Switch one: DNA methylation
DNA methylation is the most-studied mark. An enzyme called a DNA methyltransferase attaches a small methyl group (-CH₃) onto a cytosine base — almost always where a C sits right next to a G in the sequence, written CpG. Stretches dense with these are called CpG islands, and they cluster near the start (promoter) of many genes.
The rule of thumb: heavy methylation on a promoter usually means the gene is switched off. The methyl tags either physically block the transcription factors that would start reading, or recruit proteins that pack the DNA away tightly. This is one of the main routes to gene silencing. Crucially, the pattern is copied to both daughter strands during DNA replication, so the “off” memory survives cell division.
Unmethylated promoter (CpG island):
5'- ...C G...C G...C G... -3' <- transcription factors land
| | | -> GENE ON
Methylated promoter:
5'- ...Cme G...Cme G...Cme G -3' <- factors blocked, DNA packed
x x x -> GENE OFF (silenced)Switch two: histones and chromatin
DNA does not float free. It winds around spool-like histone proteins, and each spool-plus-DNA unit is a nucleosome; strung together they make chromatin. The histones have flexible “tails” that stick out, and cells decorate these tails with chemical tags — collectively histone modifications. Some tags (like certain acetyl groups) loosen the packing so genes become readable; others tighten it so genes go quiet.
When motor proteins actively slide, eject, or repack nucleosomes to expose or hide a gene, that process is chromatin remodeling. Open, loosely packed chromatin is generally active; tightly condensed chromatin is generally silent. Methylation and histone marks talk to each other constantly — a methyl tag can attract histone-tightening enzymes, locking in the “off” state from both directions.