2.5 Chromatin Structure
Packaging the Genome
Human DNA (~2 meters) must fit into a nucleus (~10 Ξm). This requires ~10,000-fold compaction, achieved through hierarchical packaging into chromatin. The basic unit is the nucleosome.
The Nucleosome
- Core histones: H2A, H2B, H3, H4 (2 copies each = octamer)
- DNA wrapped: 147 bp, 1.65 left-handed superhelical turns
- Linker DNA: ~20-80 bp between nucleosomes
- Histone H1: Linker histone, stabilizes higher-order structure
"Beads on a string" â 11 nm fiber
Levels of Compaction
DNA double helix
2 nm1x
Nucleosome (beads on string)
11 nm6x
30 nm chromatin fiber
30 nm40x
Looped domains
300 nm1000x
Metaphase chromosome
1400 nm10000x
Histone Modifications
Post-translational modifications regulate chromatin state:
Activating
- âĒ H3K4me3 (promoters)
- âĒ H3K36me3 (gene bodies)
- âĒ Acetylation (opens chromatin)
Repressive
- âĒ H3K9me3 (heterochromatin)
- âĒ H3K27me3 (Polycomb silencing)
- âĒ Deacetylation (closes chromatin)