Every ~147bp of DNA wraps 1.7 turns around a histone octamer to form a nucleosome ("bead"); a short linker (20–80bp) connects each bead to the next. Low histone acetylation lets adjacent nucleosomes stack into a compact 30nm zig-zag/solenoid fibre (heterochromatin); high acetylation neutralises histone charge and keeps the fibre open ("beads-on-a-string", euchromatin) so transcription machinery can access the DNA.
Nucleosome repeat length (NRL) = 147bp + linker(20-80bp)
Linear compaction: ~6x (open fibre) → ~40x (30nm solenoid)
Loop extrusion: cohesin reels DNA in from both sides until
stopped by convergent CTCF sites → a stable TAD loop
- Histone acetylation — drives the fibre continuously between open beads-on-a-string and the tightly wound 30nm solenoid.
- Nucleosomes in chain — how many beads of this chromatin segment are rendered (view stays framed either way).
- Extrusion progress / Auto-extrude — slides the cohesin ring (teal ring) along the fibre, growing a loop anchored at two convergent CTCF sites (flags) until it locks into a topologically associating domain (TAD).
- Folding state — switch between a straight fibre segment and the same fibre bent into a cohesin-extruded loop.
Real-world relevance: Hi-C chromosome-conformation maps reveal exactly these TAD loops; disrupting CTCF sites or cohesin can fuse neighbouring TADs and switch on the wrong genes — a mechanism implicated in some cancers and developmental disorders.