What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Chromatin in a Cell: Structure, Function, and Dynamics

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Chromatin in a Cell

Chromatin is the complex of DNA and proteins that fills the nucleus of every eukaryotic cell. In a single human cell, roughly two meters of DNA must fit into a nucleus that is typically 5–10 micrometers in diameter. This extraordinary compaction is achieved not by random folding but through a highly organized, hierarchical packaging system built around histone proteins. The term "chromatin" was coined by Walther Flemming in 1882, who observed that certain nuclear material stained strongly with basic dyes—a property that reflects the high phosphate content of DNA.

Chromatin is not merely a storage solution. It is the functional template for all DNA-templated processes, including transcription, replication, and repair. The packaging state of chromatin determines whether genes are accessible to the transcriptional machinery or sequestered in an inactive state. This regulatory layer sits above the genetic code itself, meaning that two cells with identical DNA sequences can express entirely different sets of genes depending on how their chromatin is organized. Understanding chromatin structure is therefore essential for understanding development, cellular differentiation, and disease.

The fundamental repeating unit of chromatin is the nucleosome, which consists of approximately 147 base pairs of DNA wrapped around an octamer of histone proteins. Between nucleosomes lies linker DNA, which is associated with linker histones such as H1. This beads-on-a-string arrangement is the first level of compaction, reducing the linear length of DNA by about six-fold. Further folding produces higher-order structures that achieve the thousands-fold compaction observed in metaphase chromosomes.

Importantly, chromatin is dynamic. Nucleosomes can slide, be evicted, or be exchanged with variant histones. Histone proteins carry post-translational modifications that recruit effector proteins and alter chromatin compaction. ATP-dependent remodeling complexes actively reposition nucleosomes, and histone chaperones assemble and disassemble nucleosomes during replication and transcription. This dynamic behavior allows cells to respond rapidly to developmental cues, environmental stress, and DNA damage.

Levels of Chromatin Organization

Nucleosome Core Particle

The nucleosome core particle is the fundamental repeating unit of chromatin. It comprises 147 base pairs of DNA wrapped in 1.65 left-handed superhelical turns around a histone octamer. The octamer contains two copies each of histones H2A, H2B, H3, and H4. The structure was solved at atomic resolution by Karolin Luger and colleagues in 1997, revealing that the histone fold domains—a conserved three-helix motif—mediate histone–histone interactions and form the protein surface against which DNA wraps.

The DNA double helix contacts the histone octamer at 14 distinct binding sites, each separated by about 10 base pairs. These contacts occur primarily through the phosphodiester backbone, with arginine residues of histones inserting into the minor groove of DNA. This arrangement means that the nucleosome is largely sequence-independent in its binding, although certain sequences with high AT content in the minor groove, such as those found in centromeric DNA, can position nucleosomes more stably.

The wrapping of DNA around the octamer imposes severe constraints on the DNA. The helical repeat of DNA on the nucleosome surface is approximately 10.2 base pairs per turn, compared to 10.5 base pairs per turn in free B-form DNA. This underwinding creates torsional stress that must be managed by topoisomerases during transcription and replication. The nucleosome also occludes the access of DNA-binding proteins to their recognition sites, which is why nucleosome positioning is a critical determinant of gene regulation.

Histone Tails and Linker DNA

Each core histone has an N-terminal tail that extends outward from the nucleosome core. These tails, which range from 15 to 35 amino acids in length, are rich in basic residues (lysine and arginine) and are the primary sites of post-translational modification. The tails of H3 and H4 are particularly important; they protrude through the DNA superhelix and can contact adjacent nucleosomes, promoting higher-order folding. The H4 N-terminal tail, for example, forms a key interaction with an acidic patch on the surface of the H2A–H2B dimer of a neighboring nucleosome, an interaction that stabilizes chromatin compaction.

Linker DNA connects adjacent nucleosome core particles. Its length varies between species and cell types, typically ranging from 10 to 80 base pairs. The linker histone H1 binds to the entry and exit points of DNA on the nucleosome and to the linker DNA itself, stabilizing the nucleosome and promoting compaction. H1 binding is dynamic; it is exchanged frequently and its phosphorylation state modulates its affinity for chromatin. During mitosis, hyperphosphorylation of H1 weakens its binding, contributing to the dramatic reorganization of chromatin into condensed chromosomes.

The next level of organization is the 30-nanometer fiber. This structure forms when nucleosome arrays fold under physiological ionic conditions. Two models have been proposed: the solenoid model, in which nucleosomes are arranged in a single helical stack, and the zigzag model, in which nucleosomes alternate between two stacks connected by linker DNA. In vitro studies support the zigzag model, but the existence of a regular 30-nm fiber in vivo remains controversial. Cryo-electron tomography of native chromatin suggests that the 30-nm fiber may be rare in cells, with chromatin existing instead as a disordered, dynamic array of nucleosome clutches. This has led to a revised model in which chromatin is organized into topologically associating domains (TADs) and compartments, rather than a regular fiber.

Histone Proteins and Post-Translational Modifications

The five histone families—H1, H2A, H2B, H3, and H4—are among the most conserved proteins in eukaryotes. Histones H3 and H4 are particularly invariant; the H4 sequence differs by only two amino acids between peas and cows. This conservation reflects the precise structural constraints of nucleosome assembly. However, histone variants exist for H2A and H3 that confer specialized functions. H2A.Z is enriched at promoters and is associated with gene activation, while H3.3 is deposited at actively transcribed regions. The centromere-specific variant CENP-A (in humans) marks the site of kinetochore assembly.

Histone post-translational modifications (PTMs) are covalent additions to specific amino acid residues, predominantly on the N-terminal tails but also on the globular domains. These modifications alter chromatin structure in two ways: they can directly change the electrostatic properties of histones, and they can create binding sites for effector proteins that recognize specific modifications. The latter mechanism, known as the histone code hypothesis, posits that combinations of PTMs encode regulatory information.

Histone Acetylation and Deacetylation

Acetylation is the addition of an acetyl group to the ε-amino group of lysine residues, catalyzed by histone acetyltransferases (HATs). This modification neutralizes the positive charge of lysine, weakening the electrostatic interaction between histones and the negatively charged DNA backbone. The result is a more open, accessible chromatin structure. Acetylation is strongly associated with active transcription; promoters and enhancers of active genes are marked by acetylation of H3 lysine 27 (H3K27ac) and H3 lysine 9 (H3K9ac).

The addition of acetyl groups is reversed by histone deacetylases (HDACs). Mammalian HDACs are divided into classes I, II, and IV, which are zinc-dependent, and class III (sirtuins), which require NAD+ as a cofactor. HDAC inhibitors, such as trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA, also called vorinostat), are used experimentally to study the role of acetylation and clinically as anti-cancer drugs. SAHA is approved for the treatment of cutaneous T-cell lymphoma.

Acetyl-lysine is recognized by bromodomain-containing proteins. The bromodomain is a conserved module of approximately 110 amino acids that binds acetylated lysine with micromolar affinity. The BET family of bromodomain proteins (BRD2, BRD3, BRD4, and BRDT) plays critical roles in transcriptional elongation and is a target of the investigational drug JQ1, which displaces BET proteins from chromatin and suppresses oncogenic transcription programs.

Histone Methylation and Phosphorylation

Methylation occurs on lysine and arginine residues. Unlike acetylation, methylation does not alter the charge of the residue. Instead, it functions primarily by recruiting effector proteins. Lysine residues can be mono-, di-, or trimethylated, and each state can have distinct functional consequences. For example, trimethylation of H3 lysine 4 (H3K4me3) marks active promoters, while trimethylation of H3 lysine 27 (H3K27me3) marks silenced genes. H3K9me3 is associated with constitutive heterochromatin, and H3K36me3 is enriched in the gene bodies of actively transcribed genes.

Histone methyltransferases (HMTs) are highly specific. The SET domain family includes SUV39H1, which methylates H3K9, and EZH2, the catalytic subunit of the Polycomb repressive complex 2 (PRC2), which methylates H3K27. Histone demethylases reverse these marks. LSD1 (KDM1A) demethylates H3K4me1/2 through a flavin-dependent oxidative reaction, while the Jumonji C (JmjC) domain family, including KDM4A and KDM6A, uses an Fe(II)- and α-ketoglutarate-dependent mechanism to remove methyl groups from H3K9me3 and H3K27me3, respectively.

Phosphorylation of histone serines and threonines is catalyzed by kinases and reversed by phosphatases. The best-studied modification is phosphorylation of H3 serine 10 (H3S10ph), which is associated with both mitosis and immediate-early gene activation. During mitosis, Aurora B kinase phosphorylates H3S10, which is required for chromosome condensation and segregation. In interphase, the MAP kinase pathway can induce H3S10 phosphorylation at the promoters of genes such as FOS and JUN, contributing to their rapid induction.

Euchromatin vs. Heterochromatin

Chromatin exists in two broad states that differ in compaction, composition, and function. Euchromatin is the less condensed form and is generally transcriptionally active. It is enriched in acetylated histones, H3K4me3, and nucleosome-free regions at promoters. Euchromatin replicates early in S phase and is distributed throughout the nucleus, though it tends to occupy the nuclear interior. The decondensed state of euchromatin allows transcription factors and RNA polymerase II to access their target sequences.

Heterochromatin is more condensed and is generally transcriptionally inert. It stains densely with DNA-binding dyes and replicates late in S phase. Heterochromatin is characterized by H3K9me2/3 and H3K27me3, low levels of acetylation, and the presence of specific structural proteins such as heterochromatin protein 1 (HP1). HP1 binds H3K9me3 through its chromodomain and self-associates, promoting the spreading and maintenance of heterochromatin domains.

Heterochromatin is divided into two types. Constitutive heterochromatin is permanently condensed and includes centromeres, telomeres, and other repetitive sequences. It is marked by H3K9me3 and is maintained throughout the cell cycle. Facultative heterochromatin is conditionally silenced; it can be converted to euchromatin in response to developmental signals. The inactive X chromosome in female mammals is a classic example of facultative heterochromatin. X-inactivation is initiated by the long non-coding RNA XIST, which coats the chromosome and recruits PRC2, leading to H3K27me3 deposition and gene silencing.

The boundaries between euchromatin and heterochromatin are not absolute. Insulator elements, such as those bound by CTCF, can block the spread of heterochromatin. The chicken β-globin locus control region and the Drosophila scs/scs' elements are well-studied examples of insulator function. Additionally, some genes are poised in a bivalent state, carrying both H3K4me3 (active) and H3K27me3 (repressive) marks. These bivalent domains are common in embryonic stem cells and are resolved to a monovalent state upon differentiation.

Chromatin Remodeling and Gene Regulation

Nucleosomes are not static obstacles; they are actively repositioned, evicted, or exchanged by ATP-dependent chromatin remodeling complexes. These complexes use the energy of ATP hydrolysis to alter histone–DNA contacts. 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 confer specialized functions.

SWI/SNF Family

The SWI/SNF (switch/sucrose non-fermenting) family was first identified in yeast through genetic screens for mutants defective in mating-type switching and sucrose fermentation. Mammalian SWI/SNF complexes, also called BAF (BRG1/BRM-associated factor) complexes, contain either BRG1 (SMARCA4) or BRM (SMARCA2) as the catalytic ATPase. These complexes slide or evict nucleosomes, creating nucleosome-free regions at promoters and enhancers.

SWI/SNF complexes are critical for gene activation. They are recruited by sequence-specific transcription factors and by histone modifications such as H3K4me1 at enhancers. Mutations in SWI/SNF subunits are found in approximately 20% of human cancers, making them among the most frequently mutated chromatin regulators in malignancy. For example, ARID1A, a subunit of the BAF complex, is mutated in over 50% of ovarian clear cell carcinomas.

The mechanism of nucleosome sliding involves the ATPase domain translocating along the DNA, creating a DNA loop that propagates around the nucleosome. This process is directional and processive, allowing the remodeler to move nucleosomes by tens of base pairs in a single binding event. The related ISWI (imitation switch) family, which includes SNF2H and SNF2L, primarily spaces nucleosomes regularly, generating evenly spaced arrays. The CHD family, which contains chromodomains that bind methylated histones, also slides nucleosomes and is important for transcriptional regulation and DNA repair.

Histone Chaperones

Histone chaperones are proteins that bind histones and facilitate their deposition or removal from DNA. They are essential for nucleosome assembly during replication and for histone exchange during transcription. The chaperone ASF1 (anti-silencing function 1) binds H3–H4 dimers and delivers them to the replication-dependent assembly factor CAF-1 (chromatin assembly factor 1), which deposits histones onto newly synthesized DNA. The HIRA chaperone, in contrast, deposits H3.3–H4 dimers at active genes in a replication-independent manner.

Histone chaperones also remove histones. The FACT (facilitates chromatin transcription) complex, composed of SPT16 and SSRP1, destabilizes nucleosomes ahead of RNA polymerase II and reassembles them behind the polymerase. FACT is required for transcription through chromatin and is also involved in DNA replication. Its activity is regulated by phosphorylation, and its overexpression is associated with poor prognosis in several cancers.

The interplay between remodeling complexes and histone chaperones is exemplified during transcriptional activation. At the yeast PHO5 promoter, the transcription factor Pho4 recruits the SWI/SNF remodeler to evict nucleosomes, while the chaperone Asf1 facilitates the removal of H3–H4 dimers. This cooperative action creates a nucleosome-free region that allows the transcriptional machinery to assemble.

Methods to Study Chromatin Structure

Studying chromatin requires methods that can interrogate nucleosome positioning, histone modifications, chromatin accessibility, and three-dimensional organization. The following techniques are the most widely used.

MNase-seq uses micrococcal nuclease (MNase), an endo-exonuclease that preferentially cleaves linker DNA, to digest chromatin. After digestion, the protected DNA fragments, which correspond to nucleosome core particles (approximately 147 bp), are sequenced. The resulting map reveals nucleosome positions genome-wide. MNase digestion is typically performed at 37°C for 5–15 minutes with 0.1–1 U of enzyme per microgram of chromatin, and the reaction is stopped with EGTA, which chelates the calcium required for MNase activity.

ChIP-seq (chromatin immunoprecipitation followed by sequencing) identifies the genomic locations of specific histone modifications or DNA-binding proteins. Cells are treated with formaldehyde to cross-link proteins to DNA, chromatin is sheared by sonication to fragments of 200–600 bp, and an antibody against the protein of interest is used to immunoprecipitate the cross-linked complexes. After reversing the cross-links, the enriched DNA is sequenced. ChIP-seq requires high-quality antibodies; a typical experiment uses 1–10 million cells and 1–5 μg of antibody.

ATAC-seq (assay for transposase-accessible chromatin using sequencing) maps open chromatin regions. The hyperactive Tn5 transposase simultaneously cuts and ligates adapters into accessible DNA. Because Tn5 cannot access DNA wrapped in nucleosomes, the resulting sequencing reads mark nucleosome-free regions, promoters, and enhancers. ATAC-seq requires only 50,000 cells and can be performed in a single day, making it a popular alternative to DNase-seq.

Hi-C captures the three-dimensional organization of chromatin. Cells are cross-linked, chromatin is digested with a restriction enzyme, and the resulting DNA ends are filled in with biotin-labeled nucleotides. The fragments are then ligated under dilute conditions, favoring intramolecular ligation events. After shearing and biotin pull-down, the ligated junctions are sequenced. Hi-C maps reveal topologically associating domains (TADs), which are ~1 Mb regions of preferential internal interaction, and A/B compartments, which correspond to active and inactive chromatin, respectively.

Chromatin Dynamics in DNA Replication and Repair

DNA replication requires the complete disruption of chromatin structure, as the replication fork must access the DNA template. This process is coordinated by the replisome and involves the disassembly of nucleosomes ahead of the fork and their reassembly behind it. The MCM helicase unwinds the DNA, and the resulting single-stranded DNA is coated by RPA. Histones are displaced from the DNA and are transferred to the daughter strands, where they are distributed between the two sister chromatids.

The recycling of parental histones is not random. Old H3–H4 tetramers are preferentially deposited on the leading strand, while new histones are deposited on both strands. The deposition of new histones requires the histone chaperone CAF-1, which is recruited to the replication fork through its interaction with proliferating cell nuclear antigen (PCNA). The chaperone ASF1 supplies H3–H4 dimers to CAF-1. Disruption of this pathway leads to replication stress and genomic instability.

Chromatin also plays a central role in the DNA damage response. When a double-strand break occurs, the histone variant H2AX is rapidly phosphorylated at serine 139 by the kinases ATM, ATR, and DNA-PK. This phosphorylated form, called γ-H2AX, spreads over megabase-sized domains flanking the break and serves as a platform for the recruitment of DNA repair factors. The mediator protein MDC1 binds γ-H2AX through its BRCT domain and recruits the ubiquitin ligases RNF8 and RNF168, which ubiquitylate H2A and H2AX. This ubiquitination recruits BRCA1 and 53BP1, which direct repair toward homologous recombination or non-homologous end joining, respectively.

Chromatin remodeling is required for repair factors to access the break site. The INO80 and SWI/SNF complexes are recruited to damage sites and evict or slide nucleosomes to facilitate repair. The NuA4 acetyltransferase complex acetylates H4, which promotes chromatin relaxation and recruitment of repair proteins. After repair is complete, the chromatin structure must be restored, a process that involves the deposition of new histones and the removal of γ-H2AX.

Common Pitfalls and Misconceptions

Students frequently encounter several conceptual difficulties when studying chromatin. The following are the most common.

Confusing chromatin with chromosomes. Chromatin is the DNA–protein complex that exists throughout the cell cycle. Chromosomes are the discrete, highly condensed structures that form during mitosis. A chromosome is composed of chromatin; the terms are not interchangeable. During interphase, chromatin is present but individual chromosomes are not visible as distinct structures.

Assuming nucleosomes are static. Nucleosomes are dynamic structures that can slide, be evicted, or be exchanged. The position of a nucleosome can change in response to transcriptional activity or signaling. This dynamism is essential for gene regulation.

Overlooking the role of histone modifications. Histone modifications are not merely "on/off" switches. The same modification can have different effects depending on its genomic location and the combination of other modifications present. For example, H3K4me3 is activating at promoters but can be repressive at other locations. The context matters.

Misinterpreting euchromatin and heterochromatin. Euchromatin is not "always active" and heterochromatin is not "always inactive." Some genes within heterochromatin are expressed, and euchromatin contains many silenced genes. The distinction is about the potential for activity and the general compaction state, not an absolute binary.

Thinking that acetylation is the only relevant modification. Acetylation is important, but methylation, phosphorylation, ubiquitination, and sumoylation are equally critical. Each modification has distinct writers, readers, and erasers.

Ignoring the role of non-histone proteins. Chromatin structure is determined not only by histones but also by architectural proteins such as CTCF, cohesin, and HP1. These proteins organize chromatin into loops and domains.

Assuming that all nucleosomes are identical. Histone variants such as H2A.Z, H3.3, and CENP-A confer specialized functions. The presence of a variant can change nucleosome stability and the recruitment of effector proteins.

Frequently Asked Questions

What is chromatin in a cell?

Chromatin is the complex of DNA and proteins, primarily histones, that constitutes the genetic material in the nucleus of eukaryotic cells. It packages the DNA into a compact form that fits within the nucleus while remaining accessible for transcription, replication, and repair.

What is the function of chromatin in a cell?

Chromatin serves two primary functions. First, it compacts the DNA so that the entire genome can fit within the nucleus. Second, it regulates access to the DNA, controlling which genes are expressed and when. Chromatin also provides a platform for DNA repair and ensures the faithful segregation of chromosomes during cell division.

How does chromatin structure affect gene expression?

Chromatin structure determines whether transcription factors and RNA polymerase can access promoter and enhancer sequences. Open chromatin (euchromatin) is permissive for transcription, while condensed chromatin (heterochromatin) is generally repressive. Nucleosome positioning can occlude transcription factor binding sites, and histone modifications recruit activators or repressors that further modulate accessibility.

What is the difference between chromatin and chromosomes?

Chromatin is the DNA–protein complex present throughout the cell cycle. Chromosomes are the condensed, visible structures that form when chromatin is maximally compacted during mitosis and meiosis. A chromosome is one continuous DNA molecule packaged with histones; chromatin is the material from which chromosomes are made.

What are histones and why are they important?

Histones are small, highly basic proteins that package DNA into nucleosomes. The core histones (H2A, H2B, H3, H4) form the octamer around which DNA wraps, and the linker histone H1 stabilizes the nucleosome. Histones are important because they determine DNA accessibility and carry post-translational modifications that regulate gene expression.

What is a nucleosome?

A nucleosome is the fundamental repeating unit of chromatin. It consists of 147 base pairs of DNA wrapped around a histone octamer containing two copies each of H2A, H2B, H3, and H4. Nucleosomes are connected by linker DNA and are the first level of DNA compaction.

How is chromatin studied in the lab?

Chromatin is studied using techniques such as MNase-seq (nucleosome positioning), ChIP-seq (histone modifications and protein binding), ATAC-seq (chromatin accessibility), and Hi-C (three-dimensional organization). These methods provide complementary views of chromatin structure and function.

Key Takeaways

Chromatin is the DNA–protein complex in eukaryotic nuclei that packages DNA and regulates gene expression.
The nucleosome, consisting of 147 bp of DNA wrapped around a histone octamer, is the fundamental repeating unit of chromatin.
Histone post-translational modifications, including acetylation, methylation, and phosphorylation, regulate chromatin structure by altering histone–DNA interactions and recruiting effector proteins.
Euchromatin is generally accessible and transcriptionally active, while heterochromatin is condensed and repressive; heterochromatin is further divided into constitutive and facultative types.
ATP-dependent chromatin remodeling complexes such as SWI/SNF slide or evict nucleosomes, while histone chaperones assemble and disassemble nucleosomes.
Chromatin is dynamically disrupted and reassembled during DNA replication and repair, with histone chaperones and remodeling complexes coordinating these processes.
Chromatin structure is studied using MNase-seq, ChIP-seq, ATAC-seq, and Hi-C, each providing distinct information about nucleosome positioning, modifications, accessibility, and three-dimensional organization.