Novel steady state of a microtubule assembly in a confined geometry

We study the steady state of an assembly of microtubules in a confined volume, analogous to the situation inside a cell where the cell boundary forms a natural barrier to growth. We show that the dynamical equations for growing and shrinking microtubules predict the existence of two steady states, with either exponentially decaying or exponentially increasing distribution of microtubule lengths. We identify the regimes in parameter space corresponding to these steady states. In the latter case, the apparent catastrophe frequency near the boundary was found to be significantly larger than that in the interior. Both the exponential distribution of lengths and the increase in the catastrophe frequency near the cell margin is in excellent agreement with recent experimental observations.Comment: 8 pages, submitted to Phys. Rev.

Mitotic spindle assembly by two different pathways in vitro

Abstract. We have used Xenopus egg extracts to study spindle morphogenesis in a cell-free system and have identified two pathways of spindle assembly in vitro using methods of fluorescent analogue cytochemistry. When demembranated sperm nuclei are added to egg extracts arrested in a mitotic state, individual nuclei direct the assembly of polarized microtubule arrays, which we term half-spindles; half-spindles then fuse pairwise to form bipolar spindles. In contrast, when sperm nuclei are added to extracts that are induced to enter interphase and arrested in the following mitosis, a single sperm nucleus can direct the assembly of a complete spindle. We find that microtubule arrays in vitro are strongly biased towards chromatin, but this does not depend on specific kinetochore-microtubul

Mutations in the kinesin-like protein Eg5 disrupting localization to the mitotic spindle

Eg5, a member of the bimC subfamily of kinesin-like microtubule motor proteins, localizes to spindle microtubules in mitosis but not to interphase microtubules. We investigated the molecular basis for spindle localization by transient transfection of Xenopus A6 cells with myc-tagged derivatives of Eg5. Expressed at constitutively high levels from a cytomegalovirus promoter, mycEg5 protein is cytoplasmic throughout interphase, begins to bind microtubules in early prophase, and remains localized to spindle and/or midbody microtubules through mitosis to the end of telophase. Both N- and C-terminal regions of Eg5 are required for this cell-cycle-regulated targeting. Eg5 also contains within its C-terminal domain a sequence conserved among bimC subfamily proteins that includes a potential p34cdc2 phosphorylation site. We show that mutation of a single threonine (T937) within this site to nonphosphorylatable alanine abolishes localization of the mutant protein to the spindle, whereas mutation of T937 to serine preserves spindle localization. We hypothesize that phosphorylation of Eg5 may regulate its localization to the spindle in the cell cycle

Microtubule flux in mitosis is independent of chromosomes, centrosomes, and antiparallel microtubules

We investigated the mechanism of poleward microtubule flux in the mitotic spindle by generating spindle subassemblies in Xenopus egg extracts in vitro and assaying their ability to flux by photoactivation of fluorescence and low-light multichannel fluorescence video-microscopy. We find that monopolar intermediates of in vitro spindle assembly (half-spindles) exhibit normal poleward flux, as do astral microtubule arrays induced by the addition of dimethyl sulfoxide to egg extracts in the absence of both chromosomes and conventional centrosomes. Immunodepletion of the kinesin-related microtubule motor protein Eg5, a candidate flux motor, suggests that Eg5 is not required for flux. These results suggest that poleward flux is a basic element of microtubule behavior exhibited by even simple self-organized microtubule arrays and presumably underlies the most elementary levels of spindle morphogenesis

Transport enhancement from incoherent coupling between one-dimensional quantum conductors

We study the non-equilibrium transport properties of a highly anisotropic two-dimensional lattice of spin-1/2 particles governed by a Heisenberg XXZ Hamiltonian. The anisotropy of the lattice allows us to approximate the system at finite temperature as an array of incoherently coupled one-dimensional chains. We show that in the regime of strong intrachain interactions, the weak interchain coupling considerably boosts spin transport in the driven system. Interestingly, we show that this enhancement increases with the length of the chains, which is related to superdiffusive spin transport. We describe the mechanism behind this effect, compare it to a similar phenomenon in single chains induced by dephasing, and explain why the former is much stronger

Weak measurement takes a simple form for cumulants

A weak measurement on a system is made by coupling a pointer weakly to the system and then measuring the position of the pointer. If the initial wavefunction for the pointer is real, the mean displacement of the pointer is proportional to the so-called weak value of the observable being measured. This gives an intuitively direct way of understanding weak measurement. However, if the initial pointer wavefunction takes complex values, the relationship between pointer displacement and weak value is not quite so simple, as pointed out recently by R. Jozsa. This is even more striking in the case of sequential weak measurements. These are carried out by coupling several pointers at different stages of evolution of the system, and the relationship between the products of the measured pointer positions and the sequential weak values can become extremely complicated for an arbitrary initial pointer wavefunction. Surprisingly, all this complication vanishes when one calculates the cumulants of pointer positions. These are directly proportional to the cumulants of sequential weak values. This suggests that cumulants have a fundamental physical significance for weak measurement

The Susceptibility to Hydrogen Peroxide of Indian and British Isoniazid-Sensitive and Isoniazid- Resistant Tubercle Bacilli

The present work describes an attempt to modify the method of Kreis and Le Joubioux (1957a) so that it would accurately estimate the relative proportions of catalase-positive and catalase-negative organisms in strains containing mixtures of the two types. A bactericidal test was chosen in preference to a bacteriostatic test, since it is difficult to obtain quantitative measurement with the latter technique. In performing a bactericidal test residual peroxide must be inactivated or removed by dilution so that it does not inhibit the growth of surviving organisms. Knox, Meadow and Worssam (1956) removed peroxide by centrifugation and washing, but this method was considered impracticable if this test were to be used on a large scale, and likely to produce inaccurate counts on the surviving organisms. In the present work the method of removal of peroxide was studied as well as the determination of the optimal peroxide concentration and period of exposure which would kill all catalase-negative organisms, but would leave catalase-positive organisms unaffected. In addition, the method of Kreis & Le Joubioux (1957a) was modified by reducing the inoculum of organisms exposed to peroxide so that catalase-positive bacilli would not be able to destroy peroxide during the test itself. The standardised bactericidal test was then employed in comparing the susceptibility to peroxide of isoniazid-sensitive strains from British and Indian patients, and in investigating the relationship between the peroxide susceptibility and the catalase activity of their isoniazid-resistant mutant strains

Current fluctuations in open quantum systems: Bridging the gap between quantum continuous measurements and full counting statistics

Continuously measured quantum systems are characterized by an output current, in the form of a stochastic and correlated time series which conveys crucial information about the underlying quantum system. The many tools used to describe current fluctuations are scattered across different communities: quantum opticians often use stochastic master equations, while a prevalent approach in condensed matter physics is provided by full counting statistics. These, however, are simply different sides of the same coin. Our goal with this tutorial is to provide a unified toolbox for describing current fluctuations. This not only provides novel insights, by bringing together different fields in physics, but also yields various analytical and numerical tools for computing quantities of interest. We illustrate our results with various pedagogical examples, and connect them with topical fields of research, such as waiting-time statistics, quantum metrology, thermodynamic uncertainty relations, quantum point contacts and Maxwell's demons.Comment: This is a tutorial paper, submitted to PRX Quantu