Our multivariable analysis supports this explanation

apture, Grigoryev et al were able to capture hierarchical looping of nucleosome chains that are ordered in a zig-zag like fashion. The zigzag stacking of PP 242 nucleosomes is also present in interphase chromatin, however the higher order organization of loops of nucleosomes appears to be unique for mitosis. This emphasizes the importance of nucleosomes, their positioning and their modifications in mitosis, which will be more elaborately discussed below. Chromatin accessibility in mitosis–Mitotic chromatin is highly condensed and appears to have universal organization as detected by Hi-C. This does not mean that there are no differences in levels of local condensation and chromatin accessibility along the mitotic chromosome, as described for the interphase chromatin. A study by Hsuing et al observed variability in the accessibility of the mitotic chromatin using DNA hypersensitivity assay. Hsuing and colleagues showed that certain elements like promoters are maintained accessible during mitosis, where other elements such as enhancers are only accessible in interphase but not during mitosis. This is interesting, because this suggests there are chromatin features present in the interphase chromatin that are maintaining these promoter sites accessible during mitosis. The accessibility of promoter during mitosis sites also implies that the motifs, although temporarily less stable bound by factors,, maintain their open conformation and are accessible for these factors upon mitotic exit. This was surprising as the 3D organization of the mitotic chromatin does not seem to suggest differences across the mitotic chromatin. Furthermore, again seemingly contradictory with the data on the 3D organization of the mitotic chromatin, this locus specific accessibility can be different between cell types and therefore enable epigenetic memory of chromatin accessibility after mitosis. The mechanism that enables the maintenance of chromatin accessibility in mitosis however still remains to be resolved and might be different for individual loci. Author Manuscript Author Manuscript Author Manuscript Author Manuscript Mitotic Bookmarking and Epigenetic Memory Mitotic chromosomes have their own, temporary, chromosomal organization with unique characteristics. Interestingly, this organization is highly similar between different cell types, whereas interphase chromosomal organization is on some levels highly cell type specific. Even though most transcription activity ceases and at least a subset of proteins dissociate from the chromosomes during mitosis, cells are capable of rearranging their chromosomes Crit Rev Biochem Mol Biol. Author manuscript; available in PMC 2017 June 02. Oomen and Dekker Page 9 back into cell type specific conformations in early G1. This suggest that the information to rearrange chromosomes into their interphase chromosomal organization after condensation is contained in the mitotic chromosomes, in the soluble fraction, or both even though it is temporary overruled by machineries in mitosis that enable chromosome condensation and segregation. One way cell type-specific information can be maintained could be through mitotic bookmarking of cell type specific gene regulatory PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19858123 elements by patterns of local histone modifications and other epigenetic marks. These mitotic bookmarks can be histone modifications and variants, DNA methylation, noncoding RNA and less frequently specific transcription factors and histone reader complexes that remain bound