Foundations

Epigenetics

Epigenetics studies chemical marks on DNA and chromatin that are associated with gene activity without changing the base sequence. These include DNA methylation and histone modifications. Patterns can differ by cell type, development and age; identical twins can also develop differences over time.

Key points

  • Epigenetic mechanisms can influence gene activity without changing the sequence.
  • DNA methylation and histone modifications are the main marks.
  • Patterns depend in part on cell type and age and can differ between identical twins.
EPIGENETICS · METHYLATION SWITCHES GENESPromoter freePromoter methylatedRNAGene activeGene silencedThe sequence is identical in both; only the marks (dots) decide whether the gene is read.

Marks on the genome

Well-studied mechanisms include DNA methylation, in which chemical groups are added to particular DNA bases. Its effect depends on the location and cellular context; methylation does not always lower gene activity. Chemical changes to histone proteins also affect how accessible DNA regions are to gene regulation.

Response to environment and age

Epigenetic patterns can change with development and age. Environmental factors are also studied, but an individual methylation pattern cannot readily be attributed to one specific experience or cause. So-called epigenetic clocks estimate selected age-related measures from methylation data; they do not measure a universal biological age or a person's health.

What genotyping does not see

Genome analyses DNA sequence data. Standard sequencing does not reliably measure methylation or histone modifications; those require suitable epigenetic assays. A DNA sequence alone therefore does not show which genes are active in a particular cell.

What Genome measures. Genome reads the DNA sequence, not its epigenetic marks. This article explains a layer that genotyping does not capture.

Related topics

What is a gene?Gene structure: exon, intron, promoterFrom gene to proteinGWAS and polygenic scores

Sources

  1. 1Jaenisch & Bird, 2003 Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals. Nature Genetics 33:245-254. doi.org/10.1038/ng1089
  2. 2Fraga et al., 2005 Epigenetic differences arise during the lifetime of monozygotic twins. PNAS 102:10604-10609. doi.org/10.1073/pnas.0500398102