全文获取类型
收费全文 | 98篇 |
免费 | 0篇 |
专业分类
系统科学 | 3篇 |
现状及发展 | 11篇 |
研究方法 | 14篇 |
综合类 | 54篇 |
自然研究 | 16篇 |
出版年
2017年 | 1篇 |
2016年 | 1篇 |
2013年 | 1篇 |
2012年 | 2篇 |
2011年 | 25篇 |
2010年 | 1篇 |
2008年 | 5篇 |
2007年 | 6篇 |
2006年 | 5篇 |
2005年 | 7篇 |
2004年 | 10篇 |
2003年 | 7篇 |
2002年 | 7篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有98条查询结果,搜索用时 15 毫秒
61.
Blaga CI Xu J DiChiara AD Sistrunk E Zhang K Agostini P Miller TA DiMauro LF Lin CD 《Nature》2012,483(7388):194-197
Establishing the structure of molecules and solids has always had an essential role in physics, chemistry and biology. The methods of choice are X-ray and electron diffraction, which are routinely used to determine atomic positions with sub-?ngstr?m spatial resolution. Although both methods are currently limited to probing dynamics on timescales longer than a picosecond, the recent development of femtosecond sources of X-ray pulses and electron beams suggests that they might soon be capable of taking ultrafast snapshots of biological molecules and condensed-phase systems undergoing structural changes. The past decade has also witnessed the emergence of an alternative imaging approach based on laser-ionized bursts of coherent electron wave packets that self-interrogate the parent molecular structure. Here we show that this phenomenon can indeed be exploited for laser-induced electron diffraction (LIED), to image molecular structures with sub-?ngstr?m precision and exposure times of a few femtoseconds. We apply the method to oxygen and nitrogen molecules, which on strong-field ionization at three mid-infrared wavelengths (1.7, 2.0 and 2.3?μm) emit photoelectrons with a momentum distribution from which we extract diffraction patterns. The long wavelength is essential for achieving atomic-scale spatial resolution, and the wavelength variation is equivalent to taking snapshots at different times. We show that the method has the sensitivity to measure a 0.1?? displacement in the oxygen bond length occurring in a time interval of ~5?fs, which establishes LIED as a promising approach for the imaging of gas-phase molecules with unprecedented spatio-temporal resolution. 相似文献
62.
Churchill GA Airey DC Allayee H Angel JM Attie AD Beatty J Beavis WD Belknap JK Bennett B Berrettini W Bleich A Bogue M Broman KW Buck KJ Buckler E Burmeister M Chesler EJ Cheverud JM Clapcote S Cook MN Cox RD Crabbe JC Crusio WE Darvasi A Deschepper CF Doerge RW Farber CR Forejt J Gaile D Garlow SJ Geiger H Gershenfeld H Gordon T Gu J Gu W de Haan G Hayes NL Heller C Himmelbauer H Hitzemann R Hunter K Hsu HC Iraqi FA Ivandic B Jacob HJ Jansen RC Jepsen KJ Johnson DK Johnson TE Kempermann G 《Nature genetics》2004,36(11):1133-1137
The goal of the Complex Trait Consortium is to promote the development of resources that can be used to understand, treat and ultimately prevent pervasive human diseases. Existing and proposed mouse resources that are optimized to study the actions of isolated genetic loci on a fixed background are less effective for studying intact polygenic networks and interactions among genes, environments, pathogens and other factors. The Collaborative Cross will provide a common reference panel specifically designed for the integrative analysis of complex systems and will change the way we approach human health and disease. 相似文献
63.
Mitotic chromosome segregation is facilitated by the cohesin complex, which maintains physical connections between sister chromatids until anaphase. Meiotic cell division is considerably more complex, as cohesion must be released sequentially to facilitate orderly segregation of chromosomes at both meiosis I and meiosis II. This necessitates meiosis-specific cohesin components; recent studies in rodents suggest that these influence chromosome behavior during both cell division and meiotic prophase. To elucidate the role of the meiosis-specific cohesin SMC1beta (encoded by Smc1l2) in oogenesis, we carried out meiotic studies of female SMC1beta-deficient mice. Our results provide the first direct evidence that SMC1beta acts as a chiasma binder in mammals, stabilizing sites of exchange until anaphase. Additionally, our observations support the hypothesis that deficient cohesion is an underlying cause of human age-related aneuploidy. 相似文献
64.
Identification of the gene (BBS1) most commonly involved in Bardet-Biedl syndrome,a complex human obesity syndrome 总被引:11,自引:0,他引:11
Mykytyn K Nishimura DY Searby CC Shastri M Yen HJ Beck JS Braun T Streb LM Cornier AS Cox GF Fulton AB Carmi R Lüleci G Chandrasekharappa SC Collins FS Jacobson SG Heckenlively JR Weleber RG Stone EM Sheffield VC 《Nature genetics》2002,31(4):435-438
Bardet-Biedl syndrome (BBS, OMIM 209900) is a genetic disorder with the primary features of obesity, pigmentary retinopathy, polydactyly, renal malformations, mental retardation and hypogenitalism. Individuals with BBS are also at increased risk for diabetes mellitus, hypertension and congenital heart disease. What was once thought to be a homogeneous autosomal recessive disorder is now known to map to at least six loci: 11q13 (BBS1), 16q21 (BBS2), 3p13 p12 (BBS3), 15q22.3 q23 (BBS4), 2q31 (BBS5) and 20p12 (BBS6). There has been considerable interest in identifying the genes that underlie BBS, because some components of the phenotype are common. Cases of BBS mapping ro BBS6 are caused by mutations in MKKS; mutations in this gene also cause McKusick-Kaufman syndrome (hydrometrocolpos, post-axial polydactyly and congenital heart defects). In addition, we recently used positional cloning to identify the genes underlying BBS2 (ref. 16) and BBS4 (ref. 17). The BBS6 protein has similarity to a Thermoplasma acidophilum chaperonin, whereas BBS2 and BBS4 have no significant similarity to chaperonins. It has recently been suggested that three mutated alleles (two at one locus, and a third at a second locus) may be required for manifestation of BBS (triallelic inheritance). Here we report the identification of the gene BBS1 and show that a missense mutation of this gene is a frequent cause of BBS. In addition, we provide data showing that this common mutation is not involved in triallelic inheritance. 相似文献
65.
抗巨片形吸虫同型抗体在动物巨片形吸虫感染所致的抗体依赖性细胞介导的细胞毒反应( ADCC )免疫应答作用的研究 总被引:1,自引:0,他引:1
以印尼短尾羊(ITT)和美利奴细毛羊(Merino)两种品系的绵羊为实验动物,研究动物感染巨片形吸虫后所产生的抗体依赖性细胞介导的细胞毒反应的免疫应答。结果显示:对巨片形吸虫感染具有抗性的印尼短尾羊不产生特异性的IgG2,而易感品系的美利奴细毛羊感染巨片形吸虫后体内产生高滴度特异性IgG2抗体、特异性IgG2抗体在免疫应答中起着封闭抗体的作用,抑制巨噬细胞抗体依赖性细胞介导的细胞毒反应。由于印尼短尾羊不产生特异性IgG2抗体,对巨噬细胞抗体依赖性细胞介导的细胞毒反应不产生抑制作用,因此印尼短尾羊对巨片形吸虫的感染具有一定的抵抗力。使用纯化的IgG1和IgG2进行体外杀虫试验,结果IgG1有很强的促进体外杀虫效果,而自感染巨片形吸虫的美利奴细毛羊血清标本纯化的IgG2没有体外杀虫作用。体外实验还显示嗜酸性粒细胞在免疫血清、补体和IL-5的共同参与下具有较强的杀虫作用。 相似文献
66.
Korn T Bettelli E Gao W Awasthi A Jäger A Strom TB Oukka M Kuchroo VK 《Nature》2007,448(7152):484-487
On activation, naive T cells differentiate into effector T-cell subsets with specific cytokine phenotypes and specialized effector functions. Recently a subset of T cells, distinct from T helper (T(H))1 and T(H)2 cells, producing interleukin (IL)-17 (T(H)17) was defined and seems to have a crucial role in mediating autoimmunity and inducing tissue inflammation. We and others have shown that transforming growth factor (TGF)-beta and IL-6 together induce the differentiation of T(H)17 cells, in which IL-6 has a pivotal function in dictating whether T cells differentiate into Foxp3+ regulatory T cells (T(reg) cells) or T(H)17 cells. Whereas TGF-beta induces Foxp3 and generates T(reg) cells, IL-6 inhibits the generation of T(reg) cells and induces the production of IL-17, suggesting a reciprocal developmental pathway for T(H)17 and T(reg) cells. Here we show that IL-6-deficient (Il6-/-) mice do not develop a T(H)17 response and their peripheral repertoire is dominated by Foxp3+ T(reg) cells. However, deletion of T(reg) cells leads to the reappearance of T(H)17 cells in Il6-/- mice, suggesting an additional pathway by which T(H)17 cells might be generated in vivo. We show that an IL-2 cytokine family member, IL-21, cooperates with TGF-beta to induce T(H)17 cells in naive Il6-/- T cells and that IL-21-receptor-deficient T cells are defective in generating a T(H)17 response. 相似文献
67.
All 25 sablefish ( Anoplopoma fimbria ) examined from two bays near Sitka, Alaska, were infected with anisakid larvae. There were 1 to 11 larvae per infected fish, with worms encysted in the musculature of the body wall of 5 fish and in the liver of 4 fish. For the other hosts the viscera was the site of infection. Chinook salmon ( Onchorhynchus tshawytscha ) from Barrow, Alaska, and Obsidian, Idaho, were also infected with anisakid larvae. These data extend the known northern distribution of the anisakids along the Pacific Coast for sablefish and chinook salmon. The pathogenesis of the migratory pathway of anisakid larvae is described, and comments on human health implications are presented. 相似文献
68.
Barbieri CE Baca SC Lawrence MS Demichelis F Blattner M Theurillat JP White TA Stojanov P Van Allen E Stransky N Nickerson E Chae SS Boysen G Auclair D Onofrio RC Park K Kitabayashi N MacDonald TY Sheikh K Vuong T Guiducci C Cibulskis K Sivachenko A Carter SL Saksena G Voet D Hussain WM Ramos AH Winckler W Redman MC Ardlie K Tewari AK Mosquera JM Rupp N Wild PJ Moch H Morrissey C Nelson PS Kantoff PW Gabriel SB Golub TR Meyerson M Lander ES Getz G Rubin MA Garraway LA 《Nature genetics》2012,44(6):685-689
69.
The worldwide leaf economics spectrum 总被引:48,自引:0,他引:48
Wright IJ Reich PB Westoby M Ackerly DD Baruch Z Bongers F Cavender-Bares J Chapin T Cornelissen JH Diemer M Flexas J Garnier E Groom PK Gulias J Hikosaka K Lamont BB Lee T Lee W Lusk C Midgley JJ Navas ML Niinemets U Oleksyn J Osada N Poorter H Poot P Prior L Pyankov VI Roumet C Thomas SC Tjoelker MG Veneklaas EJ Villar R 《Nature》2004,428(6985):821-827
Bringing together leaf trait data spanning 2,548 species and 175 sites we describe, for the first time at global scale, a universal spectrum of leaf economics consisting of key chemical, structural and physiological properties. The spectrum runs from quick to slow return on investments of nutrients and dry mass in leaves, and operates largely independently of growth form, plant functional type or biome. Categories along the spectrum would, in general, describe leaf economic variation at the global scale better than plant functional types, because functional types overlap substantially in their leaf traits. Overall, modulation of leaf traits and trait relationships by climate is surprisingly modest, although some striking and significant patterns can be seen. Reliable quantification of the leaf economics spectrum and its interaction with climate will prove valuable for modelling nutrient fluxes and vegetation boundaries under changing land-use and climate. 相似文献
70.
Relation of fish communities to environmental conditions in urban streams of the Wasatch Front, Utah
Twenty-eight sites along the Wasatch Front, north central Utah, representing the range of urban land use intensity for wadeable streams of the area, were sampled in September 2000. Fish communities were assessed by single-pass electrofishing, and physical habitat and water-quality characteristics were measured. On average, nonnative species comprised 54% of species richness and 53% of relative abundance, although only Salmo trutta and Pimephales promelas were very abundant at any 1 site. Salmo trutta and Catostomus platyrhynchus , a native species, were the most widely distributed and abundant species captured. Analysis of fish communities using nonmetric multidimensional scaling (NMDS) revealed a community gradient from sites dominated by Salmo trutta and Cottus species ( C. bairdi , C. beldingi ) to sites dominated by Catostomus platyrhynchus . Sites dominated by C. platyrhynchus were smaller in size and had less habitat cover, smaller average substrate size, higher concentrations of dissolved constituents, and higher water temperature than sites dominated by S. trutta . Sites dominated by C. platyrhynchus were located in more intensely urbanized watersheds at lower elevations. Stream size and associated instream habitat availability appear to limit S. trutta distribution and abundance, while native species appear more tolerant of decreased water quality and increased water temperatures in more urbanized streams. Most of the study sites are affected by development of water infrastructure for human water use (e.g., dams and diversions), and this infrastructure may play a role in dispersal of species. 相似文献