Chromatids of mitotic chromosomes were suggested to coil into a helix in early cytological studies and this assumption was recently supported by chromosome conformation capture (3C) sequencing. Still, direct differential visualization of a condensed chromatin fibre confirming the helical model was lacking. Here, we combined Hi-C analysis of purified metaphase chromosomes, biopolymer modelling and spatial structured illumination microscopy of large fluorescently labeled chromosome segments to reveal the chromonema - a helically-wound, 400 nm thick chromatin thread forming barley mitotic chromatids. Chromatin from adjacent turns of the helix intermingles due to the stochastic positioning of chromatin loops inside the chromonema. Helical turn size varies along chromosome length, correlating with chromatin density. Constraints on the observable dimensions of sister chromatid exchanges further supports the helical chromonema model.

Center of Integrated Breeding Research Department of Crop Sciences Georg August University D 37075 Göttingen Germany

Chemical Engineering Department University of Michigan Ann Arbor MI USA

Daicel Arbor Biosciences Ann Arbor MI USA

German Centre for Integrative Biodiversity Research Halle Jena Leipzig D 04103Leipzig Germany

Institute of Experimental Botany of the Czech Academy of Sciences Centre of the Region Haná for Biotechnological and Agricultural Research Olomouc 77900 Czech Republic

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

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