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This lesson covers epigenetics — heritable changes in gene expression that do not involve changes to the DNA nucleotide sequence. This is a key topic in the Edexcel A-Level Biology specification (9BI0, Topic 7).
Epigenetics literally means "above genetics". It refers to modifications that affect gene expression without altering the underlying DNA sequence. These modifications can be:
The two main epigenetic mechanisms are DNA methylation and histone modification.
Exam Tip: The crucial distinction is that epigenetic changes alter gene expression (whether genes are switched on or off) without changing the base sequence of DNA. Mutations, by contrast, do change the base sequence.
To understand epigenetics, you need to understand how DNA is packaged in the nucleus.
DNA in eukaryotic cells is wound around proteins called histones to form a structure called chromatin. The basic unit of chromatin is the nucleosome, which consists of approximately 147 base pairs of DNA wrapped around an octamer of eight histone proteins (two copies each of H2A, H2B, H3 and H4).
Chromatin exists in two states:
| State | Description | Gene expression |
|---|---|---|
| Euchromatin | Loosely packed chromatin; DNA is accessible to transcription machinery | Genes are active (can be transcribed) |
| Heterochromatin | Tightly packed chromatin; DNA is not accessible | Genes are silenced (cannot be transcribed) |
The conversion between euchromatin and heterochromatin is controlled by epigenetic modifications.
DNA methylation involves the addition of a methyl group (–CH₃) to the cytosine base in DNA, specifically to cytosines that are followed by guanine (called CpG sites or CpG dinucleotides).
Clusters of CpG sites near gene promoters are called CpG islands. About 70% of human gene promoters are associated with CpG islands. When a CpG island is:
Exam Tip: DNA methylation = gene silencing. This is the most commonly tested aspect. Remember that it is a methyl group added to cytosine, and it typically occurs at CpG sites in promoter regions.
Histones have tails that protrude from the nucleosome. These tails can be chemically modified, and these modifications alter how tightly the DNA is wound around the histones.
| Modification | Effect | Enzyme |
|---|---|---|
| Acetylation | Loosens chromatin → euchromatin → gene activation | Histone acetyltransferase (HAT) |
| Deacetylation | Tightens chromatin → heterochromatin → gene silencing | Histone deacetylase (HDAC) |
| Methylation | Can either activate or silence genes depending on position | Histone methyltransferase (HMT) |
| Phosphorylation | Often associated with chromosome condensation during mitosis | Kinases |
Conversely, histone deacetylation removes acetyl groups, restoring the positive charge on histone tails, causing DNA to wind more tightly around the histones and silencing gene expression.
Exam Tip: Acetylation = activation (both start with 'A'). This is a useful memory aid. Histone acetylation loosens chromatin and switches genes on; deacetylation tightens chromatin and switches genes off.
In female mammals (XX), one X chromosome in each cell is inactivated early in development to equalise the gene dosage between males (XY) and females. This is called X chromosome inactivation or Lyonisation.
The inactivated X chromosome is condensed into a dense structure called a Barr body, which is visible at the edge of the nucleus. Inactivation is maintained by:
X inactivation is random — in some cells, the maternal X is inactivated; in others, the paternal X is inactivated. This creates a mosaic pattern of gene expression and explains why female calico cats have patches of different fur colours.
Genomic imprinting is an epigenetic phenomenon in which certain genes are expressed in a parent-of-origin-specific manner. For imprinted genes, only one allele is expressed — either the maternal or the paternal copy — while the other is silenced by DNA methylation.
Examples:
Errors in imprinting can cause genetic disorders:
Epigenetic modifications can be influenced by environmental factors:
| Environmental factor | Epigenetic effect |
|---|---|
| Diet | Folate and methionine in the diet provide methyl groups for DNA methylation |
| Stress | Chronic stress alters methylation patterns in stress-response genes |
| Toxins | Chemicals such as bisphenol A (BPA) can alter DNA methylation |
| Smoking | Alters methylation patterns; linked to increased cancer risk |
| Ageing | Global DNA methylation decreases with age; specific genes become hypermethylated |
One of the most compelling studies of environmental epigenetics involved individuals who were in utero during the Dutch famine of 1944–45. Decades later, these individuals showed:
Exam Tip: When discussing epigenetics and the environment, you should explain that environmental factors can cause epigenetic changes that alter gene expression without changing the DNA sequence, and that some of these changes may be heritable.
Abnormal epigenetic modifications can contribute to cancer:
Because epigenetic changes are reversible (unlike mutations), they are potential targets for cancer therapy. Drugs such as azacitidine (a DNA methyltransferase inhibitor) and vorinostat (a histone deacetylase inhibitor) are used to treat certain cancers.
| Mechanism | Effect on gene expression | Reversible? |
|---|---|---|
| DNA methylation (at promoter) | Silences genes | Yes |
| Histone acetylation | Activates genes | Yes |
| Histone deacetylation | Silences genes | Yes |
| X inactivation | Silences one X chromosome | Generally permanent in somatic cells |
| Genomic imprinting | Parent-of-origin silencing | Reset between generations |
Exam Tip: Epigenetics is often examined in the context of nature vs nurture or the interaction between genes and environment. Be prepared to discuss how identical twins can develop different phenotypes due to different epigenetic modifications caused by different environmental exposures.
This material sits in Edexcel 9BI0 Topic 8 (Grey Matter — Coordination, Response and Gene Technology), which expects candidates to describe how DNA methylation and histone modifications establish heritable patterns of gene expression without altering the DNA sequence, and to explain X chromosome inactivation and genomic imprinting as worked examples of stable silencing. Synoptic links run backwards to lesson 4 on gene expression and regulation (epigenetics is the mitotically heritable layer controlling whether transcription factors can access a promoter at all) and to lesson 1 on DNA structure (5-methylcytosine — methyl group at the 5' carbon of cytosine within a CpG dinucleotide); laterally to Topic 2 (cells) for stem-cell differentiation as the establishment of lineage-specific epigenetic states, Topic 6 (cancer) for global hypomethylation plus tumour-suppressor hypermethylation as drivers of oncogenesis, and Topic 5 (energy, exercise and coordination) for the metabolite supply (SAM, folate, B12) that fuels methylation; and forwards to lesson 6 on gene technology (CRISPR-dCas9 fusions enable programmable epigenetic editing) and lesson 8 on gene therapy (epigenetic drugs as clinical reality). Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) Explain how DNA methylation at a CpG island and histone deacetylation at the same gene promoter cooperate to silence transcription. (4)
(b) A female mammal is heterozygous for an X-linked coat-colour gene (one black allele, one orange allele) and shows a tortoiseshell pattern of discrete patches. Explain this phenotype with reference to X chromosome inactivation. (4)
Solution with mark scheme:
(a) M1 (AO1) — DNA methylation chemistry. DNA methyltransferase (DNMT) transfers a methyl group from S-adenosyl methionine (SAM) onto the 5' carbon of cytosine within a CpG dinucleotide, generating 5-methylcytosine. The DNA sequence itself is unchanged.
A1 (AO1) — recruitment. Methyl-CpG-binding proteins (e.g. MeCP2) recognise 5-methylcytosine and recruit histone deacetylases (HDACs) and chromatin-condensing factors. Methylation acts as a binding platform — it does not silence transcription on its own.
A1 (AO1) — histone deacetylation. HDAC removes acetyl groups from lysine residues on histone tails (e.g. H3K9, H3K27). Loss of acetylation restores the positive charge on histone tails, increasing electrostatic attraction to negatively charged DNA phosphate. The chromatin compacts toward heterochromatin.
A1 (AO2) — combined outcome. The compacted chromatin excludes transcription factors and RNA polymerase II. The two mechanisms form a self-reinforcing loop, and maintenance methyltransferase (DNMT1) copies methylation onto the daughter strand at every replication, so the silenced state is mitotically heritable.
(b) M1 (AO1) — X inactivation principle. In female mammalian somatic cells, one X is randomly inactivated in early development to equalise X-linked dosage with males. Inactivation is mediated by the non-coding XIST RNA, which coats the chosen X in cis and recruits DNA methylation, H3K27me3 and chromatin condensation to produce a transcriptionally silent Barr body.
A1 (AO1) — clonal heritability. Once an X is silenced in an embryonic cell, all descendants maintain the same inactivated X through every subsequent mitotic division, producing clonal patches.
A1 (AO2) — coat colour. In a heterozygous female with one black-allele X and one orange-allele X, cells inactivating the orange X express only black; cells inactivating the black X express only orange. Random inactivation plus clonal expansion produces the patchwork.
A1 (AO3.1) — synoptic. Tortoiseshell coat colour is a visible read-out of epigenetics — two alleles expressed in different cells despite identical DNA. Male tortoiseshells are virtually always XXY (Klinefelter) — direct evidence that X inactivation drives the pattern.
Total: 8 marks (M2 A6).
Question (6 marks): A scientist examines a tumour-suppressor gene in two samples. In healthy tissue the promoter is unmethylated and the gene is transcribed. In the tumour, the promoter shows hypermethylated CpG islands and H3K9 trimethylation, with no detectable transcript; the DNA sequence is identical in both samples. Treatment with azacitidine (a DNMT inhibitor) partially restores expression. Explain the molecular basis of silencing and the rationale for azacitidine therapy, commenting on the distinction between mutation and epigenetic change.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that hypermethylation of CpG islands at a tumour-suppressor promoter silences transcription without altering the DNA sequence |
| 2 | AO1.2 | Linking H3K9 trimethylation to heterochromatin formation, recruitment of HP1 or chromatin-condensing complexes, and exclusion of RNA polymerase |
| 3 | AO2.1 | Interpreting the identical DNA sequence as evidence that the silencing is epigenetic (chemical decoration), not mutational (sequence change) |
| 4 | AO2.7 | Explaining that azacitidine is incorporated in place of cytosine during replication and traps DNMT, lowering methylation and re-permitting transcription |
| 5 | AO3.1 | Concluding that epigenetic silencing is reversible in a way mutation is not, which is precisely why epigenetic targets are clinically attractive |
| 6 | AO3.2 | Synoptic — connecting tumour-suppressor silencing to lesson 4 (transcription regulation), Topic 6 (cancer hallmarks), and the broader principle that "same DNA, different output" is the central problem of cell biology |
Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Edexcel reliably tests epigenetics through "same DNA sequence, different expression" prompts; candidates who treat hypermethylation as a mutation, or who fail to articulate why reversibility matters clinically, lose AO3 marks. The mark scheme rewards candidates who keep the mutation vs epigenetic-change distinction sharp.
Lesson 4 (gene expression and regulation). Transcription factors and RNA polymerase II can only act on accessible chromatin. Epigenetic marks — DNA methylation at CpG islands and histone modifications (H3K4me3 active; H3K27me3/H3K9me3 silent) — set the accessibility ground state, on top of which transcription factors choose which accessible genes to switch on. Without epigenetics, eukaryotic regulation has no stable memory.
Lesson 1 (DNA structure). A methyl group is covalently added to the 5' carbon of cytosine, exclusively at CpG dinucleotides. Because methylation is a covalent modification of an existing base, not a sequence change, it is invisible to standard sequencing — bisulfite sequencing is needed to read the methylome.
Lesson 6 (gene technology) — engineered epigenetics. CRISPR-dCas9 fused to DNMT3A enables targeted methylation of any chosen promoter; fused to TET1, targeted demethylation; fused to HDACs or HATs, targeted histone modification. The engineering counterpart of natural epigenetic regulation.
Lesson 8 (gene therapy and pharmacology). Azacitidine and decitabine (DNMT inhibitors) are licensed for myelodysplastic syndrome and AML. Vorinostat and romidepsin (HDAC inhibitors) are licensed for cutaneous T-cell lymphoma. Epigenetic drugs exploit the reversibility of silencing — a property mutations lack.
Topic 2 (cells — stem cells and differentiation). Pluripotent stem cells have broadly permissive chromatin with bivalent domains carrying both active (H3K4me3) and silencing (H3K27me3) marks at developmental regulators. Differentiation is the resolution of bivalency, keeping appropriate marks and gaining DNA methylation at lineage-inappropriate genes. Identity is an epigenetic phenomenon.
Topic 6 (cancer — epigenetic dysregulation). Tumours show global hypomethylation (genome instability) plus focal hypermethylation of tumour-suppressor promoters (MLH1 in colorectal cancer, BRCA1 in breast cancer, CDKN2A/p16 broadly). Cancer is at least as much an epigenetic disease as a genetic one.
Topic 5 (energy, exercise and coordination — diet and methylation). Folate, B12 and methionine feed the methylation cycle generating S-adenosyl methionine (SAM) — the universal methyl donor. Maternal folate status during pregnancy is linked to offspring methylation; the Dutch Hunger Winter cohort shows altered IGF2 methylation decades after in-utero famine exposure.
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