The centromere paradox: stable inheritance with rapidly evolving DNA

Every eukaryotic chromosome has a centromere, the locus responsible for poleward movement at mitosis and meiosis. Although conventional loci are specified by their DNA sequences, current evidence favors a chromatin-based inheritance mechanism for centromeres. The chromosome segregation machinery is highly conserved across all eukaryotes, but the DNA and protein components specific to centromeric chromatin are evolving rapidly. Incompatibilities between rapidly evolving centromeric components may be responsible for both the organization of centromeric regions and the reproductive isolation of emerging species.     

Surface vibrational spectroscopic studies of hydrogen bonding and hydrophobicity

Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the molecular level at various interfaces: water-surfactant-coated quartz, water-hexane, and water-air. In all cases, hydrophobicity was characterized by the appearance of dangling hydroxyl bonds on 25 percent of the surface water molecules. At the water-quartz interface, packing restrictions force the water surface layer to have a more ordered, ice-like structure. A partly wettable water-quartz interface was also studied.     

Orc6 involved in DNA replication,chromosome segregation,and cytokinesis

Origin recognition complex (ORC) proteins serve as a landing pad for the assembly of a multiprotein prereplicative complex, which is required to initiate DNA replication. During mitosis, the smallest subunit of human ORC, Orc6, localizes to kinetochores and to a reticular-like structure around the cell periphery. As chromosomes segregate during anaphase, the reticular structures align along the plane of cell division and some Orc6 localizes to the midbody before cells separate. Silencing of Orc6 expression by small interfering RNA (siRNA) resulted in cells with multipolar spindles, aberrant mitosis, formation of multinucleated cells, and decreased DNA replication. Prolonged periods of Orc6 depletion caused a decrease in cell proliferation and increased cell death. These results implicate Orc6 as an essential gene that coordinates chromosome replication and segregation with cytokinesis.     

On the mitotic movements of chromosomes

My conclusions, which, I confess, are tentative and based mainly on studies of one kind of cell, the grasshopper neuroblast, may be summarized as follows. The late prophase orientation of the chromosomes is a carry-over from the late telophase orientation. It is apparently maintained by means of the centromeres, which appear to be attached within a limited region of the nucleus throughout telophase, interphase, and prophase. Metaphase orientation of the chromosomes may be explained as the resultant of two forces: a force involving the centromere and spindle, which is responsible for keeping the centromeres in the equatorial plane of the spindle, and a repulsion force involving the noncentromeric portion of the chromosomes, which results in a tendency toward uniform spacing of the chromosomes outside the spindle. Anaphase separation of sister chromatids and their subsequent movement toward the poles of the spindle involves at least four distinct phases: (i) the initial poleward movement of the centromeres, which may be due to intrinsic repulsion or to a force acting between spindle and centromeres that produces an angle of almost 90 degrees between the separated and unseparated portions of the chromatids; (ii) the autonomous separation of the noncentromeric part of the chromosome; (iii) elongation of the spindle, beginning just after the sister chromatids are separated proximally and ending when the longer chromatids are about to lose contact distally; and (iv) the later movement apart of the daughter chromosomes, probably resulting from a pushing force exerted by elongation of the interzonal fibers.     

黑沙蒿与粉土质砂根-土及土-土界面摩阻特性

以鄂尔多斯上湾煤矿塌陷区水土保持先锋灌木之一黑沙蒿和粉土质砂为研究对象,通过直剪试验和根-土复合体单根拉拔试验,探究土壤质量含水率、根径对黑沙蒿根-土界面及土-土界面摩阻特性的影响。结果表明:在土壤埋深72 cm和144 cm左右,2种剪切界面的抗剪强度值均随质量含水率增加呈先增后减趋势;黑沙蒿根-土复合体最大拉拔力与根径呈明显线性正相关,其抗剪强度随根径增加而减小;土壤质量含水率从3.94%增至19.94%,黑沙蒿根-土界面摩擦系数由0.48减至0.38,土-土界面摩擦系数由0.43减至0.35;2种剪切界面的黏聚力随土壤质量含水率增加呈现先增后减趋势,土壤质量含水率为11.94%时,黑沙蒿根-土界面黏聚力峰值为4.51 kPa,土-土界面黏聚力峰值为4.56 kPa。     

甘蔗茎尖细胞有丝分裂过程中微管骨架的变化

笔者拟从微管列阵变化及其参与染色体运动的关系方面探讨甘蔗茎增粗机理,为单子叶植物的微管与染色体相关研究提供一定的例证。     

甘蔗茎尖细胞有丝分裂过程中微管骨架的变化（英文）

In order to understand the microtubule change of monocotyls stem-tip during mitosis,the arrangement,transformation of microtubule array and its relation with chromosome movement during mitosis were studied with freezing microtome,indirect immunofluorescence,DAPI staining and fluorescence microscopy.The results showed that nucleolus was intact when the cortical microtubules formed;cortical microtubules were changed into phramoplast microtubule bands at mitosis prophase.When phramoplast microtubule came into being,nuclear membrane was ruptured and chromosome was arranged at the position of cell plate;subsequently,phramoplast microtubules were changed into phragmoplast microtubules,phramoplast microtubules were shortening and microtubules on the sides of cell plate were increasing gradually,during this course sister chromatid was separated by microtubules at cell plate and tract to the two poles,forming phragmoplast microtubules.Then the nucleolus of two daughter cells formed and separated in the end with the increase of cells numbers.Therefore,cell division orientation could be judged from the arrangement of cell microtubules in different periods in order to understand its growth status.     

甘蔗茎尖细胞有丝分裂过程中微管骨架的变化

[目的]研究单子叶植物茎尖有丝分裂细胞的微管动态变化情况。[方法]采用冰冻切片法结合间接免疫荧光技术及DAPI染色,利用荧光显微镜观察甘蔗茎尖细胞有丝分裂时微管列阵的排列、转换及与染色体运动的关系。[结果]当周质微管形成时,细胞核保持完整;有丝分裂前期,周质微管转变为早前期微管带;当纺锤体微管形成时,细胞核膜破裂,染色体排列在细胞板位置;之后纺锤体微管向成膜体微管转换,纺锤体微管逐渐缩短而细胞板两侧微管逐渐增加,在这个过程中姊妹染色体被微管从细胞板处分离并牵引至两极,从而形成成膜体微管;之后两个子细胞的细胞核形成,并最终分裂,细胞数量增加。[结论]从细胞微管各时期的排列就可以判断出细胞分裂方向,了解其生长情况,为甘蔗增粗机理的研究提供例证。     

Requirement of Mis6 centromere connector for localizing a CENP-A-like protein in fission yeast

Mammalian kinetochores contain the centromere-specific histone H3 variant CENP-A, whose incorporation into limited chromosomal regions may be important for centromere function and chromosome segregation during mitosis. However, regulation of CENP-A localization and its role have not been clear. Here we report that the fission yeast homolog SpCENP-A is essential for establishing centromere chromatin associated with equal chromosome segregation. SpCENP-A binding to the nonrepetitious inner centromeres depended on Mis6, an essential centromere connector protein acting during G1-S phase of the cell cycle. Mis6 is likely required for recruiting SpCENP-A to form proper connection of sister centromeres.     

Gating of retinal transmission by afferent eye position and movement signals

Vision in most vertebrates is an active process that requires the brain to combine retinal signals with information about eye movement. Eye movement information may feed forward from the motor control areas of the brain or feed back from the extrinsic eye muscles. Feedback signals elicited by passive eye movement selectively gate retinal outflow at the first relay, the dorsal lateral geniculate nucleus. The gating predominantly facilitates retinogeniculate transmission immediately after eye movement and inhibits transmission when a new steady-state eye position is achieved. These two gating effects are distributed in a complementary fashion across the dorsal lateral geniculate nucleus such that the spatiotemporal activity profile could contribute to object detection and localization.     

Two histone marks establish the inner centromere and chromosome bi-orientation

For proper partitioning of chromosomes in mitosis, the chromosomal passenger complex (CPC) including Aurora B and survivin must be localized at the center of paired kinetochores, at the site called the inner centromere. It is largely unknown what defines the inner centromere and how the CPC is targeted to this site. Here, we show that the phosphorylation of histone H3-threonine 3 (H3-pT3) mediated by Haspin cooperates with Bub1-mediated histone 2A-serine 121 (H2A-S121) phosphorylation in targeting the CPC to the inner centromere in fission yeast and human cells. H3-pT3 promotes nucleosome binding of survivin, whereas phosphorylated H2A-S121 facilitates the binding of shugoshin, the centromeric CPC adaptor. Haspin colocalizes with cohesin by associating with Pds5, whereas Bub1 localizes at kinetochores. Thus, the inner centromere is defined by intersection of two histone kinases.     

A mitotic septin scaffold required for Mammalian chromosome congression and segregation

Coordination of cytokinesis with chromosome congression and segregation is critical for proper cell division, but the mechanism is unknown. Here, septins, a conserved family of polymerizing guanosine triphosphate-binding proteins, localized to the metaphase plate during mitosis. Septin depletion resulted in chromosome loss from the metaphase plate, lack of chromosome segregation and spindle elongation, and incomplete cytokinesis upon delayed mitotic exit. These defects correlated with loss of the mitotic motor and the checkpoint regulator centromere-associated protein E (CENP-E) from the kinetochores of congressing chromosomes. Mammalian septins may thus form a mitotic scaffold for CENP-E and other effectors to coordinate cytokinesis with chromosome congression and segregation.     

Polymer brushes

Polymers attached by one end to an interface at relatively high coverage stretch away from the interface to avoid overlapping, forming a polymer "brush." This simple picture may serve as the basis for models in diverse interfacial systems in polymer science, such as polymeric surfactants, stabilized suspensions of colloidal particles, and structures formed by block copolymers. The structure and dynamics of polymer brushes have been the subject of considerable theoretical and experimental activity in recent years. An account is given of recent advances in theoretical understanding of stretched polymers at interfaces, and the diverse experimental probes of systems modeled by brushes are briefly reviewed.     

Separase regulates INCENP-Aurora B anaphase spindle function through Cdc14

The inner centromere-like protein (INCENP) forms a complex with the evolutionarily conserved family of Aurora Bkinases. The INCENP-Aurora complex helps coordinate chromosome segregation, spindle behavior, and cytokinesis during mitosis. INCENP-Aurora associates with kinetochores in metaphase and with spindle microtubules in anaphase, yet the trigger for this abrupt transfer is unknown. Here we show that the conserved phosphatase Cdc14 regulated the yeast INCENP-Aurora complex, Sli15-Ipl1. Cdc14 dephosphorylated Sli15 and thereby directed the complex to spindles. Activation of Cdc14 by separase was sufficient for Sli15 dephosphorylation and relocalization. Cdc14 not only regulates mitotic exit but also modulates spindle midzone assembly through Sli15-Ipl1.     

The dynamics of the onset of frictional slip

The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip. Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.     

The global atmospheric circulation on moist isentropes

The global atmospheric circulation transports energy from the equatorial regions to higher latitudes through a poleward flow of high-energy and -entropy parcels and an equatorward flow of air with lower energy and entropy content. Because of its turbulent nature, this circulation can only be described in some averaged sense. Here, we show that the total mass transport by the circulation is twice as large when averaged on moist isentropes than when averaged on dry isentropes. The additional mass transport on moist isentropes corresponds to a poleward flow of warm moist air near Earth's surface that rises into the upper troposphere within mid-latitudes and accounts for up to half of the air in the upper troposphere in polar regions.     

设施黄瓜膜下滴灌土壤中无机阴离子的运移和分布规律

试验在黄瓜定植后的不同时期分别从水平方向和垂直方向取土样并测定土壤中的无机阴离子含量,研究膜下滴灌方式下无机阴离子NO3-、SO42-和Cl-在土壤中的运移和分布规律。结果表明:在追肥时期,土壤中NO3-、SO42-和Cl-呈上升趋势,定植后40天测得土壤各层阴离子含量为整个生长期最高。定植40天以后不再向土壤中追施肥料,不同土层中的阴离子含量呈下降趋势。垂直方向,NO3-较快,最终3个土层中的NO3-含量基本相同;SO42-和Cl-开始下降幅度大,后变得缓慢。水平方向,距离滴头较远(10~15cm)土层中的阴离子含量较高。     

Light propagation with phase discontinuities: generalized laws of reflection and refraction

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat's principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.     

Closing of the nucleotide pocket of kinesin-family motors upon binding to microtubules

We have used adenosine diphosphate analogs containing electron paramagnetic resonance (EPR) spin moieties and EPR spectroscopy to show that the nucleotide-binding site of kinesin-family motors closes when the motor.diphosphate complex binds to microtubules. Structural analyses demonstrate that a domain movement in the switch 1 region at the nucleotide site, homologous to domain movements in the switch 1 region in the G proteins [heterotrimeric guanine nucleotide-binding proteins], explains the EPR data. The switch movement primes the motor both for the free energy-yielding nucleotide hydrolysis reaction and for subsequent conformational changes that are crucial for the generation of force and directed motion along the microtubule.     

Simulations of a quasi-Taylor state geomagnetic field including polarity reversals on the Earth Simulator

High-resolution, low-viscosity geodynamo simulations have been carried out on the Earth Simulator, one of the fastest supercomputers, in a dynamic regime similar to that of Earth's core, that is, in a quasi-Taylor state. Our dynamo models exhibit features of the geodynamo not only in spatial and temporal characteristics but also in dynamics. Polarity reversals occurred when magnetic flux patches at high latitudes moved poleward and disappeared; patches with reversed field at low and mid-latitudes then moved poleward.