AP® Biology · Unit 6 · Topic 6.5
Epigenetic Regulation
Epigenetic regulation controls gene expression by chemically marking DNA and histones, switching genes on or off without changing the base sequence itself. These marks are reversible and explain why genetically identical cells can look and act completely differently.
Every cell in your body carries the same genome, but a skin cell and a neuron look and act nothing alike. The difference isn't in the DNA sequence, it's in which genes get read. Epigenetic marks are chemical tags added to DNA or to the proteins DNA wraps around, and they decide which genes are open for business in a given cell. Add tags that silence, and a gene goes quiet. Remove them, and it can switch back on.
DNA methylation works by attaching a methyl group (CH3) directly to cytosine bases, usually in gene promoter regions rich in CG sequences. A heavily methylated promoter blocks transcription factors and RNA polymerase from binding, so the gene gets silenced even though its sequence is untouched. Histone modification works differently: DNA wraps tightly around histone proteins to form chromatin, and adding acetyl groups to histones loosens that packing, exposing DNA so genes can be transcribed. Adding methyl groups to histones often does the opposite, tightening the wrap and hiding genes from the transcription machinery. Strip the methylation marks away, from DNA or histones, and the gene regains access to transcription factors and can turn back on, which is exactly what demethylating enzymes do during development to activate genes a cell needs.
This sits right next to transcriptional regulation: epigenetics controls whether a gene's promoter is even reachable before transcription factors ever get involved. It's also the mechanism behind cell differentiation, since epigenetic patterns, not DNA sequence changes, explain why a liver cell and a muscle cell built from the same genome stay so different.
Key terms
- DNA methylation
- Addition of a methyl group to cytosine bases, typically silencing nearby genes by blocking transcription machinery.
- Histone
- A protein that DNA wraps around to form chromatin, whose chemical modification affects how tightly DNA is packed.
- Histone acetylation
- Addition of acetyl groups to histones that loosens chromatin packing and promotes transcription.
- Chromatin
- The combined structure of DNA wound around histone proteins.
- Epigenetic mark
- A reversible chemical modification to DNA or histones that changes gene expression without altering the DNA sequence.
Watch it explained
- How it worksHow does adding a methyl group to DNA shut a gene down without changing its sequence?
- How it worksWalk me through histone modification loosening or tightening the DNA wrapped around it.
- Going furtherWhat happens to a gene's expression when its methylation marks are stripped away?
- Common mistakeWhy isn't an epigenetic change the same thing as a mutation?
Check yourself
End of topic.