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 Modification: How Cells Control Gene Activity

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

Every human cell contains roughly two meters of DNA, yet that DNA must fit inside a nucleus that is only about six micrometers across. The solution to this packaging problem is chromatin—the complex of DNA and proteins that organizes the genome into a compact, manageable form. But chromatin is far more than a storage solution. The way DNA is wrapped around proteins determines which genes are accessible to the cellular machinery that reads them. Chemical modifications to chromatin—collectively called chromatin modification—provide cells with a dynamic system for turning genes on and off without changing the underlying DNA sequence.

What Is Chromatin Modification?

Chromatin modification refers to chemical changes made to DNA or to the histone proteins around which DNA is wound. These modifications alter the physical structure of chromatin, making genes either more or less accessible to the transcription machinery. Because these changes affect gene activity without altering the DNA sequence itself, they are a central mechanism of epigenetic regulation—regulation that is "above" or "on top of" the genome.

Chromatin structure and packaging

To understand chromatin modification, you first need to understand chromatin structure. The fundamental repeating unit of chromatin is the nucleosome. Each nucleosome consists of approximately 147 base pairs of DNA wrapped around a core of eight histone proteins—two copies each of H2A, H2B, H3, and H4. Histones are small, positively charged proteins rich in lysine and arginine residues. Their positive charge allows them to bind tightly to the negatively charged phosphate backbone of DNA.

Nucleosomes are connected by short stretches of linker DNA, forming a structure that resembles beads on a string. This 10-nanometer fiber is the first level of compaction. Further folding produces a 30-nanometer fiber, and higher-order structures eventually compact the DNA into the metaphase chromosome visible during cell division. For a deeper look at how these levels of organization work, see Chromatin Structure and Nucleosome Chromatin.

The degree of compaction is not uniform across the genome. Some regions are tightly packed and transcriptionally silent, while others are loosely packed and actively transcribed. This difference is not static—cells can remodel chromatin in response to developmental signals, environmental cues, and metabolic demands.

The role of modifications in gene expression

Chromatin modification is the primary mechanism by which cells control the accessibility of genes. When chromatin is tightly packed, transcription factors and RNA polymerase cannot reach the DNA, and genes remain silent. When chromatin is relaxed, these proteins can bind and initiate transcription.

The key insight is that these modifications are reversible and dynamic. Enzymes add chemical groups to DNA and histones, and other enzymes remove them. This allows cells to respond quickly to changing conditions. A muscle cell and a neuron contain the same DNA, but they express different sets of genes because their chromatin is modified differently. Once established, these patterns can be inherited through cell division, allowing daughter cells to maintain their identity.

Types of Chromatin Modifications

There are three major categories of chromatin modification: DNA methylation, histone modification, and ATP-dependent chromatin remodeling. Each operates through distinct mechanisms and has different effects on gene expression.

DNA methylation

DNA methylation is the addition of a methyl group (–CH₃) to the carbon-5 position of cytosine residues, producing 5-methylcytosine. This reaction is catalyzed by enzymes called DNA methyltransferases (DNMTs). The methyl group is donated by S-adenosylmethionine (SAM), the universal methyl donor in cells.

In mammals, DNA methylation occurs predominantly at CpG dinucleotides—cytosine residues followed by guanine in the DNA sequence. Regions of the genome that are rich in CpG dinucleotides are called CpG islands. Approximately 60–70% of human gene promoters contain CpG islands, and methylation of these regions is strongly associated with transcriptional repression.

There are three main DNMTs in mammals:

DNMT1 is the maintenance methyltransferase. During DNA replication, it recognizes hemimethylated DNA (where the parental strand is methylated but the newly synthesized strand is not) and adds methyl groups to the daughter strand. This ensures that methylation patterns are faithfully copied to daughter cells.
DNMT3A and DNMT3B are de novo methyltransferases. They establish new methylation patterns during development and differentiation, adding methyl groups to previously unmethylated CpG sites.

DNA methylation represses transcription through two main mechanisms. First, methylated cytosines physically interfere with the binding of certain transcription factors. Second, methylated DNA recruits methyl-CpG-binding domain (MBD) proteins, such as MeCP2, which in turn recruit histone-modifying enzymes that compact chromatin.

Histone modifications

Histone modifications are covalent chemical changes to the amino acid residues of histone proteins, particularly in their N-terminal "tail" regions that protrude from the nucleosome core. These tails are subject to a remarkable variety of modifications, including acetylation, methylation, phosphorylation, ubiquitination, and sumoylation. For a comprehensive overview, see Histone Modification.

Acetylation is the addition of an acetyl group (–COCH₃) to lysine residues. This reaction is catalyzed by histone acetyltransferases (HATs) and reversed by histone deacetylases (HDACs). Acetylation neutralizes the positive charge on lysine, weakening the electrostatic interaction between histones and DNA. This causes chromatin to relax, generally promoting transcription. The classic example is acetylation of lysine 9 on histone H3 (H3K9ac) and lysine 16 on histone H4 (H4K16ac), both associated with active gene promoters.

Methylation is the addition of one, two, or three methyl groups to lysine or arginine residues. Unlike acetylation, methylation does not change the charge of the amino acid. Its effect depends on which residue is modified and how many methyl groups are added. Histone methyltransferases (HMTs) add methyl groups, and histone demethylases remove them. Some methylation marks are associated with activation, such as H3K4me3 (trimethylation of lysine 4 on H3), which is found at active gene promoters. Others are associated with repression, such as H3K9me3 and H3K27me3, which are hallmarks of silenced chromatin.

Phosphorylation is the addition of a phosphate group (–PO₄²⁻) to serine, threonine, or tyrosine residues. This modification adds a large, negatively charged group that can alter chromatin structure and create binding sites for other proteins. Phosphorylation of H3S10 (serine 10 on H3) is associated with chromosome condensation during mitosis and with the activation of immediate-early genes.

Ubiquitination is the attachment of ubiquitin, a 76-amino-acid protein, to lysine residues. Monoubiquitination of H2A at lysine 119 (H2AK119ub) is associated with gene silencing, while monoubiquitination of H2B at lysine 120 (H2BK120ub) is associated with active transcription. Ubiquitination is reversed by deubiquitinases (DUBs).

The complexity of histone modifications has led to the "histone code" hypothesis, which proposes that combinations of modifications on a given nucleosome or genomic region determine the functional state of that region. Different modifications recruit different effector proteins, creating a sophisticated regulatory system.

ATP-dependent chromatin remodeling

ATP-dependent chromatin remodeling is distinct from chemical modification. Rather than adding or removing chemical groups, chromatin remodelers use the energy from ATP hydrolysis to physically move, eject, or restructure nucleosomes. These multi-protein complexes are called Chromatin Remodelers, and the process they carry out is Chromatin Remodeling.

There are four major families of chromatin remodeling complexes in eukaryotes:

SWI/SNF (mating-type switching/sucrose non-fermenting) complexes slide or eject nucleosomes, creating nucleosome-free regions at promoters and enhancers. They generally promote transcription.
ISWI (imitation SWI) complexes slide nucleosomes along DNA without ejecting them, often promoting nucleosome spacing and chromatin assembly.
CHD (chromodomain-helicase-DNA binding) complexes slide or eject nucleosomes and can also recognize methylated histones.
INO80 (inositol requiring 80) complexes are involved in nucleosome exchange and DNA repair.

These remodelers contain an ATPase subunit that uses the energy from ATP hydrolysis to transiently break histone–DNA contacts and move the nucleosome. The direction and extent of movement depend on the specific complex and its associated regulatory subunits.

How Chromatin Modifications Affect Gene Expression

Chromatin modifications influence gene expression by controlling the physical accessibility of DNA to the transcription machinery. The genome can be broadly divided into two states: euchromatin and heterochromatin.

Euchromatin vs. heterochromatin

Euchromatin is the loosely packed, transcriptionally active form of chromatin. It is characterized by:

Low levels of DNA methylation
High levels of histone acetylation (H3K9ac, H3K27ac)
H3K4me3 at promoters
Nucleosome-free regions at transcription start sites
Sensitivity to digestion by DNase I

Heterochromatin is the tightly packed, transcriptionally silent form of chromatin. It is characterized by:

High levels of DNA methylation
Low levels of histone acetylation
High levels of H3K9me3 and H3K27me3
Binding of heterochromatin protein 1 (HP1)
Resistance to DNase I digestion

Heterochromatin can be further divided into constitutive heterochromatin, which is permanently silenced (such as centromeres and telomeres), and facultative heterochromatin, which is silenced in some cell types but active in others (such as developmentally regulated genes).

The transition between these states is governed by the coordinated action of chromatin modifiers. For example, the addition of H3K9me3 by the methyltransferase SUV39H1 creates a binding site for HP1. HP1 then recruits additional SUV39H1, spreading the repressive mark along the chromatin fiber. This positive feedback loop ensures robust silencing.

Recruitment of transcription factors and repressors

Chromatin modifications affect gene expression not only by changing chromatin structure but also by recruiting specific proteins. Acetylated lysine residues are recognized by bromodomain-containing proteins, many of which are components of transcription activator complexes. Methylated lysine residues are recognized by chromodomain-containing proteins, Tudor domain proteins, and PHD finger proteins.

For example, the bromodomain protein BRD4 binds to acetylated histones at enhancers and promoters, recruiting the positive transcription elongation factor b (P-TEFb), which phosphorylates RNA polymerase II to promote transcriptional elongation. Conversely, the polycomb repressive complex 1 (PRC1) recognizes H3K27me3 marks deposited by PRC2, leading to chromatin compaction and gene silencing.

The interplay between activators and repressors creates a dynamic equilibrium. Transcription factors can recruit chromatin modifiers to specific genomic locations, and the resulting modifications can either stabilize or counteract the effects of other factors. This allows cells to integrate multiple signals and fine-tune gene expression.

Examples of Chromatin Modification in Action

Chromatin modification is not an abstract concept—it underlies some of the most fundamental processes in biology.

X-inactivation

Female mammals have two X chromosomes, while males have one X and one Y. To equalize X-linked gene expression between the sexes, female cells randomly inactivate one X chromosome in a process called X-chromosome inactivation. This process is initiated by the long non-coding RNA XIST, which coats the future inactive X chromosome and recruits chromatin modifiers.

The inactive X chromosome becomes densely packed with repressive marks, including:

H3K27me3, deposited by PRC2
H3K9me3
DNA methylation at CpG islands
Depletion of active marks such as H3K4me3 and histone acetylation

The result is a highly condensed structure called a Barr body, visible under the microscope. Once established, the inactive state is stably maintained through cell division, ensuring that all daughter cells inactivate the same X chromosome.

Genomic imprinting

Genomic imprinting is a phenomenon where certain genes are expressed only from the maternal or paternal allele, not both. This is achieved through differential DNA methylation established during gamete formation.

The classic example is the insulin-like growth factor 2 (IGF2) gene and its receptor IGF2R. The IGF2 gene is expressed only from the paternal allele, while the IGF2R gene is expressed only from the maternal allele. This is controlled by an imprinting control region (ICR) that is methylated on one parental chromosome but not the other.

On the unmethylated maternal allele, the ICR binds the insulator protein CTCF, which blocks the interaction between the IGF2 promoter and its enhancer, preventing expression. On the methylated paternal allele, CTCF cannot bind, allowing the enhancer to activate the IGF2 promoter. This example illustrates how a single methylation mark can have opposite effects on different genes depending on the context.

Stem cell differentiation

Stem cells are defined by two properties: self-renewal and the ability to differentiate into multiple cell types. Chromatin modifications play a central role in maintaining pluripotency and in directing differentiation.

In embryonic stem cells, the promoters of many developmental genes are marked with both H3K4me3 (an activating mark) and H3K27me3 (a repressive mark). These "bivalent domains" keep genes poised for activation while preventing premature expression. When a stem cell commits to a particular lineage, the appropriate bivalent domains resolve: genes needed for the chosen lineage lose H3K27me3 and become active, while genes for other lineages lose H3K4me3 and become permanently silenced.

The transcription factors OCT4, SOX2, and NANOG maintain pluripotency by binding to enhancers and recruiting chromatin modifiers that maintain an open chromatin state. During differentiation, these factors are downregulated, and lineage-specific transcription factors take over, establishing new chromatin states.

How Scientists Study Chromatin Modifications

Studying chromatin modifications requires specialized techniques that can identify where modifications occur in the genome and how they change under different conditions.

Chromatin immunoprecipitation (ChIP)

Chromatin immunoprecipitation is the gold standard for mapping histone modifications and DNA-binding proteins. The basic protocol involves:

Crosslinking: Cells are treated with formaldehyde to covalently crosslink proteins to DNA.
Shearing: The chromatin is fragmented by sonication or enzymatic digestion to produce fragments of approximately 200–600 base pairs.
Immunoprecipitation: An antibody specific to the modification of interest (e.g., anti-H3K4me3) is used to pull down chromatin fragments containing that modification.
Reverse crosslinking: The crosslinks are reversed by heating, and the DNA is purified.
Analysis: The purified DNA can be analyzed by quantitative PCR (ChIP-qPCR) to examine specific loci, or by high-throughput sequencing (ChIP-seq) to map modifications genome-wide.

ChIP-seq requires careful controls, including an input sample (total chromatin before immunoprecipitation) and often an IgG control (a non-specific antibody). The sequencing data are aligned to the reference genome, and peaks of enrichment indicate regions where the modification is present.

DNA methylation analysis

DNA methylation can be analyzed by several methods. The gold standard is bisulfite sequencing. Treatment of DNA with sodium bisulfite converts unmethylated cytosines to uracil, while methylated cytosines are protected and remain as cytosine. After PCR amplification, the converted uracils are read as thymines, allowing the methylation status of each CpG to be determined by comparing the sequence to the reference genome.

Whole-genome bisulfite sequencing (WGBS) provides single-base resolution of methylation across the entire genome but is expensive. Reduced representation bisulfite sequencing (RRBS) enriches for CpG-rich regions, reducing cost while still providing genome-wide coverage of promoters and CpG islands.

Alternative methods include methylation-specific PCR (MSP), which uses primers that distinguish methylated from unmethylated DNA, and array-based methods such as the Illumina Infinium MethylationEPIC array, which measures methylation at approximately 850,000 CpG sites.

Accessibility assays

Chromatin accessibility assays measure how open the chromatin is at different genomic locations. The most widely used method is ATAC-seq (Assay for Transposase-Accessible Chromatin using sequencing).

The protocol is remarkably simple:

Cells are lysed to release nuclei.
The nuclei are incubated with the hyperactive Tn5 transposase, which simultaneously fragments accessible DNA and ligates sequencing adapters.
The tagged DNA fragments are purified, amplified by PCR, and sequenced.

Regions of open chromatin produce more fragments than regions of closed chromatin, allowing genome-wide mapping of accessible regions. ATAC-seq requires only 500–50,000 cells, making it suitable for rare cell populations.

DNase-seq and MNase-seq are older methods that use DNase I or micrococcal nuclease, respectively, to fragment chromatin. DNase-seq maps DNase I hypersensitive sites (open chromatin), while MNase-seq maps nucleosome positions by digesting linker DNA and sequencing the protected nucleosome-associated DNA.

The Role of Chromatin Modifications in Disease

Errors in chromatin modification contribute to a wide range of human diseases, from cancer to developmental disorders. Understanding these mechanisms has opened new avenues for therapy.

Cancer epigenetics

Cancer was traditionally viewed as a genetic disease caused by mutations in oncogenes and tumor suppressor genes. We now know that epigenetic changes are equally important. Cancer cells often have:

Global DNA hypomethylation: Loss of methylation at repetitive sequences and transposons, leading to genomic instability.
Promoter hypermethylation: Silencing of tumor suppressor genes such as p16INK4a (CDKN2A), BRCA1, and MLH1.
Altered histone modifications: Changes in H3K4me3, H3K9me3, and H3K27me3 patterns that disrupt normal gene regulation.

For example, the tumor suppressor gene p16INK4a is silenced by promoter hypermethylation in many cancers, including melanoma, colorectal cancer, and non-small cell lung cancer. This silencing prevents cells from entering senescence or apoptosis, contributing to uncontrolled proliferation.

Mutations in chromatin-modifying enzymes are also common in cancer. The histone methyltransferase EZH2, which deposits H3K27me3, is overexpressed or mutated in lymphoma, prostate cancer, and breast cancer. Mutations in the SWI/SNF complex subunits ARID1A and SMARCA4 are found in ovarian, gastric, and lung cancers.

Epigenetic drugs

The reversibility of chromatin modifications makes them attractive therapeutic targets. Several epigenetic drugs are already in clinical use:

DNMT inhibitors: 5-azacitidine and decitabine are nucleoside analogs that incorporate into DNA and trap DNMTs, leading to their degradation. They are used to treat myelodysplastic syndromes and acute myeloid leukemia.
HDAC inhibitors: Vorinostat and romidepsin inhibit histone deacetylases, increasing histone acetylation and reactivating silenced genes. They are used to treat cutaneous T-cell lymphoma.
EZH2 inhibitors: Tazemetostat inhibits EZH2 and is approved for epithelioid sarcoma and follicular lymphoma.
IDH inhibitors: Ivosidenib and enasidenib target mutant isocitrate dehydrogenase, which produces the oncometabolite 2-hydroxyglutarate that inhibits demethylases.

These drugs are often used in combination with conventional chemotherapy or immunotherapy, and ongoing research aims to identify biomarkers that predict which patients will respond.

Common Misconceptions and Pitfalls

Chromatin modification is a complex field, and several misconceptions are common among newcomers.

Myth: All DNA methylation silences genes

While promoter methylation is generally repressive, methylation in other contexts can have different effects. Methylation within gene bodies is actually associated with active transcription. Methylation at enhancers can either activate or repress depending on the context. Furthermore, some CpG methylation is required for normal gene expression—for example, methylation of the CTCF binding site at the IGF2/H19 locus is necessary for paternal allele expression.

Myth: Epigenetic changes are irreversible

Although some modifications are stable through cell division, all known chromatin modifications are reversible. Enzymes exist to remove every modification: demethylases remove methyl groups from DNA and histones, deacetylases remove acetyl groups, and phosphatases remove phosphate groups. The reversibility of these modifications is what makes epigenetic therapy possible.

Myth: Chromatin modification is the only mechanism of gene regulation

Chromatin modification is one layer of regulation, but gene expression is controlled at many levels. Transcription factor binding, RNA polymerase activity, mRNA stability, translation efficiency, and protein degradation all contribute to the final level of gene expression. Chromatin modifications often work in concert with these other mechanisms rather than acting alone.

Pitfall: Confusing correlation with causation

Many studies show that a particular modification is associated with a gene expression state, but association does not prove causation. For example, H3K4me3 is found at active promoters, but it is not always required for transcription. Determining the functional role of a modification requires perturbation experiments, such as deleting the enzyme that deposits the mark or using targeted epigenetic editing.

Pitfall: Overlooking cell-to-cell variability

Chromatin modifications are often studied in bulk cell populations, which averages out cell-to-cell variability. Single-cell techniques have revealed that epigenetic states are heterogeneous even within a seemingly uniform cell population. This variability can be functionally important, contributing to phenomena such as drug resistance in cancer.

Summary and Key Takeaways

Chromatin modification is a fundamental mechanism of gene regulation that allows cells to control which genes are expressed without changing the DNA sequence. The three main types of modification—DNA methylation, histone modification, and ATP-dependent chromatin remodeling—work together to create a dynamic chromatin landscape that responds to developmental and environmental signals.

The clinical importance of chromatin modification is now well established. Errors in these processes contribute to cancer, developmental disorders, and many other diseases, and drugs that target chromatin modifiers are already improving patient outcomes. As our understanding deepens, chromatin modification will likely become an even more important target for therapy.

Frequently Asked Questions

What is chromatin modification?

Chromatin modification is the process by which cells chemically alter DNA or histone proteins to change the structure of chromatin. These changes affect whether genes are accessible to the transcription machinery, thereby controlling gene expression without altering the DNA sequence itself.

What are the types of chromatin modification?

The three main types are DNA methylation (addition of methyl groups to cytosine bases), histone modification (acetylation, methylation, phosphorylation, ubiquitination, and other changes to histone proteins), and ATP-dependent chromatin remodeling (physical movement or ejection of nucleosomes).

Can you give examples of chromatin modification?

Examples include methylation of CpG islands in gene promoters, acetylation of histone H3 lysine 9 (H3K9ac), trimethylation of histone H3 lysine 4 (H3K4me3) at active promoters, and trimethylation of histone H3 lysine 27 (H3K27me3) at silenced genes. X-chromosome inactivation and genomic imprinting are biological processes that depend on chromatin modification.

How does chromatin modification affect gene expression?

Chromatin modifications affect gene expression by changing the physical accessibility of DNA. Activating modifications such as histone acetylation relax chromatin, allowing transcription factors and RNA polymerase to bind. Repressive modifications such as DNA methylation and H3K9me3 compact chromatin, blocking access to the transcription machinery.

What is a chromatin modification diagram?

A chromatin modification diagram typically shows a nucleosome with histone tails extending outward, with symbols indicating different modifications (e.g., Ac for acetylation, Me for methylation, P for phosphorylation). The diagram may also show the relationship between open euchromatin and closed heterochromatin, and how modifications recruit different proteins.

Are chromatin modifications permanent?

No. All chromatin modifications are reversible. Enzymes that add modifications (writers) are balanced by enzymes that remove them (erasers). This reversibility allows cells to respond to signals and is the basis for epigenetic therapies.

Why are chromatin modifications important?

Chromatin modifications are essential for normal development, cell differentiation, and maintaining cellular identity. They allow cells with identical DNA to express different genes, and they enable rapid responses to environmental changes. Errors in chromatin modification contribute to many diseases, making them important therapeutic targets.

Key Takeaways

Chromatin modification is a dynamic, reversible system for controlling gene accessibility without changing the DNA sequence.
DNA methylation, histone modification, and ATP-dependent chromatin remodeling are the three main types of chromatin modification.
Activating modifications (e.g., histone acetylation, H3K4me3) open chromatin, while repressive modifications (e.g., DNA methylation, H3K9me3, H3K27me3) compact it.
Chromatin modification underlies fundamental processes including X-chromosome inactivation, genomic imprinting, and stem cell differentiation.
Techniques such as ChIP-seq, bisulfite sequencing, and ATAC-seq allow scientists to map chromatin modifications genome-wide.
Errors in chromatin modification contribute to cancer and other diseases, and epigenetic drugs targeting these processes are already in clinical use.
Chromatin modifications are reversible and are only one layer of the complex regulatory network that controls gene expression.