Gether. A recent study suggested that the positively charged protein Ki-67 could act as such an agent to keep the mitotic chromosomes separated. Ki-67 is associated with nucleoli in interphase cells, however it has been known for many years that Ki-67 is associated with the outside of the mitotic chromosome. This makes Ki-67 a good candidate to be a key mediator in the individualization of mitotic chromosomes. Another key player that regulates the organization of the mitotic chromosome is the cohesin complex, which holds sister chromatids together until late mitosis when the sister chromatids are separated and the DNA is divided between the new daughter cells. Sister chromatids are tightly intertwined after DNA replication in S-phase up till prophase. This implies that the initial stages of condensation occur while the sister chromatids are entangled and interacting. However, sister chromatids need to be separated to enable proper segregation, equal distribution of chromosomes over the daughter cells and to prevent DNA breaks caused by the strong forces on the DNA by the mitotic spindle. It has been known that cohesin removal from the mitotic chromosome has two distinct pathways. Cohesin is first removed from the chromosome arms during prophase; however localization of cohesin at the centromeres is maintained. This enables coupled progression of the condensation and sister separation of the chromosome arms, while they are still attached at the centromeres necessary for metaphase plate alignment. Cohesin complexes located at the centromeres are removed after completion of the condensation process and formation of the mitotic spindle in metaphase to anaphase transition. However, the question how the topological entanglement of the sister chromatids is resolved after removal from cohesin in prophase, remains to be addressed. A recent study from Liang et al showed how topoisomerase II functions to disentangle the sister chromatids. In addition to this, Goloborodko et al showed in their computational study how addition of topoisomerase II-activity to the loop extrusion model leads to separation of the sister chromatids. The function of topoisomerase in sister segregation was later also confirmed using a labeling LOXO 101 site technique to visualize the chromatids separately by super resolution microscopy. It was shown that sister chromatid separation is initiated in early prophase, however further condensation is halted when topoisomerase II is inhibited. Entinostat Author Manuscript Author Manuscript Author Manuscript Author Manuscript Crit Rev Biochem Mol Biol. Author manuscript; available in PMC 2017 June 02. Oomen and Dekker Page 8 Chromatin condensation not only occurs at the level of chromatin loops and whole chromosomes, at the nucleosomal level changes are observed during mitosis as well. As many chromatin binding factors are thought to migrate off the chromatin during mitosis, the nucleosomes can rearrange their relative positioning. Although it is unknown how the activity and movement of ring-like complexes like condensin, as proposed for loop extrusion, are affected by the presence 
nucleosomes on the DNA, it has been shown that nucleosomes are evicted from the chromatin in order for PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19858123 condensin to load and start forming loops. One can imagine that the forces on the chromatin fiber during the condensation process might cause the nucleosomes to redistribute along the chromatin. Using an electron microscopy technique called EM-assisted nucleosome interaction c.Gether. A recent study suggested that the positively charged protein Ki-67 could act as such an agent to keep the mitotic chromosomes separated. Ki-67 is associated with nucleoli in interphase cells, however it has been known for many years that Ki-67 is associated with the outside of the mitotic chromosome. This makes Ki-67 a good candidate to be a key mediator in the individualization of mitotic chromosomes. Another key player that regulates the organization of the mitotic chromosome is the cohesin complex, which holds sister chromatids together until late mitosis when the sister chromatids are separated and the DNA is divided between the new daughter cells. Sister chromatids are tightly intertwined after DNA replication in S-phase up till prophase. This implies that the initial stages of condensation occur while the sister chromatids are entangled and interacting. However, sister chromatids need to be separated to enable proper segregation, equal distribution of chromosomes over the daughter cells and to prevent DNA breaks caused by the strong forces on the DNA by the mitotic spindle. It has been known that cohesin removal from the mitotic chromosome has two distinct pathways. Cohesin is first removed from the chromosome arms during prophase; however localization of cohesin at the centromeres is maintained. This enables coupled progression of the condensation and sister separation of the chromosome arms, while they are still attached at the centromeres necessary for metaphase plate alignment. Cohesin complexes located at the centromeres are removed after completion of the condensation process and formation of the mitotic spindle in metaphase to anaphase transition. However, the question how the topological entanglement of the sister chromatids is resolved after removal from cohesin in prophase, remains to be addressed. A recent study from Liang et al showed how topoisomerase II functions to disentangle the sister chromatids. In addition to this, Goloborodko et al showed in their computational study how addition of topoisomerase II-activity to the loop extrusion model leads to separation of the sister chromatids. The 
function of topoisomerase in sister segregation was later also confirmed using a labeling technique to visualize the chromatids separately by super resolution microscopy. It was shown that sister chromatid separation is initiated in early prophase, however further condensation is halted when topoisomerase II is inhibited. Author Manuscript Author Manuscript Author Manuscript Author Manuscript Crit Rev Biochem Mol Biol. Author manuscript; available in PMC 2017 June 02. Oomen and Dekker Page 8 Chromatin condensation not only occurs at the level of chromatin loops and whole chromosomes, at the nucleosomal level changes are observed during mitosis as well. As many chromatin binding factors are thought to migrate off the chromatin during mitosis, the nucleosomes can rearrange their relative positioning. Although it is unknown how the activity and movement of ring-like complexes like condensin, as proposed for loop extrusion, are affected by the presence nucleosomes on the DNA, it has been shown that nucleosomes are evicted from the chromatin in order for PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19858123 condensin to load and start forming loops. One can imagine that the forces on the chromatin fiber during the condensation process might cause the nucleosomes to redistribute along the chromatin. Using an electron microscopy technique called EM-assisted nucleosome interaction c.
