What Is DNA Methylation, and How Does It Affect Gene Expression?

A neuron and a muscle cell do very different things. One participates in electrical signaling; the other contracts to produce movement. They have different shapes, different responsibilities, and different collections of proteins that make their specialized functions possible.

Yet, in an ordinary mammalian body, these two cells contain essentially the same DNA. Almost all the genetic instructions needed to build either kind of cell are present in both.

How can the same genetic information produce such different results?

Part of the answer is gene regulation. Cells do not use every gene in their genomes equally. Instead, they express some genes at high levels, keep others relatively quiet, and adjust their activity according to their identity and circumstances. When biologists say a gene is expressed, they mean that its information is being used to produce RNA, which in many cases provides the instructions for making a protein.

Cells have several interacting mechanisms for regulating this process. One involves something rather remarkable: making small chemical changes to DNA itself, without changing the sequence of genetic instructions.

This process is called DNA methylation.

DNA methylation is one of the molecular mechanisms studied under epigenetics, a field concerned with how cells regulate and sometimes maintain patterns of gene activity without altering the underlying DNA sequence. Not every epigenetic modification is permanent or inherited, but some can contribute to regulatory states that persist as cells divide.

Understanding methylation begins with a surprisingly small change to an ordinary DNA base.

A small chemical addition to DNA

DNA is built from four principal chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Their order along a DNA molecule forms the sequence that carries genetic information.

In mammals, the most common form of DNA methylation involves adding a small chemical group to cytosine.

A methyl group consists of one carbon atom bonded to three hydrogen atoms (CH₃). During DNA methylation, this group is attached to the fifth carbon position of cytosine's molecular ring. The result is a modified base called 5-methylcytosine, often abbreviated 5mC.

This addition does not change the identity of the base in the genetic sequence. The modified cytosine still pairs with guanine in the DNA double helix, and the underlying order of DNA bases remains unchanged.

That is an important distinction. A mutation alters the DNA sequence itself, perhaps by replacing one base with another. DNA methylation instead changes the chemical properties of a base already present. Proteins interacting with that region of DNA may respond differently to the modification, even though the sequence of genetic instructions has not been rewritten.

In mammalian DNA, methylation occurs predominantly at locations called CpG sites. A CpG is a cytosine followed by a guanine along the same DNA strand. The lowercase p represents the phosphate linkage connecting the two bases. It does not refer to the C–G base pair formed between opposite strands of the DNA double helix.

Some regions of the genome contain unusually high concentrations of CpG sites. These are called CpG islands, and many are found near the beginnings of genes. Their importance becomes clearer when we examine how methylation affects transcription.

Cells use enzymes called DNA methyltransferases, or DNMTs, to add methyl groups to DNA. The enzymes transfer a methyl group from a donor molecule called S-adenosylmethionine (SAM) to a cytosine base. Two enzymes called DNMT3A and DNMT3B play important roles in establishing new patterns of methylation.

The essential chemistry is straightforward: an enzyme attaches a methyl group to cytosine, producing a modified DNA base without changing the nucleotide sequence.

The question is why that small chemical addition matters.

From a chemical modification to gene regulation

For many genes, expression begins with transcription, the process by which cellular machinery uses DNA as a template to produce an RNA molecule. In the case of protein-coding genes, this RNA can subsequently be used to make a protein.

Transcription does not occur indiscriminately. Cells regulate when genes are transcribed and how much RNA they produce. Among the proteins involved are transcription factors, which recognize particular DNA sequences and help control the activity of genes.

An important region in this process is the promoter, a stretch of DNA associated with the beginning of transcription. Promoters help determine where transcription begins and provide sites where regulatory proteins can interact with the DNA.

Now consider what happens when cytosines within or near a promoter become methylated.

For some transcription factors, the presence of methyl groups changes how effectively they can recognize and bind to their DNA targets. If methylation interferes with the binding of proteins that would otherwise help initiate transcription, the corresponding gene may become less active.

But methylation can influence transcription in another way.

Cells contain proteins that specifically recognize methylated DNA. Some of these methylation-reading proteins recruit other molecular machinery that alters the local environment around a gene.

To understand why this matters, we need to consider how DNA is organized inside a cell.

DNA is not simply a long, exposed molecule floating inside the nucleus. It is associated with proteins, particularly histones, around which sections of DNA are wrapped. Together, DNA and its associated proteins form a material called chromatin.

The organization of chromatin influences how readily proteins can interact with particular DNA regions. Methylation-reading proteins can recruit machinery that modifies histones or changes the organization of chromatin, sometimes helping maintain conditions that are unfavorable for transcription.

In this way, DNA methylation can contribute to gene repression through several interconnected mechanisms. It may interfere with the binding of certain transcription factors, or it may help recruit other proteins that reinforce a less transcriptionally active state.

The methyl group itself does not physically lock a gene away. Instead, it changes the molecular characteristics of DNA in ways that other proteins can recognize and respond to.

This is also why DNA methylation cannot be understood as entirely independent of other epigenetic mechanisms. Chemical modifications to histones and molecular machinery that reorganizes chromatin can interact with methylation, sometimes reinforcing one another. These processes are distinguishable, but cells frequently use them together.

For an introduction, however, the key principle is simple: adding a methyl group to DNA can change how regulatory proteins interact with that region, and those changes can influence transcription.

Why methylation is not simply an off switch

DNA methylation is often described as a mechanism that turns genes off.

There is a good reason for that description. In many circumstances, methylation of a gene's promoter is associated with transcriptional repression, and methylation can help establish or maintain a state in which a gene is largely inactive.

But the description becomes misleading if we treat it as a universal rule.

The relationship between methylation and gene expression depends strongly on where the methylation occurs.

Many promoters contain CpG islands that are normally unmethylated. When certain CpG-rich promoters become heavily methylated, the corresponding genes may be strongly repressed.

Yet an unmethylated promoter does not guarantee that a gene will be actively transcribed. Genes can be inactive for many other reasons, even when their promoters contain little or no DNA methylation.

Meanwhile, methylation is also frequently found within the bodies of actively transcribed genes. A gene body is the portion of a gene that is transcribed into RNA, as distinct from nearby regulatory regions such as its promoter.

This is an important contrast. Methylation within a gene does not necessarily indicate that the gene has been silenced.

Researchers have investigated whether gene-body methylation influences processes such as the use of alternative transcription start sites or the processing of RNA. Some specific mechanisms have experimental support, but their significance varies across biological contexts. The presence of gene-body methylation alone does not establish what effect, if any, it is having on transcription.

There is also the question of cause and effect.

Sometimes methylation directly contributes to reducing transcription. In other situations, a gene may become inactive through other regulatory processes, with methylation subsequently helping to stabilize its repressed state.

Observing that a methylated region is associated with low gene expression does not, by itself, establish that methylation originally caused the gene to become inactive.

These distinctions are central to Peter Jones's review of DNA methylation across different genomic regions, and to more recent research examining how cells interpret methylated DNA.

The useful generalization, then, is not that methylation always turns genes off. It is that DNA methylation changes the regulatory properties of particular DNA regions, with consequences that depend on their location and molecular context.

That still leaves an important question unanswered. If methylation patterns influence gene regulation, how does a cell establish those patterns, and what happens to them when the DNA is copied?

How methylation patterns are established, maintained, and changed

DNA methylation is not distributed randomly across the genome. Different regions contain different levels of methylation, and different cell types can have distinct patterns reflecting their developmental histories and regulatory needs.

Some patterns are established by enzymes such as DNMT3A and DNMT3B. This is called de novo methylation, meaning that new methylation is added rather than copied from a preexisting pattern.

Where these enzymes act depends partly on their interactions with chromatin and other regulatory proteins. They do not independently decide which genes a cell should express. Instead, DNA methylation patterns emerge as part of a larger system of gene regulation.

Once established, some of those patterns can persist through cell division.

This is where the molecular arrangement of CpG sites becomes especially important.

DNA consists of two complementary strands. When a CpG sequence occurs on one strand, the opposing strand also contains a CpG at the corresponding position when each strand is read in its usual direction. As a result, the corresponding cytosines on both strands can be methylated.

That symmetry provides the basis for a remarkable maintenance mechanism.

Before a cell divides, it must copy its DNA. During replication, the two strands of the original DNA molecule separate, and each serves as a template for a new complementary strand.

The resulting DNA molecules each contain one original strand and one newly synthesized strand.

Now imagine that a CpG site was methylated on both strands before replication.

The original strand retains its methylated cytosine. But the newly synthesized strand does not automatically receive the corresponding methyl group merely because the DNA sequence has been copied.

For a time, the resulting DNA is hemimethylated: methylation is present on the original strand but absent from the corresponding cytosine on the new strand.

Cells have machinery capable of recognizing and correcting this asymmetry.

An enzyme called DNMT1 plays a central role in maintaining DNA methylation patterns. Working with other proteins, including one called UHRF1, DNMT1 helps restore methylation to the newly synthesized strand at previously methylated CpG sites.

The original methylation pattern can therefore guide the establishment of a corresponding pattern on newly copied DNA.

This mechanism is not perfectly automatic or infallible. Maintenance can be incomplete, and other molecular processes can alter the pattern. Nevertheless, it provides a way for some chemical modifications to persist through successive rounds of DNA replication.

The ability to maintain methylation patterns does not mean they are permanent.

Methylation can be lost when the maintenance machinery does not restore it during DNA replication. If this happens over repeated cell divisions, existing methylation can gradually become diluted. This is called passive demethylation.

Cells also possess enzymatic pathways that can contribute to removing DNA methylation through chemical modification of the affected bases.

An important family of enzymes involved in these pathways is called TET. TET enzymes oxidize 5-methylcytosine, producing modified forms including 5-hydroxymethylcytosine. Some of these products can undergo further processing that ultimately restores an unmodified cytosine, while others may persist and have regulatory significance of their own.

This is not a matter of simply pulling an intact methyl group off DNA. It involves additional chemical reactions, sometimes followed by DNA repair or replication-dependent processes.

The mechanisms of methylation establishment, maintenance, and removal are examined in detail in Wei and Wu's review of mammalian DNA methylome dynamics.

Together, these processes give cells two complementary capabilities: they can preserve certain methylation patterns over time, and they can change those patterns when circumstances require it.

That combination helps explain why DNA methylation is important not only for regulating genes in the present, but sometimes for preserving regulatory decisions made in the past.

When gene regulation becomes cellular memory

We began with an observation about cell identity: neurons and muscle cells contain essentially the same genetic information, yet they use that information differently.

Their differences arise through complex interactions among transcription factors, chromatin organization, DNA methylation, and other regulatory mechanisms. DNA methylation is not solely responsible for establishing or maintaining a cell's identity.

But its ability to preserve certain chemical patterns through DNA replication makes it one mechanism by which established regulatory states can persist.

A particularly striking example is X-chromosome inactivation.

In humans and many other placental mammals, most cells with two X chromosomes largely silence one of them. This helps balance the expression of X-linked genes between cells with different numbers of X chromosomes.

The process begins during early development and involves several interacting mechanisms. An RNA molecule called XIST helps initiate chromosome-wide silencing, while changes to chromatin contribute to establishing the inactive state.

DNA methylation becomes part of this regulatory system. Methylation at many promoters on the inactive X chromosome helps maintain their repression after the silenced state has been established.

As cells divide, their descendants generally preserve the identity of the inactive X chromosome. Not every gene on that chromosome is silenced, and DNA methylation is only one component of the maintenance machinery. Nevertheless, the example illustrates how an established pattern of gene regulation can persist through many generations of cells without requiring a change to the underlying DNA sequence.

This is one meaning of epigenetic memory: the preservation of a regulatory state through cell divisions without an accompanying alteration of the genetic sequence.

It is important not to take the idea too far.

Not every methylation mark constitutes a durable memory. Some patterns change during development or in response to changes within cells, and some methylation differences may have little direct influence on gene expression. Simply observing a methylated cytosine does not establish that a stable regulatory state exists.

Nor should inheritance through ordinary cell division be confused with inheritance between generations of organisms. Mammalian reproduction and early development involve extensive reorganization of DNA methylation patterns. Some specialized marks can survive particular stages of this process, but the transmission of environmentally acquired methylation changes across generations is a separate and substantially more qualified phenomenon.

DNA methylation is therefore both simpler and more versatile than the familiar idea of a chemical switch that turns genes on or off.

At the molecular level, it begins with a small addition to a DNA base. At the level of gene regulation, it changes how proteins can interact with particular regions of DNA. And because cells can establish, maintain, and modify methylation patterns, it can contribute to regulatory states that endure without altering the DNA sequence.

The same genome can be used in different ways, and some of those ways of using it can persist.

DNA methylation is one part of the molecular machinery that makes this possible.

Further Reading

• Jones, P. A. (2012). Functions of DNA methylation: islands, start sites, gene bodies and beyond. Nature Reviews Genetics, 13, 484–492. A foundational review explaining why methylation has different relationships with gene expression depending on its genomic location.

• Greenberg, M. V. C., & Bourc'his, D. (2019). The diverse roles of DNA methylation in mammalian development and disease. Nature Reviews Molecular Cell Biology, 20, 590–607. A broad review of methylation patterns, their regulation, and their biological functions.

• Wei, A., & Wu, H. (2022). Mammalian DNA methylome dynamics: mechanisms, functions and new frontiers. Development, 149(24), dev182683. An open-access review of how methylation patterns are established, maintained, and modified, including their interactions with other epigenetic systems.

• Buckley, T. J., et al. (2026). Gene regulatory mechanisms downstream of DNA methylation. Nature Reviews Genetics. A contemporary review of the molecular machinery that recognizes DNA methylation and connects it to transcriptional regulation.

For a more detailed exploration of the concluding example, see Jacobson, Pandya-Jones, and Plath (2022), A lifelong duty: how Xist maintains the inactive X chromosome.