177 research outputs found
Cleavage furrow positioning
To complete the cell cycle, the cleavage furrow draws the plasma membrane toward the cell center, pinching the cytoplasm into two lobes that are subsequently separated into two cells. The position of the cleavage furrow is induced by the mitotic spindle during early anaphase. Although the mechanism of cleavage furrow positioning is not understood at a molecular level, recent results suggest that it might be mediated by local relief from the inhibitory effects of microtubules
Astral Signals Spatially Bias Cortical Myosin Recruitment to Break Symmetry and Promote Cytokinesis
SummaryBackgroundAfter anaphase, the segregated chromosomes are sequestered by cytokinesis into two separate daughter cells by a cleavage furrow formed by the actomyosin-based contractile ring. The failure to properly position the contractile ring between the segregated chromosomes can result in aneuploidy. In both C. elegans embryos and human cells, the central spindle regulates division-plane positioning in parallel with a second pathway that involves astral microtubules.ResultsWe combined genetic and pharmacological manipulations with live cell imaging to spatially separate the two division cues in a single cell. We demonstrate that the two pathways for furrow formation are mechanistically and genetically distinct. By following the distribution of green fluorescent protein (GFP)-tagged nonmuscle myosin, we have found that the astral pathway for furrow formation involves the negative regulation of cortical myosin recruitment. An asymmetrically positioned spindle induces the asymmetric cortical accumulation of myosin. This cortical myosin behaves as a coherent contractile network. If the cortical network is nonuniform over the cell, the cortical contractile elements coalesce into a single furrow. This coalescence requires interconnections among contractile elements.ConclusionsWe conclude that the two pathways of cleavage-furrow formation are mechanistically distinct. In particular, we conclude that the astral pathway for cleavage-furrow formation involves the negative regulation of myosin distribution by astral cues
Particle rearrangements during transitions between local minima of the potential energy landscape of a supercooled Lennard-Jones liquid
The potential energy landscape (PEL) of supercooled binary Lennard-Jones
(BLJ) mixtures exhibits local minima, or inherent structures (IS), which are
organized into meta-basins (MB). We study the particle rearrangements related
to transitions between both successive IS and successive MB for a small 80:20
BLJ system near the mode-coupling temperature T_MCT. The analysis includes the
displacements of individual particles, the localization of the rearrangements
and the relevance of string-like motion. We find that the particle
rearrangements during IS and MB transitions do not change significantly at
T_MCT. Further, it is demonstrated that IS and MB dynamics are spatially
heterogeneous and facilitated by string-like motion. To investigate the
mechanism of string-like motion, we follow the particle rearrangements during
suitable sequences of IS transitions. We find that most strings observed after
a series of transitions do not move coherently during a single transition, but
subunits of different sizes are active at different times. Several findings
suggest that the occurrence of a successful string enables the system to exit a
MB. Moreover, we show that the particle rearrangements during two consecutive
MB transitions are basically uncorrelated. Specifically, different groups of
particles are highly mobile during subsequent MB transitions. Finally, the
relation between the features of the PEL and the relaxation processes in
supercooled liquids is discussed.Comment: 13 pages, 10 figure
Complexity in surfaces of densest packings for families of polyhedra
Packings of hard polyhedra have been studied for centuries due to their
mathematical aesthetic and more recently for their applications in fields such
as nanoscience, granular and colloidal matter, and biology. In all these
fields, particle shape is important for structure and properties, especially
upon crowding. Here, we explore packing as a function of shape. By combining
simulations and analytic calculations, we study three 2-parameter families of
hard polyhedra and report an extensive and systematic analysis of the densest
packings of more than 55,000 convex shapes. The three families have the
symmetries of triangle groups (icosahedral, octahedral, tetrahedral) and
interpolate between various symmetric solids (Platonic, Archimedean, Catalan).
We find that optimal (maximum) packing density surfaces that reveal unexpected
richness and complexity, containing as many as 130 different structures within
a single family. Our results demonstrate the utility of thinking of shape not
as a static property of an object in the context of packings, but rather as but
one point in a higher dimensional shape space whose neighbors in that space may
have identical or markedly different packings. Finally, we present and
interpret our packing results in a consistent and generally applicable way by
proposing a method to distinguish regions of packings and classify types of
transitions between them.Comment: 16 pages, 8 figure
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