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Mitotic chromosome assembly despite nucleosome depletion in Xenopus egg extracts

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Science  23 Jun 2017:
Vol. 356, Issue 6344, pp. 1284-1287
DOI: 10.1126/science.aam9702

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Packaging without nucleosomes

The fundamental compaction of DNA in chromosomes is thought to start with the wrapping of DNA around nucleosomes, but is nucleosome wrapping required for the condensation of mitotic chromosomes? Shintomi et al. combined Xenopus egg extracts and mouse sperm nuclei and found that chromosome-like structures could be assembled in the near-complete absence of nucleosomes (see the Perspective by Kakui and Uhlmann). These “nucleosome-depleted” chromosomes were composed of condensin-enriched discrete axes and poorly organized chromatin loops. This finding challenges the textbook view of mitotic chromosome organization.

Science, this issue p. 1284; see also p. 1233

Abstract

The nucleosome is the fundamental structural unit of eukaryotic chromatin. During mitosis, duplicated nucleosome fibers are organized into a pair of rod-shaped structures (chromatids) within a mitotic chromosome. However, it remains unclear whether nucleosome assembly is indeed an essential prerequisite for mitotic chromosome assembly. We combined mouse sperm nuclei and Xenopus cell-free egg extracts depleted of the histone chaperone Asf1 and found that chromatid-like structures could be assembled even in the near absence of nucleosomes. The resultant “nucleosome-depleted” chromatids contained discrete central axes positive for condensins, although they were more fragile than normal nucleosome-containing chromatids. Combinatorial depletion experiments underscored the central importance of condensins in mitotic chromosome assembly, which sheds light on their functional cross-talk with nucleosomes in this process.

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