Research on the formation of mitotic chromosomes from interphase chromatin domains, ongoing for several decades, made significant progress in recent years. It was stimulated by the development of advanced microscopic techniques and implementation of chromatin conformation capture methods that provide new insights into chromosome ultrastructure. This review aims to summarize and compare several models of chromatin fiber folding to form mitotic chromosomes and discusses them in the light of the novel findings. Functional genomics studies in several organisms confirmed condensins and cohesins as the major players in chromosome condensation. Here we compare available data on the role of these proteins across lower and higher eukaryotes and point to differences indicating evolutionary different pathways to shape mitotic chromosomes. Moreover, we discuss a controversial phenomenon of the mitotic chromosome ultrastructure - chromosome cavities - and using our super-resolution microscopy data, we contribute to its elucidation.

Institute of Experimental Botany Czech Acad Sci Centre of the Region Haná for Biotechnological and Agricultural Research Šlechtitelů 31 CZ 77900 Olomouc Czech Republic

The Leibniz Institute of Plant Genetics and Crop Plant Research Gatersleben Corrensstrasse 3 D 06466 Seeland Germany

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Advanced environmental scanning electron microscopy reveals natural surface nano-morphology of condensed mitotic chromosomes in their native state

Helical coiling of metaphase chromatids

Image analysis workflows to reveal the spatial organization of cell nuclei and chromosomes

Prospects and limitations of expansion microscopy in chromatin ultrastructure determination