Molecular Biology
Epigenetics
Heritable changes in gene expression without DNA sequence changes
Epigenetics is the study of heritable changes in gene expression that don't alter DNA sequence. Mechanisms: (1) DNA methylation — methyl groups added to cytosines, silencing genes. (2) Histone modifications — chemical tags on histone tails affect chromatin accessibility. (3) Non-coding RNAs — regulate expression. (4) Higher-order chromatin structure. Epigenetic patterns: established during development, propagated through cell divisions, can respond to environment. Examples: imprinting (parent-of-origin effects), X chromosome inactivation, transgenerational effects. Not just classical inheritance — shapes how genome works.
- Etymology"Epi" (above) + "genetics" — beyond DNA sequence
- DNA methylation5-methylcytosine; silences genes
- Histone modificationsAcetylation, methylation, phosphorylation; affect accessibility
- ImprintingParent-of-origin gene silencing
- X-inactivationOne X silenced in females (XIST RNA)
- HeritableThrough cell divisions; some across generations
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Why epigenetics matters
- Development. Cell type specification.
- Cancer. Aberrant methylation drives.
- Inheritance. Beyond classical genetics.
- Aging. Methylation patterns change.
- Environmental health. Toxins alter epigenome.
- Drug development. Epigenetic therapies emerging.
- Reprogramming. iPSC technology relies on epigenetics.
Common misconceptions
- Epigenetics changes DNA. No — chemical modifications.
- All epigenetic changes inherited. Some across cell divisions; rare across generations.
- Lifestyle directly rewrites DNA. Modifies methylation/histones; not sequence.
- Epigenetics replaces genetics. Complementary.
- All methylation silences. Mostly; some activates.
- Epigenetic = environmental. Both genetic + environmental factors.
Frequently asked questions
What's DNA methylation?
Methyl group added to cytosine → 5-methylcytosine. Done by DNA methyltransferases. Most common at CpG dinucleotides (especially in CpG islands at gene promoters). Methylation usually silences genes — blocks transcription factor binding, recruits repressors. Pattern inherited through cell division (semi-conservative). Tumor suppressor genes often hypermethylated (silenced) in cancer.
What are histone modifications?
Histones (proteins around which DNA wraps) have tails with various amino acids. Modified by enzymes: acetylation (HATs), methylation, phosphorylation, ubiquitination, etc. Effects: relax chromatin (active genes — H3K4me3, H3K27ac) or compact (silent — H3K27me3, H3K9me3). Code: combinations of marks read by other proteins to activate/repress. "Histone code" hypothesis.
What's imprinting?
Some genes expressed only from one parent. Maternal copy silenced; paternal expressed. Or vice versa. Established via DNA methylation in germline. ~70 imprinted genes in humans. Important: Igf2 (growth, paternal-expressed), H19 (maternal-expressed). Failure causes diseases: Prader-Willi (paternal 15q), Angelman (maternal 15q). Imprinting: epigenetic memory of parent.
What's X chromosome inactivation?
Females (XX) have two X chromosomes; males (XY) have one. To equalize gene dosage: females silence one X in each cell. Random — different X silenced in different cells. Mechanism: XIST RNA (long non-coding RNA) coats one X; recruits silencing machinery; condenses chromosome. Calico cats: classic example (X-linked coat color genes; mosaic expression). One of best-studied examples of epigenetic regulation.
How can environment affect epigenetics?
Multiple ways. Stress, diet, toxins can alter DNA methylation, histone modifications. Examples: famine in WWII Netherlands — children of pregnant mothers had altered methylation persisting decades. Smoking changes methylation. Royal jelly determines bee caste (queen vs worker) via methylation. Some changes heritable across generations (transgenerational epigenetic inheritance) — controversial in mammals.
How do non-coding RNAs work?
Many types regulate expression. (1) miRNAs (microRNAs): short; bind mRNAs; promote degradation or block translation. ~1500 in human genome; regulate ~60% of genes. (2) lncRNAs: long; XIST is famous; multiple roles. (3) piRNAs: silence transposons in germline. (4) eRNAs from enhancers; activate transcription. Major regulatory layer beyond protein-coding genes.
What's the relationship to disease?
Cancer: aberrant methylation common (hypermethylation of tumor suppressors, hypomethylation of oncogenes). Drugs that modify methylation: 5-azacytidine (anticancer). Imprinting disorders: Prader-Willi, Angelman, Beckwith-Wiedemann. Chronic disease: epigenetic changes link environment to disease (smoking → lung cancer not just genetic). Therapeutic potential: epigenetic drugs.