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1.
CARLSON JG 《Science (New York, N.Y.)》1956,124(3214):203-206
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. 相似文献
2.
Cross-striated muscle fibers may contract reversibly to less than 30 percent of their rest length and it is not easy to reconcile this fact with the sliding filament model of muscular contraction. The mechanism of supercontraction has been studied in fibrils obtained from the giant muscle fibers of the barnacle Balanus nubilus. They were examined by phase-contrast light microscopy and electron microscopy. Contraction beyond the 50-percent stage was found to be achieved largely by the passage of thick filaments through the Z-disks, which are perforated. The overlap of thick filaments from adjacent sarcomeres causes the appearance of the contraction bands about the Z-disks. Subsequent contraction is associated with a folding and loose coiling, but not a shortening, of the thick filaments. 相似文献
3.
Polymorphism of sickle cell hemoglobin aggregates: structural basis for limited radial growth 总被引：2，自引：0，他引：2
Fibers composed of molecules of deoxygenated sickle cell hemoglobin are the basic cause of pathology in sickle cell disease. The hemoglobin molecules in these fibers are arranged in double strands that twist around one another with a long axial repeat. These fibrous aggregates exhibit a pattern of polymorphism in which the ratio of their helical pitch to their radius is approximately constant. The observed ratio agrees with an estimate of its value calculated from the geometric properties of helical assemblies and the degree of distortion that a protein-protein interface can undergo. This agreement indicates that the radius of an aggregate is limited by the maximum possible stretching of double strands. The geometric properties limiting the radial extent of sickle hemoglobin fibers are fundamental to all cables of protein filaments and could contribute to the control of diameter in other biological fibers such as collagen or fibrin. 相似文献
4.
Conservation of brain amyloid proteins in aged mammals and humans with Alzheimer's disease 总被引：37，自引：0，他引：37
D J Selkoe D S Bell M B Podlisny D L Price L C Cork 《Science (New York, N.Y.)》1987,235(4791):873-877
The formation of clusters of altered axons and dendrites surrounding extracellular deposits of amyloid filaments (neuritic plaques) is a major feature of the human brain in both aging and Alzheimer's disease. A panel of antibodies against amyloid filaments and their constituent proteins from humans with Alzheimer's disease cross-reacted with neuritic plaque and cerebrovascular amyloid deposits in five other species of aged mammals, including monkey, orangutan, polar bear, and dog. Antibodies to a 28-amino acid peptide representing the partial protein sequence of the human amyloid filaments recognized the cortical and microvascular amyloid of all of the aged mammals examined. Plaque amyloid, plaque neurites, and neuronal cell bodies in the aged animals showed no reaction with antibodies to human paired helical filaments. Thus, with age, the amyloid proteins associated with progressive cortical degeneration in Alzheimer's disease are also deposited in the brains of other mammals. Aged primates can provide biochemically relevant models for principal features of Alzheimer's disease: cerebrovascular amyloidosis and neuritic plaque formation. 相似文献
5.
The continuous-filament ceramic composite is becoming recognized as necessary for new, high-temperature structural applications. Yet because of the susceptibility of the filaments to damage from traditional methods for the preparation of ceramics, vapor-phase infiltration has become the fabrication method of choice. The chemical vapor infiltration methods for producing these composites are now being studied in earnest, with the complexity of filament weaves and deposition chemistry being merged with standard heat and mass-transport relationships. Two of the most influential effects on the mechanical properties of these materials are the adhesion and frictional force between the fibers and the matrix, which can be controlled by a tailored interface coating. A variety of materials are available for producing these composites including carbide, nitride, boride, and oxide filaments and matrices. Silicon carbide-based materials are by far the most advanced and are already being used in aerospace applications. 相似文献
6.
【目的】研究生理型雄性不育小麦花粉细胞内微丝和胼胝质的结构及其相关基因的表达,并揭示其与生理型雄性不育的关系,为进一步研究化学杂交剂SQ-1诱导小麦生理型雄性不育的机理提供一定的理论依据。【方法】以化学杂交剂SQ-1诱导的生理型雄性不育系ms(A)-西农1376及对应正常可育系(A)-西农1376为试材,用TRITC-phalloidin标记细胞内微丝,苯胺蓝标记胼胝质,qRT-PCR技术分别对肌动蛋白解聚因子TaADF（Actin depolymerizing factor）、类葡聚糖合成酶TaGSL（Glucan synthase-like）进行差异表达分析。【结果】(1)在减数分裂前期Ⅰ、中期Ⅰ、后期Ⅰ这三个时期,生理型雄性不育系花粉细胞的微丝结构与可育系没有显著差异：前期Ⅰ,微丝分布于整个细胞质中,细胞核区域也可见少量微丝环绕细胞核;中期Ⅰ,微丝分布在细胞质中,在形成纺锤体部位染色更深,形成纺锤体微丝,由细胞两极发出的纺锤体微丝伸向赤道板;后期Ⅰ,在向两极移动的染色体的中间部位染色较深,微丝分布较多。(2)在早末期Ⅰ,与可育系相比,不育系花粉细胞没有形成清晰且明显可见的中国灯笼状成膜体微丝结构,且在细胞中线部位亦没有清晰可见的微丝累积。(3)晚末期Ⅰ,可育系花粉细胞在形成细胞板的部位是线性的、平滑的,成膜体微丝消失,而不育系花粉细胞在形成细胞板的部位形成了很大的缝隙,同时,可育系胼胝质在细胞板处的沉积比较平滑,而不育系胼胝质在细胞板处的沉积较可育系相比缺乏,并且是褶皱的、有裂纹的。(4)四分体时期,可育系花粉可见围绕细胞核的辐射状微丝,不育系花粉细胞中微丝呈模糊状态,并且不育系中胼胝质染色的整体荧光强度较可育系减弱。利用实时荧光定量PCR技术分析肌动蛋白解聚因子TaADF和类葡聚糖合成酶TaGSL在减数分裂期的相对表达量,结果发现,不育系中TaADF的相对表达量是可育系的4.28倍,由于TaADF表达量上调,加剧了细胞内微丝解聚,微丝结构受到破坏,同时不育系中TaGSL表达量下降,只有可育系的0.83倍,胼胝质的沉积也受到影响。【结论】TaADF在不育系中上调表达,破坏了细胞内微丝的正常结构,使微丝不能正常行使其功能,进而可能导致花药发育中与育性相关的某些代谢通路等受到影响。与此同时,微丝结构的破坏导致细胞板形成出现异常也可能是引起胼胝质在细胞板处沉积受到影响的一个重要原因。因此,微丝和胼胝质的异常变化与化学杂交剂SQ-1诱导的生理型雄性不育密切相关。 相似文献
7.
Multiple unrelated polymer systems have evolved to partition DNA molecules between daughter cells at division. To better understand polymer-driven DNA segregation, we reconstituted the three-component segregation system of the R1 plasmid from purified components. We found that the ParR/parC complex can construct a simple bipolar spindle by binding the ends of ParM filaments, inhibiting dynamic instability, and acting as a ratchet permitting incorporation of new monomers and riding on the elongating filament ends. Under steady-state conditions, the dynamic instability of unattached ParM filaments provides the energy required to drive DNA segregation. 相似文献
8.
The orbital region of the cortex in the cat is essential to the occurrence of spontaneous spindle bursts and thalamically induced recruiting responses, observed in both cortex and thalamus. Ablations of the entire dorsal convexity, and of the mesial and cingulate regions of the cortex, failed to interfere with the spindle bursts and recruiting responses, whereas ablations confined to the orbital cortex alone abolished completely these potentials in the cortex and thalamus. Therefore, the orbital cortex appears to be the only region of the neocortex to play a crucial role in the regulation of thalamocortical synchronizing and integrating functions. These functions are believed to be associated with a nonspecific system governing internal inhibition which manifests itself in inattention, drowsiness, and sleep. 相似文献
9.
Galkin VE Yu X Bielnicki J Heuser J Ewing CP Guerry P Egelman EH 《Science (New York, N.Y.)》2008,320(5874):382-385
It has been widely assumed that the atomic structure of the flagellar filament from Salmonella typhimurium serves as a model for all bacterial flagellar filaments given the sequence conservation in the coiled-coil regions responsible for polymerization. On the basis of electron microscopic images, we show that the flagellar filaments from Campylobacter jejuni have seven protofilaments rather than the 11 in S. typhimurium. The vertebrate Toll-like receptor 5 (TLR5) recognizes a region of bacterial flagellin that is involved in subunit-subunit assembly in Salmonella and many other pathogenic bacteria, and this short region has diverged in Campylobacter and related bacteria, such as Helicobacter pylori, which are not recognized by TLR5. The driving force in the change of quaternary structure between Salmonella and Campylobacter may have been the evasion of TLR5. 相似文献
10.
Nosema cuiculi: in vitro isolation 总被引：8，自引：0，他引：8
J A Shadduck 《Science (New York, N.Y.)》1969,166(904):516-517
A mammalian microsporidian was propagated for the first time in vitro with rabbit choroid plexus monolayer cultures. Tentative classification as Nosema cuniculi (Levaditi, 1923) was established by morphologic and tinctorial characteristics and by extrusion of sintgle polar filaments with attached sporoplasin from living spores. The method permits propagation of large numbers of organisms in putre cultutre. 相似文献
11.
细胞松弛素B对牛卵母细胞体外成熟过程中微管、微丝及染色体的影响 总被引：1，自引：0，他引：1
细胞松弛素B(Cytochalasin B,CB)是一种抑制微丝聚合的药物,能抑制微丝的组装从而阻止胞质分裂和极体排放,是研究细胞分裂器形成与变化的重要药物。在牛卵母细胞体外成熟培养过程中加入7.5 μg/mL的CB进行处理,分析CB对减数分裂过程中细胞骨架形态、染色体的排列与分离等方面的影响。结果显示,CB处理后卵母细胞第一极体的排放受到了抑制,染色体的排列和分离受到了影响,出现了同源染色体分离不完全或分离不均匀及分离后又聚在一起等异常情况,形成许多二倍体卵母细胞;纺锤体微管的形态发生了变化,出现了两个纺锤体、巨大纺锤体和多极纺锤体等异常结构;微丝的正常分布受到了影响,染色体周围没有或少有微丝分布,皮质下的微丝分布也变得少而不均匀;这说明微丝与微管在减数分裂过程中是协同作用的,CB通过影响微丝的动态变化,改变了纺锤体微管的形态结构,最终抑制了极体的排放。 相似文献
12.
The dynamics of the stratospheric polar vortex and its relation to springtime ozone depletions 总被引：1，自引：0，他引：1
Dramatic springtime depletions of ozone in polar regions require that polar stratospheric air has a high degree of dynamical isolation and extremely cold temperatures necessary for the formation of polar stratospheric clouds. Both of these conditions are produced within the stratospheric winter polar vortex. Recent aircraft missions have provided new information about the structure of polar vortices during winter and their relation to polar ozone depletions. The aircraft data show that gradients of potential vorticity and the concentration of conservative trace species are large at the transition from mid-latitude to polar air. The presence of such sharp gradients at the boundary of polar air implies that the inward mixing of heat and constituents is strongly inhibited and that the perturbed polar stratospheric chemistry associated with the ozone hole is isolated from the rest of the stratosphere until the vortex breaks up in late spring. The overall size of the polar vortex thus limits the maximum areal coverage of the annual polar ozone depletions. Because it appears that this limit has not been reached for the Antarctic depletions, the possibility of future increases in the size of the Antarctic ozone hole is left open. In the Northern Hemisphere, the smaller vortex and the more restricted region of cold temperatures suggest that this region has a smaller theoretical maximum for column ozone depletion, about 40 percent of the currently observed change in the Antarctic ozone column in spring. 相似文献
13.
应用光、电镜技术研究发现:新排卵母细胞的纺锤体位于动物极质膜下,长轴与质膜平行,卵周隙狭窄,除纺锤体区外,微绒毛呈倒伏状遍布于卵细胞表面,皮质颗粒排列于质膜下,线粒体多分布于卵母细胞近中区及染色体旁;排卵后2～4h,纺锤体在质膜下旋转90°,卵周隙增宽,微绒毛逐渐竖起,少数皮质颗粒提前外排,线粒体趋于分散,排卵后8h,纺锤体内移,微绒毛趋于融合,皮质颗粒减少,线粒体退化。 相似文献
14.
Kapoor TM Lampson MA Hergert P Cameron L Cimini D Salmon ED McEwen BF Khodjakov A 《Science (New York, N.Y.)》2006,311(5759):388-391
The stable propagation of genetic material during cell division depends on the congression of chromosomes to the spindle equator before the cell initiates anaphase. It is generally assumed that congression requires that chromosomes are connected to the opposite poles of the bipolar spindle ("bioriented"). In mammalian cells, we found that chromosomes can congress before becoming bioriented. By combining the use of reversible chemical inhibitors, live-cell light microscopy, and correlative electron microscopy, we found that monooriented chromosomes could glide toward the spindle equator alongside kinetochore fibers attached to other already bioriented chromosomes. This congression mechanism depended on the kinetochore-associated, plus end-directed microtubule motor CENP-E (kinesin-7). 相似文献
15.
Many routes have been developed for the synthesis of carbon nanotubes, but their assembly into continuous fibers has been achieved only through postprocessing methods. We spun fibers and ribbons of carbon nanotubes directly from the chemical vapor deposition (CVD) synthesis zone of a furnace using a liquid source of carbon and an iron nanocatalyst. This process was realized through the appropriate choice of reactants, control of the reaction conditions, and continuous withdrawal of the product with a rotating spindle used in various geometries. This direct spinning from a CVD reaction zone is extendable to other types of fiber and to the spin coating of rotating objects in general. 相似文献
16.
Baroreceptors in the adrenal gland 总被引：1，自引：0，他引：1
Afferent activity from dissected filaments of nerves to the adrenal gland was recorded. Firing rates of single fibers followed changes in systemic blood pressure. Chemically and mechanically induced changes in blood pressure were reflected by an appropriate directional change in unit activity. 相似文献
17.
Paramyosin: molecular length and assembly 总被引：1，自引：0，他引：1
J Kendrick-Jones C Cohen A G Szent-Gy?rgyi W Longley 《Science (New York, N.Y.)》1969,163(872):1196-1198
Paratnyosin paracrystails formned with divalent cations have a 725-angstrom axial period and show simple negative staining patterns in the electron microscope. The structure of the aggregates is interpreted in terms of an array of polar molecules about 1275 angstroms long, with "gaps" and "overlaps" in the molecular assembly. Antiparallel relatiotns between molecules lead to the assembly of paracrystals with opposite polarity at either end. Implications of the in vitro structure for filaments containing paramyosin in muscle are discussed. 相似文献
18.
Mitrofanov IG Sanin AB Boynton WV Chin G Garvin JB Golovin D Evans LG Harshman K Kozyrev AS Litvak ML Malakhov A Mazarico E McClanahan T Milikh G Mokrousov M Nandikotkur G Neumann GA Nuzhdin I Sagdeev R Shevchenko V Shvetsov V Smith DE Starr R Tretyakov VI Trombka J Usikov D Varenikov A Vostrukhin A Zuber MT 《Science (New York, N.Y.)》2010,330(6003):483-486
Hydrogen has been inferred to occur in enhanced concentrations within permanently shadowed regions and, hence, the coldest areas of the lunar poles. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to detect hydrogen-bearing volatiles directly. Neutron flux measurements of the Moon's south polar region from the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) spacecraft were used to select the optimal impact site for LCROSS. LEND data show several regions where the epithermal neutron flux from the surface is suppressed, which is indicative of enhanced hydrogen content. These regions are not spatially coincident with permanently shadowed regions of the Moon. The LCROSS impact site inside the Cabeus crater demonstrates the highest hydrogen concentration in the lunar south polar region, corresponding to an estimated content of 0.5 to 4.0% water ice by weight, depending on the thickness of any overlying dry regolith layer. The distribution of hydrogen across the region is consistent with buried water ice from cometary impacts, hydrogen implantation from the solar wind, and/or other as yet unknown sources. 相似文献
19.
Spencer JR Pearl JC Segura M Flasar FM Mamoutkine A Romani P Buratti BJ Hendrix AR Spilker LJ Lopes RM 《Science (New York, N.Y.)》2006,311(5766):1401-1405
The Cassini spacecraft completed three close flybys of Saturn's enigmatic moon Enceladus between February and July 2005. On the third and closest flyby, on 14 July 2005, multiple Cassini instruments detected evidence for ongoing endogenic activity in a region centered on Enceladus' south pole. The polar region is the source of a plume of gas and dust, which probably emanates from prominent warm troughs seen on the surface. Cassini's Composite Infrared Spectrometer (CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures up to 145 kelvin or higher, making Enceladus only the third known solid planetary body-after Earth and Io-that is sufficiently geologically active for its internal heat to be detected by remote sensing. If the plume is generated by the sublimation of water ice and if the sublimation source is visible to CIRS, then sublimation temperatures of at least 180 kelvin are required. 相似文献
20.
The first unambiguous full-disk radar mapping of Mercury at 3.5-centimeter wavelength, with the Goldstone 70-meter antenna transmitting and 26 antennas of the Very Large Array receiving, has provided evidence for the presence of polar ice. The radar experiments, conducted on 8 and 23 August 1991, were designed to image the half of Mercury not photographed by Mariner 10. The orbital geometry allowed viewing beyond the north pole of Mercury; a highly reflective region was clearly visible on the north pole during both experiments. This polar region has areas in which the circular polarization ratio (pt) was 1.0 to 1.4; values < approximately 0.1 are typical for terrestrial planets. Such high values of have hitherto been observed in radar observations only from icy regions of Mars and icy outer planet satellites. 相似文献