The big idea: Every cell in your body carries the same DNA — yet a nerve cell, a muscle cell and a skin cell all look and behave completely differently.
They differ because each cell switches different genes ON and OFF. The DNA letters are identical; what changes is which genes are expressed.
Epigenetics is the study of these switches. An epigenetic change alters gene expression without changing the DNA base sequence — and the change can be inherited by daughter cells when the cell divides.
The base sequence is identical in both panels — only the epigenetic switches differ. GENE OFF (silenced): methyl (CH₃) groups are added to the DNA and the DNA/histones are tightly packed, so RNA polymerase cannot bind and no mRNA is made. GENE ON: the DNA is open, so it can be transcribed.
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- Epigenetics
- Heritable changes in gene expression that do NOT change the DNA base sequence (the A, T, C, G order stays the same).
- Gene expression
- Whether a gene is switched on (transcribed into mRNA and used) or off.
- Heritable
- Passed on — here, copied to the daughter cells when a cell divides, so the same genes stay on or off.
- Methyl group
- A small chemical tag, written CH₃, that can be attached to DNA.
- Histone
- A protein that DNA is wound around; how tightly the DNA wraps controls whether genes can be read.
Epigenetic ≠ mutation: A mutation changes the actual DNA base sequence (it rewrites the letters).
An epigenetic change does not touch the sequence — it only changes whether a gene is read, like a sticky note on a page rather than crossing words out.
Memory hook: epi- means 'on top of' — epigenetic marks sit on top of the DNA, they don't rewrite it.
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There are two main epigenetic mechanisms, and both work by controlling whether the transcription machinery can reach a gene.
Read each as a chain of cause and effect: a chemical mark is added, that changes how the DNA is exposed, and that decides whether the gene is read or silenced.
1 — DNA methylation (usually SILENCES a gene)
- Methyl groups (CH₃) are attached to the DNA, often at the gene's promoter (the start signal).
- The methyl marks block RNA polymerase from settling on the promoter.
- With the start signal blocked, the gene cannot be transcribed — no mRNA is made.
- So methylation at a promoter usually switches the gene OFF (silenced).
Focus on the lower panel: methyl (CH₃) groups on the DNA plus tightly packed DNA/histones keep RNA polymerase out — the gene is silenced. Remove those marks and loosen the packing and the same gene can be read again.
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2 — Histone modification (changes how tightly DNA is packed)
- DNA is wound around histone proteins; chemical tags added to the histones change how tightly it wraps.
- Tightly packed (condensed) DNA is hidden — RNA polymerase cannot reach the gene, so it is OFF.
- Loosely packed (open) DNA is exposed — RNA polymerase can reach the gene, so it is ON.
- So the packing state set by histone tags decides whether a gene is accessible and expressed.
| DNA packing | Is the gene accessible? | Can RNA polymerase bind? | Gene state |
|---|---|---|---|
| Loosely packed (open) | Yes — the DNA is exposed | Yes | ON (expressed) |
| Tightly packed (condensed) | No — the DNA is hidden | No | OFF (silenced) |
Both come down to one thing: access: Both mechanisms control whether RNA polymerase can reach the gene.
Methylation blocks the promoter directly; tight histone packing hides the whole stretch of DNA. Either way, a blocked or hidden gene is silenced (OFF).
Open, unmethylated DNA is accessible, so the gene is expressed (ON).
Crucially, in none of this does the base sequence change — and the marks can be copied to daughter cells, so the same genes stay on or off after division (heritable).
| DNA methylation | Histone modification | |
|---|---|---|
| What is added/changed | Methyl (CH₃) groups are attached to the DNA, often at a gene's promoter | Chemical tags are added to the histone proteins that DNA is wound around |
| What it changes | Whether RNA polymerase can read the gene | How tightly the DNA is packed around the histones |
| Effect on the gene | Methylation at a promoter usually SILENCES the gene (transcription blocked) | Tightly packed (condensed) = gene OFF; loosely packed = gene ON |
| Does the base sequence change? | No — the A, T, C, G order is untouched | No — the base sequence is untouched |
| Reversible / heritable? | Reversible, and can be copied to daughter cells (heritable) | Reversible, and can be copied to daughter cells (heritable) |
Why this matters: These switches are how one genome produces many cell types — every cell keeps all the genes, but each expresses a different subset.
Because epigenetic marks are reversible, they also let cells respond to the environment and, in some cases, change which genes are on across a lifetime — without ever altering the DNA sequence itself.
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How this is tested: Epigenetics is an HL-only understanding (D2.2, HL). A common Explain question asks how genes can be switched on and off without changing the DNA base sequence.
Score points for: the definition (heritable change in expression, sequence unchanged); DNA methylation silencing genes (CH₃ at the promoter blocks transcription); and histone modification controlling packing (tight = OFF, loose = ON).
A frequent trap is to call an epigenetic change a mutation — make it explicit that the base sequence is not changed.
IB-style question — switching genes on and off without changing the sequence
All the cells in a multicellular organism contain the same DNA, yet different cell types express different genes. Explain, using DNA methylation and histone modification, how genes can be switched off without changing the DNA base sequence. [5]
How to score all five marks
- Define epigenetics. Epigenetic changes alter gene expression but do not change the DNA base sequence; they can be inherited by daughter cells.
- Methylation adds CH₃. Methyl (CH₃) groups are added to the DNA, often at the gene's promoter.
- Methylation silences. This blocks RNA polymerase / transcription, so the gene is switched off (silenced) — but the base sequence is unchanged.
- Histones control packing. Modifying the histone proteins changes how tightly the DNA is packed.
- Tight = off, loose = on. Tightly packed (condensed) DNA hides the gene from RNA polymerase, so it is OFF; loosely packed DNA is accessible, so it is ON. (Award 1 mark per distinct point, up to 5.)
Final answer
Epigenetic changes alter gene expression without changing the base sequence and can be inherited. Methyl (CH₃) groups added to DNA (e.g. at the promoter) block RNA polymerase / transcription, silencing the gene. Modifying histones changes how tightly the DNA is packed: tightly packed DNA is inaccessible so the gene is off, loosely packed DNA is accessible so the gene is on. In neither case does the base sequence change.
Why this scores full marks: It gives the definition (expression changes, sequence does not), covers both mechanisms (methylation silencing + histone packing on/off), and states the direction of each (CH₃ → off; tight → off, loose → on).
A common way to lose marks is to describe only methylation — a 5-mark answer needs the histone-packing mechanism too — or to slip and say the change is a mutation.