Epigenetic changes are reversible modifications to DNA or histone proteins that turn genes on or off without altering the underlying DNA sequence, controlling which genes a cell expresses (CED 6.5.A.2).
Epigenetic changes are tweaks to how genes are expressed that don't touch the DNA letters themselves. Think of the DNA sequence as the text of a book. Epigenetic changes are like sticky notes and highlighter marks that tell the cell which pages to read and which to skip. The text stays the same, but what gets used changes.
The two big mechanisms you need are DNA methylation (adding methyl groups, usually to silence a gene) and histone modification (chemical tags on the proteins DNA wraps around, which open up or close off access to genes). Per EK 6.5.A.2, these modifications are reversible, which is the part that trips people up. The change isn't permanent like a mutation. It can be added or removed depending on the cell's needs.
This lives in Unit 6: Gene Expression and Regulation, specifically topic 6.5, and supports learning objective AP Bio 6.5.A (describe the interactions that regulate gene expression). Epigenetic changes are one answer to a huge question Unit 6 keeps asking: if every cell in your body has the same DNA, why is a liver cell different from a neuron? The answer is differential gene expression, and epigenetics is a major driver of it. EK 6.5.A.3 ties this directly to phenotype, because the genes a cell actually turns on determine what that cell becomes and does.
Keep studying AP Biology Unit 6
Cell Differentiation (Unit 6)
Every cell in your body shares identical DNA, yet a muscle cell and a skin cell look nothing alike. Epigenetic changes are how that happens. Methylation patterns silence the genes a cell doesn't need, so the same genome produces specialized cell types.
DNA Methylation and Histone Modification (Unit 6)
These are the two concrete mechanisms behind the abstract idea of epigenetics. Methylation tags usually shut a gene down, while acetylating histones loosens the DNA wrapping so genes become accessible for transcription.
Regulatory Sequences and Regulatory Proteins (Unit 6)
Both regulate transcription, but at different levels. Regulatory sequences and proteins control whether transcription machinery binds a gene, while epigenetic changes control whether that gene is even physically accessible in the first place.
Non-coding RNA molecules (Unit 6)
ncRNAs can guide epigenetic machinery to specific genes, helping decide which regions get silenced. They're another layer of expression control that doesn't change the DNA sequence.
Expect this in multiple-choice as a classic setup: a researcher finds the same gene with the same DNA sequence behaving differently in two cell types (say, methylated and silent in liver cells, unmethylated and active in pancreatic cells). The right answer points to epigenetic regulation, not a mutation, because the sequence is identical. Another common stem describes histone acetylation making genes more accessible for transcription, testing whether you can name that as histone modification and connect it to increased gene expression. The move you have to make is always the same: gene expression changed, DNA sequence didn't, therefore it's epigenetic. No released free-response question uses this term verbatim, but it supports the kind of gene-regulation reasoning FRQs reward when you explain how identical genomes produce different cell types.
A mutation changes the actual DNA sequence (the letters), and it's heritable through that altered sequence. An epigenetic change leaves the sequence intact and is reversible. If a question says "the DNA sequence is identical" but expression differs, that's your signal for epigenetics, not a mutation.
Epigenetic changes alter which genes are expressed without changing the DNA sequence itself.
The two mechanisms to know are DNA methylation (usually silences genes) and histone modification (acetylation usually activates genes by loosening DNA).
These modifications are reversible, which separates them from permanent mutations (EK 6.5.A.2).
Cells with identical DNA can have different phenotypes because epigenetic changes turn different gene sets on and off, driving cell differentiation.
On the exam, the giveaway phrase is 'same DNA sequence, different expression,' which always points to epigenetic regulation.
They're reversible modifications to DNA or histone proteins that change gene expression without altering the DNA sequence. The main examples are DNA methylation and histone modification, covered under EK 6.5.A.2 in Unit 6.
No. Mutations change the actual DNA sequence and aren't easily reversed, while epigenetic changes leave the sequence alone and are reversible. If a question states the DNA sequence is identical between two cells but expression differs, it's epigenetic.
Both are epigenetic, but methylation adds methyl groups directly to DNA and usually silences a gene, while histone modification (like acetylation) tags the proteins DNA wraps around to open or close access to genes.
Because of epigenetic changes plus differential gene expression. A liver cell and a neuron share identical DNA, but methylation and histone tags turn different genes on, so each cell expresses a different set of proteins (EK 6.5.A.3).
Yes, in Unit 6 topic 6.5. They show up most often in multiple-choice stems describing the same gene behaving differently in two cell types with identical DNA, where you identify the cause as epigenetic regulation.