Active gelation breaks time-reversal-symmetry of mitotic chromosome mechanics

AbstractIn cell division, mitosis is the phase in which duplicated sets of chromosomes are mechanically aligned to form the metaphase plate before being segregated in two daughter cells. Irreversibility is a hallmark of this process, despite the fundamental laws of Newtonian mechanics being time symmetric.Here we show experimentally that mitotic chromosomes receive the arrow of time by time-reversal-symmetry breaking of the underlying mechanics in prometaphase. By optically inducing hydrodynamic flows within prophase nuclei, we find that duplicated chromatid pairs initially form a fluid suspension in the nucleoplasm: although showing little motion on their own, condensed chromosomes are free to move through the nucleus in a time-reversible manner. Actively probing chromosome mobility further in time, we find that this viscous suspension of chromatin transitions into a gel after nuclear breakdown. This gel state, in which chromosomes cannot be moved by flows, persists even when chromosomes start moving to form the metaphase plate. Complemented by minimal reconstitution experiments, our active intra-nuclear micro-rheology reveals time-reversal-symmetry breaking of chromosome mechanics to be caused by the transition from a purely fluid suspension into an active gel.Graphical abstractOne sentence summaryFlows induced in living cell nuclei reveal the rheological changes that bring chromosomes under mechanical control during mitosis.

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Evidence for the coexistence of time-reversal symmetry breaking and Bardeen-Cooper-Schrieffer-like superconductivity in La7Pd3

Physical Review B ◽

10.1103/physrevb.103.024507 ◽

2021 ◽

Vol 103 (2) ◽

Author(s):

D. A. Mayoh ◽

A. D. Hillier ◽

G. Balakrishnan ◽

M. R. Lees

Keyword(s):

Symmetry Breaking ◽

Time Reversal ◽

Time Reversal Symmetry

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Unsplit superconducting and time reversal symmetry breaking transitions in Sr2RuO4 under hydrostatic pressure and disorder

Nature Communications ◽

10.1038/s41467-021-24176-8 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Vadim Grinenko ◽

Debarchan Das ◽

Ritu Gupta ◽

Bastian Zinkl ◽

Naoki Kikugawa ◽

...

Keyword(s):

Hydrostatic Pressure ◽

Symmetry Breaking ◽

Time Reversal ◽

Order Parameter ◽

Muon Spin Rotation ◽

Horizontal Line ◽

Time Reversal Symmetry ◽

Zero Field ◽

Symmetry Protected ◽

Line Node

AbstractThere is considerable evidence that the superconducting state of Sr2RuO4 breaks time reversal symmetry. In the experiments showing time reversal symmetry breaking, its onset temperature, TTRSB, is generally found to match the critical temperature, Tc, within resolution. In combination with evidence for even parity, this result has led to consideration of a dxz ± idyz order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding TTRSB = Tc, but it has a hard-to-explain horizontal line node at kz = 0. Therefore, s ± id and d ± ig order parameters are also under consideration. These avoid the horizontal line node, but require tuning to obtain TTRSB ≈ Tc. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr2RuO4 under hydrostatic pressure, and Sr1.98La0.02RuO4 at zero pressure. Both hydrostatic pressure and La substitution alter Tc without lifting the tetragonal lattice symmetry, so if the degeneracy is symmetry-protected, TTRSB should track changes in Tc, while if it is accidental, these transition temperatures should generally separate. We observe TTRSB to track Tc, supporting the hypothesis of dxz ± idyz order.

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