about 11 nanometers. The nucleosome is roughly 400 to 600 times heavier than a nucleotide.

### Are nucleotides and nucleosomes both found in the nucleus?

Nucleosomes are found exclusively in the nucleus of eukaryotic cells, because that is where chromatin is located. Nucleotides are found throughout the cell, including the nucleus, cytoplasm, and mitochondria. Nucleotides in the nucleus are used for DNA replication and transcription; cytoplasmic nucleotides participate in metabolism and signaling.

### What is the function of a nucleosome?

The nucleosome has two primary functions. First, it compacts DNA, allowing the 2-meter human genome to fit into a nucleus that is only 5 to 10 micrometers in diameter. Second, it regulates access to the genetic information. Nucleosome positioning and histone modifications determine whether a given DNA sequence is accessible to transcription factors, RNA polymerase, and the DNA replication machinery.

### What are the three parts of a nucleotide?

The three parts of a nucleotide are: (1) a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil), (2) a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and (3) one or more phosphate groups attached to the 5' carbon of the sugar.

## Key Takeaways

- A nucleotide is the monomeric unit of nucleic acids, composed of a nitrogenous base, a pentose sugar, and a phosphate group. It encodes genetic information.
- A nucleosome is the fundamental repeating unit of chromatin, composed of 147 base pairs of DNA wrapped around a histone octamer (two each of H2A, H2B, H3, and H4).
- Nucleotides are ~0.34 nm in length and ~300–500 Da in mass; nucleosomes are ~11 nm in diameter and ~206 kDa in mass.
- Nucleosomes exist only in eukaryotes; prokaryotes use different DNA-binding proteins for compaction.
- Nucleosomes regulate gene expression through positioning, histone post-translational modifications, and ATP-dependent remodeling.
- DNA sequencing determines nucleotide order; ChIP-seq, X-ray crystallography, and cryo-EM are used to study nucleosome positioning and structure.
- The most common exam error is confusing a nucleotide with a nucleoside (which lacks the phosphate group) or thinking a nucleosome is a DNA sequence rather than a protein–DNA complex.

## Related Clinical & Scientific Guides

* [MAPK Pathway: Mechanism, Function, and Clinical Relevance](/knowledge/molecular-biology/mapk-pathway)
* [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)