<title>Abstract</title>
Underlying higher order chromatin organization are Structural Maintenance of Chromosomes (SMC) complexes, large protein rings that entrap DNA. The molecular mechanism by which SMC complexes organize chromatin is as yet incompletely understood. Two prominent models posit that SMC complexes actively extrude DNA loops (loop extrusion), or that they sequentially entrap two DNAs that come into proximity by Brownian motion (diffusion capture). To explore the implications of these two mechanisms, we perform biophysical simulations of a 3.76 Mb-long chromatin chain, the size of the long Schizosaccharomyces pombe chromosome I left arm. On it, the SMC complex condensin is modeled to perform loop extrusion or diffusion capture. We then compare computational to experimental observations of mitotic chromosome formation. Both loop extrusion and diffusion capture can result in native-like contact probability distributions. In addition, the diffusion capture model more readily recapitulates mitotic chromosome axis shortening and chromatin compaction. Diffusion capture can also explain why mitotic chromatin shows reduced, as well as more anisotropic, movements, features that lack support from loop extrusion. The condensin distribution within mitotic chromosomes, visualized by stochastic optical reconstruction microscopy (STORM), shows clustering predicted from diffusion capture. Our results inform the evaluation of current models of mitotic chromosome formation.

<title>Abstract</title>
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<title>Background</title>
Structural maintenance of chromosomes (SMC) complexes are central organizers of chromatin architecture throughout the cell cycle. The SMC family member condensin is best known for establishing long-range chromatin interactions in mitosis. These compact chromatin and create mechanically stable chromosomes. How condensin contributes to chromatin organization in interphase is less well understood.


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<title>Results</title>
Here, we use efficient conditional depletion of fission yeast condensin to determine its contribution to interphase chromatin organization. We deplete condensin in G2-arrested cells to preempt confounding effects from cell cycle progression without condensin. Genome-wide chromatin interaction mapping, using Hi-C, reveals condensin-mediated chromatin interactions in interphase that are qualitatively similar to those observed in mitosis, but quantitatively far less prevalent. Despite their low abundance, chromatin mobility tracking shows that condensin markedly confines interphase chromatin movements. Without condensin, chromatin behaves as an unconstrained Rouse polymer with excluded volume, while condensin constrains its mobility. Unexpectedly, we find that condensin is required during interphase to prevent ongoing transcription from eliciting a DNA damage response.


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<title>Conclusions</title>
In addition to establishing mitotic chromosome architecture, condensin-mediated long-range chromatin interactions contribute to shaping chromatin organization in interphase. The resulting structure confines chromatin mobility and protects the genome from transcription-induced DNA damage. This adds to the important roles of condensin in maintaining chromosome stability.


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<title>Abstract</title>Underlying higher order chromatin organization are Structural Maintenance of Chromosomes (SMC) complexes, large protein rings that entrap DNA. The molecular mechanism by which SMC complexes organize chromatin is as yet incompletely understood. Two prominent models posit that SMC complexes actively extrude DNA loops (loop extrusion), or that they sequentially entrap two DNAs that come into proximity by Brownian motion (diffusion capture). To explore the implications of these two mechanisms, we perform biophysical simulations of a 3.76 Mb-long chromatin chain, the size of the long <italic>S. pombe</italic> chromosome I left arm. On it, the SMC complex condensin is modeled to perform loop extrusion or diffusion capture. We then compare computational to experimental observations of mitotic chromosome formation. Both loop extrusion and diffusion capture can result in native-like contact probability distributions. In addition, the diffusion capture model more readily recapitulates mitotic chromosome axis shortening and chromatin density enrichment. Diffusion capture can also explain why mitotic chromatin shows reduced, as well as more anisotropic, movements, features that lack support from loop extrusion. The condensin distribution within mitotic chromosomes, visualized by stochastic optical reconstruction microscopy (STORM), shows clustering predicted from diffusion capture. Our results inform the evaluation of current models of mitotic chromosome formation.

減数分裂では、遺伝情報をもつゲノムDNAを半減させるために、DNA交換反応により二価染色体が形成される。そこで本研究は、二価染色体の三次元構造を分子レベルで可視化し、減数分裂における染色体構造制御に関わる分子基盤の解明を目的とした。DNAシークエンス解析により染色体構造を決定するために、減数分裂の各ステージに細胞を同調する実験システムを構築した。これにより減数分裂の任意のステージの細胞を効率よく回収できるようになり、染色体の三次元構造解析の基盤を確立することができた。また、二価染色体を形成するためのDNA交換反応を人工的に誘導する実験も立ち上げており、二価染色体の構造制御メカニズムを解明する実験基盤が構築できた。

本研究課題は、高齢不妊の背後に潜む生物学的原因探究のため、配偶子形成を担う減数分裂における染色体の三次元構造の制御メカニズムの解明を目的とした。DNAシークエンス解析（Hi-C）により、染色体の三次元構造は、顕微鏡を超えた高解像度の立体情報が得られる。これまで大多数の細胞を用いて行われてきたHi-Cを利用して、細胞ごとに異なる染色体構造を明らかとするために、１細胞から回収した染色体ゲノムDNA由来のDNAシークエンスライブラリーの作成を行なった。本手法の開発により各細胞の染色体構造を１細胞レベルで可視化できるようになるため、本研究成果は、高齢不妊と染色体構造の繋がりを明らかとするための礎を構築したといえる。

本研究課題は、染色体構造を分子レベルで解き明かすことにより、配偶子（卵・精子）を形成するための減数分裂を理解することを目的とした。「Hi−C」による染色体構造の高解像度での可視化には、均一な細胞集団を用いてビッグデータを取得する必要がある。そこで本研究では、シンプルなゲノム構造を持つ分裂酵母を利用して、減数分裂を同調的に誘導する実験システムを確立した。本研究の成果は、減数分裂の進行に伴う染色体構造の時間変化を決定するための実験基盤であり、今後Hi−Cによって染色体の内部構造を分子レベルで解き明かすための礎となる成果といえる。