Epigenetics: Beyond the Sequence
Epigenetics involves heritable changes in gene expression (active versus inactive genes) that do not involve changes to the underlying DNA sequence — a change in phenotype without a change in genotype.
1. DNA Methylation
The addition of a methyl group (CH3) to the DNA strand itself, often at CpG sites. Typically, methylation represses gene transcription.
2. Histone Modification
DNA is wrapped around proteins called histones. If histones squeeze DNA tightly, genes are hidden (heterochromatin). If they relax, genes are accessible for transcription (euchromatin).
- Acetylation: Usually associated with active transcription (relaxes chromatin).
- Methylation: Can be associated with activation or repression, depending on which amino acid on the histone is modified.
The Agouti Mouse Experiment
Genetically identical mice can look completely different (one yellow and obese, one brown and healthy) purely based on the mother's diet during pregnancy altering the epigenetic methylation of the agouti gene.