Nucleosome Core Particle:
- DNA wrapped: ~147 bp (1.65 turns around octamer)
- Histone octamer: 2 × H2A + 2 × H2B + 2 × H3 + 2 × H4
- H1 (linker histone): binds at DNA entry/exit; NOT in octamer
- Total nucleosome (with linker DNA): ~200 bp
Levels of Chromatin Compaction:
| Level | Structure | Compaction |
|---|---|---|
| 1 | Nucleosome "beads on a string" | 11 nm fibre |
| 2 | Solenoid (30 nm fibre) | H1-mediated coiling |
| 3 | Loop domains on scaffold | 300 nm fibre |
| 4 | Metaphase chromosome | 700–800 nm |
Histone Modifications (Epigenetics):
- Acetylation of lysine → loosens chromatin → MORE transcription
- Methylation of H3K9 → tightens chromatin → LESS transcription (heterochromatin)
- Phosphorylation of H3 → chromosome condensation during mitosis