Figure 2. The genome in 3D. Chromatin, organized as arrays of the nucleosome (146 bp of DNA wrapped in 2.5 loops around the core histone octamer), is arranged into A (open, euchromatic) or B (closed, heterochromatic) compartments that can be megabases wide. Superimposed upon these structures are topologically associating domains (TADs), which are on average (median size) 185 kb, and can be within either A or B compartments. Sequences within TADs are much more likely to come into contact with each other than with loci from outside domains. TAD boundaries and chromatin loop formations (eg, 
promoter-enhancer loops) are often demarcated by CTCF (and additional proteins not shown here). Chromatin loops, or 
interactions, allow distal regulatory elements, like enhancers, to come into contact with gene promoters in order to regulate gene expression, often in cell type-specific fashion, with modulation of the RNA Polymerase II transcriptional machinery.