Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Topological domains are key architectural building blocks of chromosomes in complex genomes. Their functional importance and evolutionary dynamics are however not well defined. Here we performed comparative Hi-C in liver cells from four mammalian species, and characterized the conservation and divergence of chromosomal contact insulation and the resulting domain architectures within distantly related genomes. We show that the modular organization of chromosomes is robustly conserved in syntenic regions. This overall conservation is compatible with conservation of the binding landscape of the insulator protein CTCF. Specifically, conserved CTCF sites are co-localized with cohesin, enriched at strong topological domain borders and bind to DNA motifs with orientations that define the directionality of CTCF’s long-range interactions. Interestingly, CTCF binding sites which are divergent between species are strongly correlated with divergence of internal domain structure. This divergence is likely driven by local CTCF binding sequence changes, demonstrating how genome evolution can be linked directly with a continuous flux of local chromosome conformation changes. Conversely, we provide evidence that large-scale domains are harder to break and that they are reorganized during genome evolution as intact modules. Hi-C and 4C-seq experiments were conducted in primary liver cells obtained from mouse, rabbit, macaque and dog