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排序方式: 共有150条查询结果,搜索用时 31 毫秒
131.
应用自编子程序与通用软件ABAQUSTM联机模拟计算了考虑潜热影响的激光涂敷过程温度场的变化.为定量地考察潜热的影响,使用了考虑潜热及不考虑潜热两种分析模型.计算结果表明,在激光涂敷过程的温度场分析中,潜热的影响不是总可以忽略的,其误差取决于材料在物相转变时交界面温差的范围及材料熔解体积的大小.考虑潜热的有限元模型分析结果和忽略潜热的计算结果比较,前者显示出较低的最高温度、较小的熔池范围和较窄的热影响域.另外,在移动热源作用下潜热的影响比固定热源更为突出.与实验结果比较,证明考虑潜热的计算模型会得出更精确的结果. 相似文献
132.
Calmodulin interacts with MLO protein to regulate defence against mildew in barley 总被引:29,自引:0,他引:29
Kim MC Panstruga R Elliott C Müller J Devoto A Yoon HW Park HC Cho MJ Schulze-Lefert P 《Nature》2002,416(6879):447-451
In plants, defence against specific isolates of a pathogen can be triggered by the presence of a corresponding race-specific resistance gene, whereas resistance of a more broad-spectrum nature can result from recessive, presumably loss-of-regulatory-function, mutations. An example of the latter are mlo mutations in barley, which have been successful in agriculture for the control of powdery mildew fungus (Blumeria graminis f. sp. hordei; Bgh). MLO protein resides in the plasma membrane, has seven transmembrane domains, and is the prototype of a sequence-diversified family unique to plants, reminiscent of the seven-transmembrane receptors in fungi and animals. In animals, these are known as G-protein-coupled receptors and exist in three main families, lacking sequence similarity, that are thought to be an example of molecular convergence. MLO seems to function independently of heterotrimeric G proteins. We have identified a domain in MLO that mediates a Ca2+-dependent interaction with calmodulin in vitro. Loss of calmodulin binding halves the ability of MLO to negatively regulate defence against powdery mildew in vivo. We propose a sensor role for MLO in the modulation of defence reactions. 相似文献
133.
The fundamental mechanism behind laser action leads in general only to narrowband, single-wavelength emission. Several approaches for achieving spectrally broadband laser action have been put forward, such as enhancing the optical feedback in the wings of the gain spectrum, multi-peaked gain spectra, and the most favoured technique at present, ultrashort pulse excitation. Each of these approaches has drawbacks, such as a complex external laser cavity configuration, a non-flat optical gain envelope function, or an inability to operate in continuous mode, respectively. Here we present a monolithic, mid-infrared 'supercontinuum' semiconductor laser that has none of these drawbacks. We adopt a quantum cascade configuration, where a number of dissimilar intersubband optical transitions are made to cooperate in order to provide broadband optical gain from 5 to 8 microm wavelength. Laser action with a Fabry-Pérot spectrum covering all wavelengths from 6 to 8 microm simultaneously is demonstrated with this approach. Lasers that emit light over such an extremely wide wavelength range are of interest for applications as varied as terabit optical data communications or ultra-precision metrology and spectroscopy. 相似文献
134.
Lee C Hong B Choi JM Kim Y Watanabe S Ishimi Y Enomoto T Tada S Kim Y Cho Y 《Nature》2004,430(7002):913-917
To maintain chromosome stability in eukaryotic cells, replication origins must be licensed by loading mini-chromosome maintenance (MCM2-7) complexes once and only once per cell cycle. This licensing control is achieved through the activities of geminin and cyclin-dependent kinases. Geminin binds tightly to Cdt1, an essential component of the replication licensing system, and prevents the inappropriate reinitiation of replication on an already fired origin. The inhibitory effect of geminin is thought to prevent the interaction between Cdt1 and the MCM helicase. Here we describe the crystal structure of the mouse geminin-Cdt1 complex using tGeminin (residues 79-157, truncated geminin) and tCdt1 (residues 172-368, truncated Cdt1). The amino-terminal region of a coiled-coil dimer of tGeminin interacts with both N-terminal and carboxy-terminal parts of tCdt1. The primary interface relies on the steric complementarity between the tGeminin dimer and the hydrophobic face of the two short N-terminal helices of tCdt1 and, in particular, Pro 181, Ala 182, Tyr 183, Phe 186 and Leu 189. The crystal structure, in conjunction with our biochemical data, indicates that the N-terminal region of tGeminin might be required to anchor tCdt1, and the C-terminal region of tGeminin prevents access of the MCM complex to tCdt1 through steric hindrance. 相似文献
135.
Giovannoni SJ Bibbs L Cho JC Stapels MD Desiderio R Vergin KL Rappé MS Laney S Wilhelm LJ Tripp HJ Mathur EJ Barofsky DF 《Nature》2005,438(7064):82-85
Proteorhodopsins are light-dependent proton pumps that are predicted to have an important role in the ecology of the oceans by supplying energy for microbial metabolism. Proteorhodopsin genes were first discovered through the cloning and sequencing of large genomic DNA fragments from seawater. They were later shown to be widely distributed, phylogenetically diverse, and active in the oceans. Proteorhodopsin genes have not been found in cultured bacteria, and on the basis of environmental sequence data, it has not yet been possible to reconstruct the genomes of uncultured bacterial strains that have proteorhodopsin genes. Although the metabolic effect of proteorhodopsins is uncertain, they are thought to function in cells for which the primary mode of metabolism is the heterotrophic assimilation of dissolved organic carbon. Here we report that SAR11 strain HTCC1062 ('Pelagibacter ubique'), the first cultivated member of the extraordinarily abundant SAR11 clade, expresses a proteorhodopsin gene when cultured in autoclaved seawater and in its natural environment, the ocean. The Pelagibacter proteorhodopsin functions as a light-dependent proton pump. The gene is expressed by cells grown in either diurnal light or in darkness, and there is no difference between the growth rates or cell yields of cultures grown in light or darkness. 相似文献
136.
L. K. N. Cho J. A. Lowe R. B. Maguire J. C. Tsang 《Cellular and molecular life sciences : CMLS》1987,43(4):397-399
Summary Prodigiosin condensing enzyme (PCE) activities were present inSerratia marcescens wild type 08, mutants OF, WF and 9-3-3. Their specific activities exhibited different maxima and at different times during the late log phase or the early stationary phase of cell growth. The levels of prodigiosin and its precursors also showed a significant increase at this period. The results support that prodigiosin and/or its precursors are secondary metabolites. The ubiquity of the PCE activity in mutants deficient in prodigiosin biosynthesis suggest that this particular enzyme may also be present in non-pigmented clinical isolates. 相似文献
137.
Recent advances in strategies for synthesizing nanoparticles--such as semiconductor quantum dots, magnets and noble-metal clusters--have enabled the precise control of composition, size, shape, crystal structure, and surface chemistry. The distinct properties of the resulting nanometre-scale building blocks can be harnessed in assemblies with new collective properties, which can be further engineered by controlling interparticle spacing and by material processing. Our study is motivated by the emerging concept of metamaterials-materials with properties arising from the controlled interaction of the different nanocrystals in an assembly. Previous multi-component nanocrystal assemblies have usually resulted in amorphous or short-range-ordered materials because of non-directional forces or insufficient mobility during assembly. Here we report the self-assembly of PbSe semiconductor quantum dots and Fe2O3 magnetic nanocrystals into precisely ordered three-dimensional superlattices. The use of specific size ratios directs the assembly of the magnetic and semiconducting nanoparticles into AB13 or AB2 superlattices with potentially tunable optical and magnetic properties. This synthesis concept could ultimately enable the fine-tuning of material responses to magnetic, electrical, optical and mechanical stimuli. 相似文献
138.
Structure of the extracellular region of HER2 alone and in complex with the Herceptin Fab 总被引:40,自引:0,他引:40
HER2 (also known as Neu, ErbB2) is a member of the epidermal growth factor receptor (EGFR; also known as ErbB) family of receptor tyrosine kinases, which in humans includes HER1 (EGFR, ERBB1), HER2, HER3 (ERBB3) and HER4 (ERBB4). ErbB receptors are essential mediators of cell proliferation and differentiation in the developing embryo and in adult tissues, and their inappropriate activation is associated with the development and severity of many cancers. Overexpression of HER2 is found in 20-30% of human breast cancers, and correlates with more aggressive tumours and a poorer prognosis. Anticancer therapies targeting ErbB receptors have shown promise, and a monoclonal antibody against HER2, Herceptin (also known as trastuzumab), is currently in use as a treatment for breast cancer. Here we report crystal structures of the entire extracellular regions of rat HER2 at 2.4 A and human HER2 complexed with the Herceptin antigen-binding fragment (Fab) at 2.5 A. These structures reveal a fixed conformation for HER2 that resembles a ligand-activated state, and show HER2 poised to interact with other ErbB receptors in the absence of direct ligand binding. Herceptin binds to the juxtamembrane region of HER2, identifying this site as a target for anticancer therapies. 相似文献
139.
Transcriptional regulation and function during the human cell cycle 总被引:20,自引:0,他引:20
140.
CTCF-binding sites flank CTG/CAG repeats and form a methylation-sensitive insulator at the DM1 locus 总被引:15,自引:0,他引:15
Filippova GN Thienes CP Penn BH Cho DH Hu YJ Moore JM Klesert TR Lobanenkov VV Tapscott SJ 《Nature genetics》2001,28(4):335-343
An expansion of a CTG repeat at the DM1 locus causes myotonic dystrophy (DM) by altering the expression of the two adjacent genes, DMPK and SIX5, and through a toxic effect of the repeat-containing RNA. Here we identify two CTCF-binding sites that flank the CTG repeat and form an insulator element between DMPK and SIX5. Methylation of these sites prevents binding of CTCF, indicating that the DM1 locus methylation in congenital DM would disrupt insulator function. Furthermore, CTCF-binding sites are associated with CTG/CAG repeats at several other loci. We suggest a general role for CTG/CAG repeats as components of insulator elements at multiple sites in the human genome. 相似文献