What Is Epigenetics? How Genes Switch On and Off

Epigenetics explains how chemical marks on DNA and its packaging proteins switch genes on or off without changing the DNA sequence, which is why cells differ.

Category: Genetics · Created: 2026-10-15 · Updated: 2026-10-15 · 4 min read

Illustration: Schematic of epigenetic mechanisms, including DNA methylation and demethylation · Image: Wahane S, Halawani D, Zhou X and Zou H, CC BY 4.0, via Wikimedia Commons.

Epigenetics is the study of how genes are switched on or off by chemical marks on DNA and on the proteins that package it, without any change to the DNA sequence. The marks decide which genes a cell reads, and some of them are copied to daughter cells when a cell divides. Nearly every cell in the body carries the same DNA, so these marks explain much of why a liver cell and a nerve cell behave so differently.

Why do cells with the same DNA behave differently?

Every cell in the human body descends from one fertilized egg, and most carry nearly identical DNA. A liver cell and a neuron still make very different proteins, because each one reads a different set of genes. The DNA is the same score in both cells. The marks decide which passages are played.

The DNA in one cell is about two metres long when stretched out, yet it fits inside a nucleus only a few micrometres across. It is wound around proteins called histones, eight to a core, forming units called nucleosomes. Two parts carry the control: the DNA bases, some of which carry small chemical groups, and the histone spools, whose tails stick out and can carry groups of their own.

Enzymes add these groups and remove them. A mark can stay in place for the whole life of a cell, which is why a cell keeps its job after it divides. Reading a gene into a working product is called gene expression, and the marks set how easily that reading can start.

What chemical marks are involved?

The three best-studied marks are listed below.

Mark	Where it sits	Usual effect on gene activity
DNA methylation	A methyl group (CH₃) on a cytosine base, often in CpG pairs	Usually silences nearby genes
Histone acetylation	An acetyl group on a histone tail	Loosens the packaging, usually allowing reading
Histone methylation	A methyl group on a histone tail	Can activate or silence, depending on the site

Methylation is the most studied mark. Enzymes called DNA methyltransferases add methyl groups to cytosine bases, and a gene whose control region is heavily methylated is usually quiet. Acetylation works differently. Histone tails carry positive charges, and DNA is negatively charged, so acetyl groups that neutralize those charges loosen the grip and let the spool unwind.

How are epigenetic marks passed on?

When a cell divides, its methylation pattern is copied onto the new DNA strand, so the daughter cells keep their job. This copying runs alongside DNA replication, which copies the sequence itself. Most marks are erased and written again during the formation of eggs and sperm and in the early embryo.

A small number of marks survive that reset. In some plants and animals, these marks affect the next generation. The evidence in people is limited and debated, and it is not a common route for ordinary traits, which still follow the patterns described by Mendelian inheritance.

The classic animal example is the agouti gene in mice. A transposable element, a mobile piece of DNA, sits in front of the gene. When the element is unmethylated, the gene is active and the coat is yellow. When it is methylated, the coat is mottled and brown. Genetically identical mice can therefore differ, and the mother's diet during pregnancy has been shown to shift the balance.

Can epigenetic changes happen in people?

Yes. Identical twins start with the same DNA and similar marks, but studies have found that their patterns drift apart over a lifetime, and the differences grow with age. Diet, smoking, infection and stress have all been linked to changes in marks, though the size and direction of each effect varies.

Cancer cells often show widespread abnormal methylation, and tumor-suppressor genes can be silenced by it. Some cancer drugs act on the enzymes that write these marks. Studies of people born during the Dutch famine of 1944 to 1945 have reported differences in DNA methylation decades later, but how much those marks matter for health is still under study.

Is epigenetics the same as genetics?

Not quite. Genetics concerns the DNA sequence, which sets which genes exist and what their products look like. Epigenetics concerns when and where those genes are used, and the two interact. Mutations can alter marks, and marks can change how a sequence variant behaves.

Editing tools show the difference. CRISPR-Cas9 cuts and changes the sequence itself. Epigenetic editing tools aim to change marks at a chosen site without cutting the DNA. The same logic explains how stem cells become specialized cells: their sequence stays the same while their marks change.