Figure 1: Hierarchical genome folding.

~147 bp of DNA wrap around the histone octamer to form the nucleosome, the chromatin basic building block. At the few kb level, nucleosomes fold into tetranucleosme units. The zigzag topology appears dominant in small kb arrays. Further folding through hierarchical looping creates nucleosome clutches of different sizes and levels of compaction. Genes fold while maintaining the folding features of the lower scales. Compartmentalization at the few Mb produces TADs, which further fold to form segregated compartments. Finally, chromosomes separate in territories in the cell nucleus.