The eukaryotic genome consists of DNA molecules far longer than the cells that contain them. They reach their greatest compaction during chromosome condensation in mitosis. This process is aided by condensin, a structural maintenance of chromosomes (SMC) family member1,2. The spatial organization of mitotic chromosomes and how condensin shapes chromatin architecture are not yet fully understood. Here we use chromosome conformation capture (Hi-C)3,4 to study mitotic chromosome condensation in the fission yeast Schizosaccharomyces pombe5-7. This showed that the interphase landscape characterized by small chromatin domains is replaced by fewer but larger domains in mitosis. Condensin achieves this by setting up longer-range, intrachromosomal DNA interactions, which compact and individualize chromosomes. At the same time, local chromatin contacts are constrained by condensin, with profound implications for local chromatin function during mitosis. Our results highlight condensin as a major determinant that changes the chromatin landscape as cells prepare their genomes for cell division.
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