全文获取类型
收费全文 | 1388篇 |
免费 | 106篇 |
国内免费 | 16篇 |
专业分类
1510篇 |
出版年
2023年 | 11篇 |
2022年 | 10篇 |
2021年 | 34篇 |
2020年 | 21篇 |
2019年 | 25篇 |
2018年 | 36篇 |
2017年 | 31篇 |
2016年 | 46篇 |
2015年 | 63篇 |
2014年 | 58篇 |
2013年 | 82篇 |
2012年 | 109篇 |
2011年 | 82篇 |
2010年 | 60篇 |
2009年 | 52篇 |
2008年 | 57篇 |
2007年 | 59篇 |
2006年 | 52篇 |
2005年 | 58篇 |
2004年 | 51篇 |
2003年 | 65篇 |
2002年 | 37篇 |
2001年 | 38篇 |
2000年 | 40篇 |
1999年 | 27篇 |
1998年 | 16篇 |
1997年 | 12篇 |
1996年 | 7篇 |
1995年 | 10篇 |
1994年 | 10篇 |
1993年 | 13篇 |
1992年 | 23篇 |
1991年 | 21篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 20篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1976年 | 6篇 |
1974年 | 5篇 |
1972年 | 12篇 |
1970年 | 5篇 |
1968年 | 6篇 |
1967年 | 9篇 |
1965年 | 7篇 |
排序方式: 共有1510条查询结果,搜索用时 0 毫秒
991.
Badura Monika Jarosińska Marta Noryśkiewicz Agnieszka M. Kosmaczewska Agata Sady-Bugajska Agata Święta-Musznicka Joanna Pińska Katarzyna Latałowa Małgorzata 《Vegetation History and Archaeobotany》2023,32(1):85-103
Vegetation History and Archaeobotany - The aim of this article is the study of the botanical material in the fillings of 54 coffin pillows from Catholic and Protestant burials dated to the... 相似文献
992.
The increasing incidence of thromboembolic diseases has sustained the search for new agents able to stimulate the natural fibrinolytic system. The first generation of antithrombotic agents include bacterial streptokinase and human urine urokinase. Because these molecules lack specificity for the fibrin clot, important efforts have been made to produce, using recombinant DNA technology, agents presenting higher fibrin clot selectivity such as t-PA (tissue-type plasminogen activator) and scu-PA (single chain urokinase-type plasminogen activator). In parallel, several laboratories are presently attempting to create mutants and hybrids plasminogen activators displaying improved thrombolytic properties with respect to the natural molecules. In this paper, we describe briefly the mechanisms of fibrinolysis and the role of the different natural thrombolytic agents. In addition, we review the possibilities of genetic engineering for the production of natural and novel plasminogen activators. 相似文献
993.
Cryo‐electron tomography analyses of terminal organelle mutants suggest the motility mechanism of Mycoplasma genitalium 下载免费PDF全文
Margot P. Scheffer Maria Lluch‐Senar Ana M. Mariscal Enrique Querol Franziska Matthaeus Jaume Piñol Achilleas S. Frangakis 《Molecular microbiology》2018,108(3):319-329
The terminal organelle of Mycoplasma genitalium is responsible for bacterial adhesion, motility and pathogenicity. Localized at the cell tip, it comprises an electron‐dense core that is anchored to the cell membrane at its distal end and to the cytoplasm at its proximal end. The surface of the terminal organelle is also covered with adhesion proteins. We performed cellular cryoelectron tomography on deletion mutants of eleven proteins that are implicated in building the terminal organelle, to systematically analyze the ultrastructural effects. These data were correlated with microcinematographies, from which the motility patterns can be quantitatively assessed. We visualized diverse phenotypes, ranging from mild to severe cell adhesion, motility and segregation defects. Based on our observations, we propose a double‐spring ratchet model for the motility mechanism that explains our current and previous observations. Our model, which expands and integrates the previously suggested inchworm model, allocates specific functions to each of the essential components of this unique bacterial motility system. 相似文献
994.
Physicochemical Characterization of Adeno-associated Satellite Virus Type 4 and Its Nucleic Acid 总被引:12,自引:10,他引:2 下载免费PDF全文
Based on protein, deoxyribonucleic acid (DNA), and phosphorus analyses, adeno-associated satellite virus type 4 has a DNA content of 26.5%, significantly larger than type 1 which contains 18.9% DNA. Type 4 DNA contains 58 to 62% guanine + cytosine, as calculated from thermal denaturation temperature and buoyant density measurements. Simian adenovirus SV15, which serves as a helper virus for type 4 satellite, contains 13.7% DNA with a guanine + cytosine content of 55 to 57%. The DNA extracted from type 4 satellite virus is a linear, double-stranded molecule, as shown by thermal denaturation temperature profile measurements and by electron microscopy. The contour length and sedimentation coefficient of type 4 DNA indicate a molecular weight of 3.0 x 10(6) daltons. 相似文献
995.
996.
Piñero S Rivera J Romero D Cevallos MA Martínez A Bolívar F Gosset G 《Journal of molecular microbiology and biotechnology》2007,13(1-3):35-44
Tyrosinase (EC 1.14.18.1) is a monophenol oxidase responsible for the synthesis of the black pigment known as melanin. The tyrosinase gene (melA) is plasmid-encoded in many rhizobial species. In Rhizobium etli CFN42, the genetic location of melA in the symbiotic plasmid (p42d) and its RpoN-NifA regulation suggest an involvement in symbiosis. In this work, we analyzed the symbiotic phenotype of a streptomycin-resistant derivative of CFN42 (CE3), a melA mutant (SP2) and a complemented strain (SP66), demonstrating that melA inactivation reduced nodule formation rate and diminished total nodule number by 27% when compared to the CE3 strain. The nitrogen fixation capacity of the mutant strain was not affected. Also, in vitro assays were performed where the resistance of CE3, SP2 and SP66 strains to H(2)O(2) was evaluated; the melA mutant strain was consistently less resistant to peroxide. In another series of experiments, Escherichia coli W3110 strain expressing R. etli melA displayed enhanced resistance to p-hydroxybenzoic, vanillinic and syringic acids, which are phenolic compounds frequently found in the soil. Our results are the first to demonstrate a specific role for tyrosinase in R. etli: this enzyme is required during early symbiosis, apparently providing resistance against reactive oxygen species and phenolic compounds generated as part of the plant protective responses. 相似文献
997.
Pastré D Hamon L Mechulam A Sorel I Baconnais S Curmi PA Le Cam E Piétrement O 《Biomacromolecules》2007,8(12):3712-3717
Studying the influence of macromolecular crowding at high ionic strengths on assemblies of biomolecules is of particular interest because these are standard intracellular conditions. However, up to now, no techniques offer the possibility of studying the effect of molecular crowding at the single molecule scale and at high resolution. We present a method to observe double-strand DNA under macromolecular crowding conditions on a flat mica surface by atomic force microscope. By using high concentrations of monovalent salt ([NaCl] > 100 mM), we promote DNA adsorption onto NiCl 2 pretreated muscovite mica. It therefore allows analysis of DNA conformational changes and DNA compaction induced by polyethylene glycol (PEG), a neutral crowding agent, at physiological concentrations of monovalent salt. 相似文献
998.
A T-1-conotoxin, lt5d, was purified and characterized from the venom of vermivorous hunting cone snails Conus litteratus. The complete amino acid sequence of lt5d (DCCPAKLLCCNP) has been determined by Edman degradation. With two disulfide bonds, the calculated average mass is 1274.57 Da, which is confirmed by MALDI-TOF mass spectrometry (average mass 1274.8778). Under whole cell patch-clamp mode, lt5d inhibits tetrodotoxin-sensitive sodium currents on adult rat dorsal root ganglion neurons, but has no effects on tetrodotoxin-resistant sodium currents. The inhibition of TTX-sensitive sodium currents by lt5d was found to be concentration-dependent with the IC50 value of 156.16 nM. Thus, this is the first T-superfamily conotoxin identified to block TTX-sensitive sodium channels. 相似文献
999.
Boyd ES Jackson RA Encarnacion G Zahn JA Beard T Leavitt WD Pi Y Zhang CL Pearson A Geesey GG 《Applied and environmental microbiology》2007,73(20):6669-6677
Elemental sulfur (S(0)) is associated with many geochemically diverse hot springs, yet little is known about the phylogeny, physiology, and ecology of the organisms involved in its cycling. Here we report the isolation, characterization, and ecology of two novel, S(0)-reducing Crenarchaea from an acid geothermal spring referred to as Dragon Spring. Isolate 18U65 grows optimally at 70 to 72 degrees C and at pH 2.5 to 3.0, while isolate 18D70 grows optimally at 81 degrees C and pH 3.0. Both isolates are chemoorganotrophs, dependent on complex peptide-containing carbon sources, S(0), and anaerobic conditions for respiration-dependent growth. Glycerol dialkyl glycerol tetraethers (GDGTs) containing four to six cyclopentyl rings were present in the lipid fraction of isolates 18U65 and 18D70. Physiological characterization suggests that the isolates are adapted to the physicochemical conditions of Dragon Spring and can utilize the natural organic matter in the spring as a carbon and energy source. Quantitative PCR analysis of 16S rRNA genes associated with the S(0) flocs recovered from several acid geothermal springs using isolate-specific primers indicates that these two populations together represent 17 to 37% of the floc-associated DNA. The physiological characteristics of isolates 18U65 and 18D70 are consistent with their potential widespread distribution and putative role in the cycling of sulfur in acid geothermal springs throughout the Yellowstone National Park geothermal complex. Based on phenotypic and genetic characterization, the designations Caldisphaera draconis sp. nov. and Acidilobus sulfurireducens sp. nov. are proposed for isolates 18U65 and 18D70, respectively. 相似文献
1000.
Zhang H Pi R Li R Wang P Tang F Zhou S Gao J Jiang J Chen S Liu P 《Archives of biochemistry and biophysics》2007,460(1):25-32
Peroxisome proliferator-activated receptors (PPARalpha, beta/delta and gamma) are nuclear receptors and PPARgamma activation was previously reported to inhibit collagen expression in the heart, but whether PPARbeta/delta also regulates collagen expression in the heart remains unclear. In this study, we investigated the effect of PPARbeta/delta activation on angiotensin II (Ang II)-induced collagen type I expression in adult rat cardiac fibroblasts. The results showed that PPARbeta/delta was expressed at the moderate level in cardiac fibroblasts. GW501516, a selective PPARbeta/delta agonist, depressed Ang II-stimulated collagen type I expression and collagen synthesis in cardiac fibroblasts in a concentration-dependent manner. Furthermore, these inhibitory effects of GW501516 were completely reversed by the knockdown of PPARbeta/delta via RNA interference. In summary, we find that PPARbeta/delta is present in cardiac fibroblasts and PPARbeta/delta activation inhibits Ang II-induced collagen type I expression at least in part via decreasing collagen synthesis. PPARbeta/delta may be a promising therapeutic target for myocardial fibrosis. 相似文献