Fig. 2

Levels of chromatin organization. a Interphasic chromosomes occupy discrete areas inside of the nucleus, termed chromosomal territories. At the sub-chromosome level, chromatin is organized in topologically associating domains (TADs), considered as fundamental building blocks of the three-dimensional genome. TADs are remarkably conserved between cell types and even species and span hundreds of kilobases to several megabases. Notably, long-range chromatin interaction, such as enhancer-promoter contacts, does not cross TAD boundaries, meaning that intra-TAD interactions are frequent, while inter-TAD contacts are unusual. Specific architectural proteins consolidate three-dimensional contacts, including transcription factor binding, Cohesin, CTCF, Mediator, or YY1 (not shown). b Representation of a contact heat map generated from a HiC experiment. TADs visually appear as triangles limiting a portion of the genome where interactions are very frequent (red color). Each TAD pertains to either A (blue) or B (green) compartment, which are estimated by an eigenvector analysis of the genome contact matrix after normalization of the HiC data. A compartment correlates with open, transcriptionally active chromatin, while B compartment presents repressive features