Atomic force microscopy identifies filamentous structures comprising mitotic chromosomes

Chromosomes undergo dynamic structural reorganization during the cell cycle and become highly condensed during mitosis. This condensation process involves the assembly of chromatin fibers, in which chromosome-associated proteins (e.g., condensin) and post-translational modifications (e.g., histone deacetylation) play key roles. Although an increasing number of studies have advanced our understanding of chromatin fiber assembly into chromosomes, detailed morphological information on chromatids remains limited. In this report, we used atomic force microscopy to examine the nanoscale structure of chromosomes preserved in liquid and found that mitotic chromosomes are composed of a series of thin filamentous structures. Quantitative analysis of chromosomes from the chromokinesin KIF4A-depleted cells and under histone deacetylase inhibition revealed that the filament width and inter-filament spacing are regulated by KIF4A and histone deacetylation, likely reflecting their specific contributions at different hierarchical levels of chromosome assembly. These findings demonstrate that atomic force microscopy enables the detection of fine nanoscale structural alterations beyond the resolution of optical microscopy and provides new morphological insights into the organization of mitotic chromosomes.