全文获取类型
收费全文 | 1479篇 |
免费 | 160篇 |
专业分类
1639篇 |
出版年
2023年 | 8篇 |
2022年 | 25篇 |
2021年 | 43篇 |
2020年 | 13篇 |
2019年 | 24篇 |
2018年 | 39篇 |
2017年 | 24篇 |
2016年 | 45篇 |
2015年 | 72篇 |
2014年 | 67篇 |
2013年 | 81篇 |
2012年 | 115篇 |
2011年 | 86篇 |
2010年 | 69篇 |
2009年 | 65篇 |
2008年 | 89篇 |
2007年 | 88篇 |
2006年 | 80篇 |
2005年 | 71篇 |
2004年 | 73篇 |
2003年 | 76篇 |
2002年 | 68篇 |
2001年 | 12篇 |
2000年 | 13篇 |
1999年 | 17篇 |
1998年 | 18篇 |
1997年 | 12篇 |
1996年 | 13篇 |
1995年 | 12篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1991年 | 8篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1979年 | 7篇 |
1978年 | 9篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1973年 | 6篇 |
1970年 | 6篇 |
1969年 | 6篇 |
1968年 | 4篇 |
1966年 | 6篇 |
1961年 | 5篇 |
排序方式: 共有1639条查询结果,搜索用时 0 毫秒
241.
242.
The glycerophosphoinositols are diffusible phosphoinositide metabolites reported to modulate actin dynamics and tumour cell spreading. In particular, the membrane permeant glycerophosphoinositol 4-phosphate (GroPIns4P) has been shown to act at the level of the small GTPase Rac1, to induce the rapid formation of membrane ruffles. Here, we have investigated the signalling cascade involved in this process, and show that it is initiated by the activation of Src kinase. In NIH3T3 cells, exogenous addition of GroPIns4P induces activation and translocation of Rac1 and its exchange factor TIAM1 to the plasma membrane; in addition, in in-vitro assays, GroPIns4P favours the formation of a protein complex that includes Rac1 and TIAM1. Neither of these processes involves direct actions of GroPIns4P on these proteins. Thus, through the use of specific inhibitors of tyrosine kinases and phospholipase C (and by direct evaluation of kinase activities and inositol 1,4,5-trisphosphate production), we show that GroPIns4P activates Src, and as a consequence, phospholipase Cgamma and Ca(2+)/calmodulin kinase II, the last of which directly phosphorylates TIAM1 and leads to TIAM1/Rac1-dependent ruffle formation. 相似文献
243.
Croce M Meazza R Orengo AM Fabbi M Borghi M Ribatti D Nico B Carlini B Pistoia V Corrias MV Ferrini S 《Cancer immunology, immunotherapy : CII》2008,57(11):1625-1634
AIM: IL-21 is the most recently identified member of the IL-2 cytokine family. Here we studied the therapeutic efficacy of IL-21-gene-modified cells (Neuro2a/IL-21) in a syngeneic metastatic neuroblastoma (NB) model. MATERIALS AND METHODS: Neuro2a/IL-21 cells were tested as subcutaneous (sc) vaccine both in prophylactic and therapeutic settings. Depletion studies, cytotoxicity assay and immunohistochemical analyses were carried out to evaluate the mechanisms involved in tumor rejection. RESULTS: When injected sc in syngeneic A/J mice viable Neuro2a/IL-21 cells were rejected and induced resistance to a subsequent iv challenge with Neuro2a parental cells (Neuro2a/pc), suggesting the involvement of an immune response. More importantly, in mice bearing Neuro2a/pc micrometastases, a single sc injection of Neuro2a/IL-21 cells significantly increased the mean tumor-free survival of treated animals (43 vs. 22 days) and cured 14% of them. The administration of two or three doses of Neuro2a/IL-21 cell vaccine further increased the mean survival time to 54 and 75 days, and the cure rate to 27 and 33%, respectively, whereas the use of unmodified Neuro2a or mock-transfected cells had no effect. In vivo cell subset depletion and a Winn-assay indicated the involvement of CD8 + CTLs. Immunohistochemical analysis indicated a reduction of CD31+ and VEGFR2+ microvessels in late metastases from therapeutically vaccinated mice. A role of survivin as antigen was suggested by in vitro assays using survivin-synthetic CTL-epitopes. CONCLUSIONS: Our present data indicate that IL-21-secreting NB cells are effective as therapeutic vaccine in mice bearing metastatic NB, through a specific CTL response involving survivin as antigen, and suggest a potential interest for IL-21 in NB immuno-gene therapy. 相似文献
244.
245.
Julia Xu Mary A A McRae Scott Harron Beatrice Rob Reuben E Huber 《Biochimie et biologie cellulaire》2004,82(2):275-284
The interactions between Na+ (and K+) and Asp-201 of beta-galactosidase were studied. Analysis of the changes in Km and Vmax showed that the Kd for Na+ of wild type beta-galactosidase (0.36 +/- 0.09 mM) was about 10x lower than for K+ (3.9 +/- 0.6 mM). The difference is probably because of the size and other physical properties of the ions and the binding pocket. Decreases of Km as functions of Na+ and K+ for oNPG and pNPG and decreases of the Ki of both shallow and deep mode inhibitors were similar, whereas the Km and Ki of substrates and inhibitors without C6 hydroxyls remained constant. Thus, Na+ and K+ are important for binding galactosyl moieties via the C6 hydroxyl throughout catalysis. Na+ and K+ had lesser effects on the Vmax. The Vmax of pNPF and pNPA (substrates that lack a C6 hydroxyl) did not change upon addition of Na+ or K+, showing that the catalytic effects are also mediated via the C6 hydroxyl. Arrhenius plots indicated that Na+, but not K+, caused k3 (degalactosylation) to increase. Na+ also caused the k2 (galactosylation) with oNPG, but not with pNPG, to increase. In contrast, K+ caused the k2 values with both oNPG and pNPG to increase. Na+ and K+ mainly altered the entropies of activation of k2 and k3 with only small effects on the enthalpies of activation. This strongly suggests that only the positioning of the substrate, transition states, and covalent intermediate are altered by Na+ and K+. Further evidence that positioning is important was that substitution of Asp-201 with a Glu caused the Km and Ki values to increase significantly. In addition, the Kd values for Na+ or K+ were 5 to 8 fold higher. The negative charge of Asp-201 was shown to be vital for Na+ and K+ binding. Large amounts of Na+ or K+ had no effect on the very large Km and Ki values of D201N-beta-galactosidase and the Vmax values changed minimally and in a linear rather than hyperbolic way. D201F-beta-galactosidase, with a very bulky hydrophobic side chain in place of Asp, essentially obliterated all binding and catalysis. 相似文献
246.
247.
Structural changes in bacteriorhodopsin during the photocycle measured by time-resolved polarized Fourier transform infrared spectroscopy. 下载免费PDF全文
The structural changes in bacteriorhodopsin during the photocycle are investigated. Time resolved polarized infrared spectroscopy in combination with photoselection is used to determine the orientation and motion of certain structural units of the molecule: Asp-85, Asp-96, Asp-115, the Schiff base, and several amide I vibrations. The results are compared with recently published x-ray diffraction data with atomic resolution about conformational motions during the photocycle. The orientation of the measured vibrations are also calculated from the structure data, and based on the comparison of the values from the two techniques new information is obtained: several amide I bands in the infrared spectrum are assigned, and we can also identify the position of the proton in the protonated Asp residues. 相似文献
248.
Beatrice Belenghi Filippo Acconcia Maurizio Trovato Michele Perazzolli Alessio Bocedi Fabio Polticelli Paolo Ascenzi Massimo Delledonne 《European journal of biochemistry》2003,270(12):2593-2604
In plants, cysteine protease inhibitors are involved in the regulation of protein turnover and play an important role in resistance against insects and pathogens. AtCYS1 from Arabidopsis thaliana encodes a protein of 102 amino acids that contains the conserved motif of cysteine protease inhibitors belonging to the cystatin superfamily (Gln-Val-Val-Ala-Gly). Recombinant A. thaliana cystatin-1 (AtCYS1) was expressed in Escherichia coli and purified. AtCYS1 inhibits the catalytic activity of papain (Kd = 4.0 x 10-2 micro m, at pH 7.0 and 25 degrees C), generally taken as a molecular model of cysteine proteases. The molecular bases for papain inhibition by AtCYS1 have been analysed taking into account the three-dimensional structure of the papain-stefin B complex. AtCYS1 is constitutively expressed in roots and in developing siliques of A. thaliana. In leaves, AtCYS1 is strongly induced by wounding, by challenge with avirulent pathogens and by nitric oxide (NO). The overexpression of AtCYS1 blocks cell death activated by either avirulent pathogens or by oxidative and nitrosative stress in both A. thaliana suspension cultured cells and in transgenic tobacco plants. The suppression of the NO-mediated cell death in plants overexpressing AtCYS1 provides the evidence that NO is not cytotoxic for the plant, indicating that NO functions as cell death trigger through the stimulation of an active process, in which cysteine proteases and theirs proteinaceous inhibitors appear to play a crucial role. 相似文献
249.
Delledonne Massimo Allegro Gianni Belenghi Beatrice Balestrazzi Alma Picco Franco Levine Alex Zelasco Samantha Calligari Paolo Confalonieri Massimo 《Molecular breeding : new strategies in plant improvement》2001,7(1):35-42
Transgenic white poplar (Populus alba L.) plants expressing a novel Arabidopsis thaliana cysteine proteinase inhibitor (Atcys) gene have been produced using Agrobacterium tumefaciens-mediated gene transfer. Internodal stem segments of cv. Villafranca were co-cultivated with the EHA105 pBI-Atcys A. tumefaciens strain. Sixteen putative transgenic plant lines were regenerated from different calli with a transformation efficiency of 11%. The integration and expression of the cysteine proteinase inhibitor (Atcys) gene into the plant genome was confirmed by Southern and northern blot analyses. Papain inhibitory activity was detected in poplar transgenic tissues by means of a specific in vitro assay. Such activity was sufficient to inhibit most of the digestive proteinase activity of chrysomelid beetle (Chrysomela populi L.) and confer resistance to C. populi larvae on selected transgenic plants. A close correspondence between the inhibition of papain and resistance to poplar leaf beetle was observed in all tested transgenic lines. Our results indicate that Atcys could be succesfully employed in breeding programmes aimed at the selection of new poplar genotypes resistant to major insect pests. 相似文献
250.