全文获取类型
收费全文 | 38392篇 |
免费 | 2478篇 |
国内免费 | 14篇 |
专业分类
40884篇 |
出版年
2023年 | 224篇 |
2022年 | 520篇 |
2021年 | 908篇 |
2020年 | 518篇 |
2019年 | 673篇 |
2018年 | 939篇 |
2017年 | 784篇 |
2016年 | 1315篇 |
2015年 | 1992篇 |
2014年 | 2137篇 |
2013年 | 2928篇 |
2012年 | 3349篇 |
2011年 | 3191篇 |
2010年 | 1949篇 |
2009年 | 1680篇 |
2008年 | 2383篇 |
2007年 | 2314篇 |
2006年 | 2054篇 |
2005年 | 1851篇 |
2004年 | 1739篇 |
2003年 | 1677篇 |
2002年 | 1514篇 |
2001年 | 304篇 |
2000年 | 208篇 |
1999年 | 296篇 |
1998年 | 378篇 |
1997年 | 253篇 |
1996年 | 251篇 |
1995年 | 230篇 |
1994年 | 224篇 |
1993年 | 219篇 |
1992年 | 134篇 |
1991年 | 144篇 |
1990年 | 147篇 |
1989年 | 109篇 |
1988年 | 100篇 |
1987年 | 90篇 |
1986年 | 70篇 |
1985年 | 95篇 |
1984年 | 95篇 |
1983年 | 71篇 |
1982年 | 83篇 |
1981年 | 80篇 |
1980年 | 79篇 |
1979年 | 72篇 |
1978年 | 39篇 |
1977年 | 50篇 |
1976年 | 36篇 |
1975年 | 38篇 |
1973年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
991.
Eukaryotic translation elongation factor 2 (eEF2) facilitates the movement of the peptidyl tRNA-mRNA complex from the A site of the ribosome to the P site during protein synthesis. ADP-ribosylation (ADPR) of eEF2 by bacterial toxins on a unique diphthamide residue inhibits its translocation activity, but the mechanism is unclear. We have employed a hormone-inducible diphtheria toxin (DT) expression system in Saccharomyces cerevisiae which allows for the rapid induction of ADPR-eEF2 to examine the effects of DT in vivo. ADPR of eEF2 resulted in a decrease in total protein synthesis consistent with a defect in translation elongation. Association of eEF2 with polyribosomes, however, was unchanged upon expression of DT. Upon prolonged exposure to DT, cells with an abnormal morphology and increased DNA content accumulated. This observation was specific to DT expression and was not observed when translation elongation was inhibited by other methods. Examination of these cells by electron microscopy indicated a defect in cell separation following mitosis. These results suggest that expression of proteins late in the cell cycle is particularly sensitive to inhibition by ADPR-eEF2. 相似文献
992.
993.
Liisa M. Uotila Maria Aatonen Carl G. Gahmberg 《The Journal of biological chemistry》2013,288(46):33494-33499
CD11c/CD18 (αXβ2, p150/95, or complement receptor 4, CR4) is a monocyte/macrophage-enriched integrin that has been reported to bind to a variety of ligands. These include cell surface proteins, extracellular matrix proteins, and soluble ligands. The regulation of ligand binding to CD11c/CD18 has remained poorly understood. Previous work has shown that both α-chain and β-chain phosphorylations of CD11a/CD18 and CD11b/CD18 are needed for activity, but no corresponding studies on CD11c/CD18 have been performed. In this study, we have identified the phosphorylation site of CD11c as Ser-1158 and show that it is pivotal for adherence and phagocytosis. 相似文献
994.
995.
996.
Sergio S. Barbosa Selma O. Klanten Jonathan B. Puritz Robert J. Toonen Maria Byrne 《Biological journal of the Linnean Society. Linnean Society of London》2013,108(4):821-833
The present study investigated the fine‐scale population genetic structure of sympatric asterinid sea stars with contrasting modes of larval development (benthic versus pelagic). Parvulastra exigua lacks a dispersive life phase yet is one of the worlds most widely distributed and abundant sea stars, whereas Meridiastra calcar, a sea star with a dispersive larva, has a more limited regional scale distribution. Populations of P. exigua sampled from tide pools on three adjacent headlands showed significant genetic substructure (mitochodrial DNA control region) at fine spatial scales (tide pools < 300 m apart: FST = 0.249, P < 0.01; headlands 5–15 km apart: FST = 0.125, P = 0.04). As expected, M. calcar populations sampled from the same headlands did not exhibit significant genetic structuring (FST = 0.029, P = 0.14). The life‐history traits of P. exigua, a mixed mating system (selfing + outcrossing), pseudocopulation among closely‐related conspecifics, and an entirely benthic life cycle with a philopatric larva, undoubtedly influence its strong genetic structure across fine spatial scales. Localized genetic structure, especially at the very fine‐scale of tide pools, would not be detected in the more typical regional scale approaches adopted by most studies of marine invertebrate populations. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, ●●, ●●–●●. 相似文献
997.
Adriana Fernanda Vizuete Daniela Fraga de Souza Maria Cristina Guerra Cristiane Batassini Márcio Ferreira Dutra Caren Bernardi Ana Paula Costa Carlos-Alberto Gonçalves 《Life sciences》2013,92(17-19):923-928
AimsWe investigated the effects of ketogenic diet (KD) on levels of tumor necrosis factor alpha (TNF-α, a classical pro-inflammatory cytokine), BDNF (brain-derived neurotrophic factor, commonly associated with synaptic plasticity), and S100B, an astrocyte neurotrophic cytokine involved in metabolism regulation.Main methodsYoung Wistar rats were fed during 8 weeks with control diet or two KD, containing different proportions of omega 6 and omega 3 polyunsaturated fatty acids. Contents of TNF-α, BDNF and S100B were measured by ELISA in two brain regions (hippocampus and striatum) as well as blood serum and cerebrospinal fluid.Key findingsOur data suggest that KD was able to reduce the levels of BDNF in the striatum (but not in hippocampus) and S100B in the cerebrospinal fluid of rats. These alterations were not affected by the proportion of polyunsaturated fatty acids offered. No changes in S100B content were observed in serum or analyzed brain regions. Basal TNF-α content was not affected by KD.SignificanceThese findings reinforce the importance of this diet as an inductor of alterations in the brain, and such changes might contribute to the understanding of the effects (and side effects) of KD in brain disorders. 相似文献
998.
Maria Luiza Felippe-Bauer Tiago do Nascimento da Silva Rosimeire Lopes da Trindade 《Memórias do Instituto Oswaldo Cruz》2013,108(1):54-58
Two new species of Culicoides Latreille of the Neotropical subgenus Mataemyia Vargas are described and illustrated based on female specimens from Juruti, Pará, Brazil and compared with their similar congeners. This paper also presents a diagnosis of the subgenus Mataemyia and a systematic key for the identification of the 19 species of the subgenus. 相似文献
999.
Fábio Takenori Higa Lucila Okuyama Fukasawa Maria Gisele Gon?alves Maristela Marques Salgado Ana Paula Silva de Lemos Lee H Harrison Priscilla Lima de Oliveira Carla Naufal da Silva Claudio Tavares Sacchi 《Memórias do Instituto Oswaldo Cruz》2013,108(2):246-247
We evaluated the use of a newly described sodC-based real-time-polymerase chain reaction (RT-PCR) assay for detecting Neisseria meningitidis in normally sterile sites, such as cerebrospinal fluid and serum. The sodC-based RT-PCR assay has an advantage over ctrA for detecting nongroupable N. meningitidis isolates, which are commonly present in asymptomatic pharyngeal carriage. However, in our study, sodC-based RT-PCR was 7.5% less sensitive than ctrA. Given the public health impact of possible false-negative results due to the use of the sodC target gene alone, sodC-based RT-PCR for the diagnosis of meningococcal meningitis should be used with caution. 相似文献
1000.
Valéria Cataneli Pereira Maria de Lourdes Ribeiro de Souza da Cunha 《Memórias do Instituto Oswaldo Cruz》2013,108(7):939-942
Coagulase-negative staphylococci (CoNS) are the microorganisms most frequently isolated from clinical samples and are commonly found in neonatal blood cultures. Oxacillin is an alternative treatment of choice for CoNS infections; however, resistance to oxacillin can have a substantial impact on healthcare by adversely affecting morbidity and mortality. The objective of this study was to detect and characterise oxacillin-resistant CoNS strains in blood cultures of newborns hospitalised at the neonatal ward of the University Hospital of the Faculty of Medicine of Botucatu. One hundred CoNS strains were isolated and the mecA gene was detected in 69 of the CoNS strains, including 73.2% of Staphylococcus epidermidis strains, 85.7% of Staphylococcus haemolyticus strains, 28.6% of Staphylococcus hominis strains and 50% of Staphylococcus lugdunensis strains. Among these oxacillin-resistant CoNS strains, staphylococcal cassette chromosome mec (SCCmec) type I was identified in 24.6%, type II in 4.3%, type III in 56.5% and type IV in 14.5% of the strains. The data revealed an increase in the percentage of CoNS strains isolated from blood cultures from 1991-2009. Furthermore, a predominant SCCmec profile of the oxacillin-resistant CoNS strains isolated from neonatal intensive care units was identified with a prevalence of SCCmec types found in hospital-acquired strains. 相似文献