Dividing eukaryotic cells package extremely long chromosomal DNA molecules into discrete bodies to enable microtubule-mediated transport of one genome copy to each of the newly forming daughter cells1–3. Assembly of mitotic chromosomes involves DNA looping by condensin4–8 and chromatin compaction by global histone deacetylation9–13. Although condensin confers mechanical resistance to spindle pulling forces14–16, it is not known how histone deacetylation affects material properties and, as a consequence, segregation mechanics of mitotic chromosomes. Here we show how global histone deacetylation at the onset of mitosis induces a chromatin-intrinsic phase transition that endows chromosomes with the physical characteristics necessary for their precise movement during cell division. Deacetylation-mediated compaction of chromatin forms a structure dense in negative charge and allows mitotic chromosomes to resist perforation by microtubules as they are pushed to the metaphase plate. By contrast, hyperacetylated mitotic chromosomes lack a defined surface boundary, are frequently perforated by microtubules and are prone to missegregation. Our study highlights the different contributions of DNA loop formation and chromatin phase separation to genome segregation in dividing cells.

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Herpesviruses must amplify their DNA to load viral particles and they do so in replication compartments. The development and functions of replication compartments during DNA amplification are poorly understood, though. Here we examine 2 functionally distinct replicons in the same cells to dissect DNA amplification within replication compartments. Using a combination of singlecell assays, computational modeling, and population approaches, we show that compartments initially were seeded by single genomes of Epstein-Barr virus (EBV). Their amplification subsequently took 13 to 14 h in individual cells during which their compartments occupied up to 30% of the nucleus and the nuclear volume grew by 50%. The compartmental volumes increased in proportion to the amount of DNA and viral replication proteins they contained. Each compartment synthesized similar levels of DNA, indicating that the total number of compartments determined the total levels of DNA amplification. Further, the amplification, which depended on the number of origins, was regulated differently early and late during the lytic phase; early during the lytic phase, the templates limited DNA synthesis, while later the templates were in excess, coinciding with a decline in levels of the viral replication protein, BMRF1, in the replication compartments. These findings show that replication compartments are factories in which EBV DNA amplification is both clonal and coordinated.Epstein-Barr virus | viral DNA replication | replication compartments M ammalian DNA synthesis is well orchestrated to achieve consistent outcomes. Each DNA template is synthesized once and only once during each S phase (1). Regions of DNA on the order of a megabase are synthesized with multiple internal sites of DNA initiation and completed independently of adjacent regions (2). Chromosomes are synthesized in their entirety with the possible exception of a small number of their telomeric repeats (3). DNA viruses, being obligate cellular parasites, have evolved to synthesize their genomes by capitalizing on different cellular components. Not only herpesviruses (4, 5) but also adenoviruses (6) and parvoviruses (7, 8) form discrete sites in the nuclei of infected cells in which they amplify their DNAs. These virus families have genomes spanning in length from 3 to 250 kbp and include both single-and double-stranded DNAs, illustrating the diversity of cellular parasites that have evolved to revamp nuclear structure for their replication. It is not clear, though, what advantages this compartmentalization provides them. We have examined Epstein-Barr virus (EBV) DNA amplification to elucidate the role of replication compartments in the amplification of its DNA.In latently infected cells, EBV DNA is synthesized similarly to cellular DNA with the significant exception that only ∼85% of its genomes are duplicated each S phase (9). During the lytic phase, EBV, as with other herpesviruses, undergoes up to several hundredfold amplification of its DNA (10-12) to provide the genomes to be housed in ...