The folding of interphase chromatin into highly compact mitotic chromosomes is one of the most recognizable changes during the cell cycle. However, the structural organization underlying this drastic compaction remains elusive. Here, we combine several super resolution methods, including structured illumination microscopy (SIM), binding-activated localization microscopy (BALM), and atomic force microscopy (AFM), to examine the structural details of the DNA within the mitotic chromosome, both in the native state and after up to 30-fold extension using single-molecule micromanipulation. Images of native chromosomes reveal widespread ∼125 nm compact granules (CGs) throughout the metaphase chromosome. However, at maximal extensions, we find exclusively ∼90 nm domains (mitotic nanodomains, MNDs) that are unexpectedly resistant to extensive forces of tens of nanonewtons. The DNA content of the MNDs is estimated to be predominantly ∼80 kb, which is comparable to the size of the inner loops predicted by a recent nested loop model of the mitotic chromosome. With this DNA content, the total volume expected of the human genome assuming closely packed MNDs is nearly identical to what is observed. Thus, altogether, these results suggest that these mechanically stable MNDs, and their higher-order assembly into CGs, are the dominant higher-level structures that underlie the compaction of chromatin from interphase to metaphase.
Keywords: AFM; BALM; mitotic chromosome; structural domains; super-resolution.