Roberti, M. J. | bioRxiv Science

bioRxiv2017

Disruption of dual zygotic spindle assembly shows epigenetic asymmetry to be chromosome intrinsic

At the beginning of mammalian life the genetic material from each parent meets when the fertilized egg divides. It was previously thought that a single microtubule spindle is responsible to spatially combine the two genomes and then segregate them to create the two-cell embryo. Utilizing light-sheet microscopy, we showed that two bipolar spindles form in the zygote, that independently congress the maternal and paternal genomes. These two spindles aligned their poles prior to anaphase but kept the parental genomes apart during the first cleavage. This spindle assembly mechanism provides a rationale for erroneous divisions into more than two blastomeric nuclei observed in mammalian zygotes and reveals the mechanism behind the observation that parental genomes occupy separate nuclear compartments in the two-cell embryo.\n\nOne Sentence Summary: After fertilization, two spindles form around pro-nuclei in mammalian zygotes and keep the parental genomes apart during the first division.

cell biology↗

bioRxiv2017

Correlative live and super-resolution imaging reveals the dynamic structure of replication domains

Chromosome organization in higher eukaryotes controls gene expression, DNA replication, and DNA repair. Genome mapping has revealed the functional units of chromatin at the sub-megabase scale as self-interacting regions called topologically associating domains (TADs) and showed they correspond to replication domains (RDs). A quantitative structural and dynamic description of RD behavior in the nucleus is however missing, as visualization of dynamic subdiffraction-sized RDs remains challenging. Using fluorescence labeling of RDs combined with correlative live and super-resolution microscopy in situ, we determined biophysical parameters to characterize the internal organization, spacing and mechanical coupling of RDs. We found that RDs are typically 150 nm in size and contain four co-replicating regions spaced 60 nm apart. Spatially neighboring RDs are spaced 300 nm apart and connected by highly flexible linker regions that couple their motion only below 550 nm. Our pipeline allows a robust quantitative characterization of chromosome structure in situ, and provides important biophysical parameters to understand general principles of chromatin organization.

cell biology↗