Mitotic chromosomes are essential structures for the faithful transmission of duplicated genomic DNA into two daughter cells during cell division (1). A long strand of DNA is wrapped around the core histone and forms a nucleosome. The nucleosome has long been assumed to be folded into 30-nm chromatin fibers (Fig. A)(1). However, it remains unclear how the nucleosome or 30-nm chromatin fiber is organized into mitotic chromosomes, although it is well known that condensins and topoisomerase IIα are implicated in this process (2-4). When we observed frozen hydrated (vitrified) human mitotic cells using cryo-electron microscopy, which enables direct high-resolution imaging of the cellular structures in a close-to-native state, we did not find any higher order structures, or even 30-nm chromatin fibers, but just a uniform disordered texture of the chromosome (Fig. B) (5-6). To further investigate the structure of mitotic chromosome, we performed small angle x-ray scattering or SAXS, which can detect regular internal structures in non-crystalline materials in solution. Mitotic chromosomes purified from HeLa cells were exposed to the synchrotron radiation beam at SPring-8 in Japan. Again, the results were striking: no structural peaks larger than 11-nm were detected. Therefore, we propose that the nucleosome fibers exist in a highly disordered, interdigitated state like a “polymer melt” that undergoes local dynamic movement (Fig. B)(5-6). We also postulate that a similar state exists in active interphase nuclei, resulting in several advantages in the transcription and DNA replication processes (6-7). The possible genomic organization in the mitotic chromosomes and nuclei is discussed.



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