The big idea: A cell can end up making a different protein, or a different amount of a protein, for two completely different reasons — and the IB wants you to tell them apart.
Mutation — the DNA base sequence is changed. The actual letters of the gene are different. This is permanent (not normally reversible) and is passed to daughter cells. It can change the structure of the protein.
Epigenetic change — the base sequence is NOT changed. Instead a chemical tag (methylation of the DNA, or a change to the histones) switches a gene up or down. This is reversible and changes how much protein is made — not what the protein is.
One liner: a mutation rewrites the gene; an epigenetic change just changes the volume at which the gene is read.
- Mutation
- A permanent change to the DNA base sequence (the order of A, T, C, G). It is heritable and not normally reversible, and it can change the structure of the protein the gene codes for.
- Epigenetic change
- A change in gene EXPRESSION caused by chemical tags (DNA methylation or histone modification) — without altering the base sequence. It is reversible and changes how much protein is made.
- Gene expression
- Whether a gene is switched on, and how strongly — i.e. how much mRNA (and protein) the gene produces.
- DNA methylation
- Adding a methyl (–CH₃) tag to the DNA, usually silencing a gene (turning expression down) without changing the base sequence.
- Reversible
- Able to be undone — an epigenetic tag can be added or removed, switching a gene back; a mutation usually cannot be reversed.
The fastest way to tell them apart: Ask one question: did the DNA base sequence change?
Yes → it's a mutation (permanent, can change the protein's structure).
No, but expression changed → it's an epigenetic change (reversible, changes the amount of protein).
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Line the two up side by side. The key contrast is what is altered — the sequence (the gene's letters) or just the expression (how loudly the gene is read) — and that single difference explains every other property in the table.
| Mutation | Epigenetic change | |
|---|---|---|
| What changes? | The DNA base sequence itself (e.g. A→G, a base added or lost) | Gene expression — whether/how strongly a gene is read — via methylation / histone tags |
| Base sequence altered? | Yes — the letters of the DNA are changed | No — the sequence is left exactly the same |
| Reversible? | Not normally — once the sequence is changed it usually stays changed | Yes — a methyl/histone tag can be added or removed, switching the gene back |
| Heritable? | Yes — copied to daughter cells (and to offspring if in a gamete) | Can be — some tags are copied to daughter cells (and sometimes to offspring) |
| Effect on the PROTEIN | Can change the protein's structure (a different amino acid → a different/faulty protein) | Changes the amount of protein made (the protein itself is normal — there is just more or less of it, or none) |
Why each property follows from that one difference: Reversibility. A mutation has changed the actual DNA letters, and cells don't normally 'un-change' a sequence — so it's permanent. An epigenetic tag is just a chemical label that an enzyme can add or strip off — so it's reversible.
Effect on the protein. A mutation can swap an amino acid, so the protein's structure can change (it may fold wrongly or not work). An epigenetic change leaves the gene's sequence intact, so any protein made is normal — there is just more or less of it (or none, if the gene is silenced).
Measuring gene expression — read the mRNA: You can't see 'expression' directly, so biologists use a proxy: how much mRNA (or finished protein) a gene produces.
The rule is simple:
More mRNA from a gene = the gene is more highly expressed.
Little or no mRNA = the gene is barely expressed / switched off.
So to compare how active a gene is in two different cell types or two conditions, you compare the amount of mRNA (or protein) each makes. The cell with more mRNA is expressing that gene more strongly.
Reading an expression data set
- Find the gene (row) and the two cells/conditions (columns) you're comparing.
- Compare the mRNA amounts: the larger value = the gene is more highly expressed there.
- If mRNA is near zero, the gene is essentially switched off in that cell.
- Same gene, different amounts in different cells → the cells differ by expression, not by their DNA — they have the same genes, but read them differently (often an epigenetic difference).
| Gene | mRNA in skin cells | mRNA in pancreas cells | What it tells you |
|---|---|---|---|
| Insulin gene | Almost none | Very high | Strongly expressed in the pancreas, essentially off in skin |
| Keratin gene | Very high | Almost none | Strongly expressed in skin, essentially off in the pancreas |
| Housekeeping gene (respiration) | Moderate | Moderate | Expressed in both — every cell respires |
What the data shows: Skin cells and pancreas cells contain the same genes, yet the insulin gene makes lots of mRNA in the pancreas and almost none in skin.
Because the mRNA amount differs, the insulin gene is expressed much more strongly in the pancreas — even though the DNA sequence is identical in both cells. That is a difference in expression, not in the genes themselves.
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How this is tested: The classic HL item is a Distinguish / Compare question: distinguish between a mutation and an epigenetic change. Score the points the table gives you — sequence changed (yes/no), reversible (no/yes), and effect (protein structure vs amount).
The other style is a data question: you're given mRNA (or protein) amounts for a gene in two cell types or conditions and asked which cell expresses the gene more, or to deduce that two cells differ by expression rather than by their DNA. State the rule plainly: more mRNA = more highly expressed.
IB-style question — distinguish a mutation from an epigenetic change
Distinguish between a mutation and an epigenetic change in terms of the DNA, reversibility and the effect on the protein. [3]
How to score all three marks
- The DNA. A mutation changes the DNA base sequence; an epigenetic change does NOT change the base sequence — it adds methyl/histone tags that alter expression.
- Reversibility. A mutation is not normally reversible (the sequence stays changed); an epigenetic change is reversible (the tag can be added or removed).
- Effect on the protein. A mutation can change the protein's structure (a different/faulty protein); an epigenetic change changes the amount of protein made — the protein itself is normal. (Award 1 mark per distinct contrast, up to 3.)
Final answer
A mutation changes the DNA base sequence, is not normally reversible, and can change the protein's structure. An epigenetic change does not change the sequence, is reversible, and changes the amount of (normal) protein made.
Why this scores full marks: Each mark is a paired contrast (mutation point AND the matching epigenetic point), covering the three axes asked for: sequence, reversibility, protein.
A common way to lose marks is to describe only the mutation — a 'distinguish' answer must give both sides of each contrast.
IB-style question — interpret gene-expression data
A gene produces 40 units of mRNA in liver cells and 2 units of mRNA in muscle cells. Both cell types contain this gene. Deduce what the data show about the gene's expression. [2]
Model answer
- Compare the mRNA. The liver makes far more mRNA (40 vs 2), so the gene is more highly expressed in the liver and barely expressed in muscle.
- Deduce the cause. Both cells have the same gene, so they differ in expression, not in their DNA — the gene is switched on more strongly in the liver. (Mark 1: more mRNA in liver = more highly expressed. Mark 2: same gene → a difference in expression, not in the DNA sequence.)
Final answer
The liver makes far more mRNA (40 vs 2), so the gene is much more highly expressed in liver than in muscle. As both cells contain the same gene, they differ in expression, not in their DNA sequence.
Watch the wording: In a data answer, anchor everything to the mRNA amount: 'more mRNA → more highly expressed'. Don't jump to 'the gene mutated' — different amounts of mRNA from the same gene is a difference in expression, not a mutation.