全文获取类型
收费全文 | 7913篇 |
免费 | 734篇 |
国内免费 | 2篇 |
专业分类
8649篇 |
出版年
2024年 | 4篇 |
2023年 | 41篇 |
2022年 | 81篇 |
2021年 | 182篇 |
2020年 | 117篇 |
2019年 | 137篇 |
2018年 | 171篇 |
2017年 | 177篇 |
2016年 | 221篇 |
2015年 | 417篇 |
2014年 | 470篇 |
2013年 | 591篇 |
2012年 | 728篇 |
2011年 | 697篇 |
2010年 | 440篇 |
2009年 | 409篇 |
2008年 | 564篇 |
2007年 | 504篇 |
2006年 | 495篇 |
2005年 | 436篇 |
2004年 | 459篇 |
2003年 | 329篇 |
2002年 | 372篇 |
2001年 | 80篇 |
2000年 | 43篇 |
1999年 | 70篇 |
1998年 | 88篇 |
1997年 | 53篇 |
1996年 | 31篇 |
1995年 | 26篇 |
1994年 | 25篇 |
1993年 | 31篇 |
1992年 | 14篇 |
1991年 | 14篇 |
1990年 | 20篇 |
1989年 | 15篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1978年 | 4篇 |
1976年 | 3篇 |
1973年 | 4篇 |
1967年 | 2篇 |
1960年 | 3篇 |
排序方式: 共有8649条查询结果,搜索用时 15 毫秒
121.
Understanding the factors contributing to expansion of nonnative populations is a critical step toward accurate risk assessment and effective management of biological invasions. Nevertheless, few studies have attempted explicitly to test hypotheses regarding factors driving invasive spread by seeking correlations between patterns of vector movement and patterns of genetic connectivity. Herein, we describe such an attempt for the invasive tunicate Styela clava in the northeastern Pacific. We utilized microsatellite data to estimate gene flow between samples collected throughout the known range of S. clava in the region, and assessed correlation of these estimates with patterns of intracoastal commercial vessel traffic. Our results suggest that recent shipping patterns have contributed to the contemporary distribution of genetic variation. However, the analysis also indicates that other factors—including a complex invasion history and the influence of other vectors—have partially obscured genetic patterns associated with intracoastal population expansion. 相似文献
122.
Nitrogen yield advantage from grass–legume mixtures is robust over a wide range of legume proportions and environmental conditions 下载免费PDF全文
Matthias Suter John Connolly John A. Finn Ralf Loges Laura Kirwan Maria‐Teresa Sebastià Andreas Lüscher 《Global Change Biology》2015,21(6):2424-2438
Current challenges to global food security require sustainable intensification of agriculture through initiatives that include more efficient use of nitrogen (N), increased protein self‐sufficiency through homegrown crops, and reduced N losses to the environment. Such challenges were addressed in a continental‐scale field experiment conducted over 3 years, in which the amount of total nitrogen yield (Ntot) and the gain of N yield in mixtures as compared to grass monocultures (Ngainmix) was quantified from four‐species grass–legume stands with greatly varying legume proportions. Stands consisted of monocultures and mixtures of two N2‐fixing legumes and two nonfixing grasses. The amount of Ntot of mixtures was significantly greater (P ≤ 0.05) than that of grass monocultures at the majority of evaluated sites in all 3 years. Ntot and thus Ngainmix increased with increasing legume proportion up to one‐third of legumes. With higher legume percentages, Ntot and Ngainmix did not continue to increase. Thus, across sites and years, mixtures with one‐third proportion of legumes attained ~95% of the maximum Ntot acquired by any stand and had 57% higher Ntot than grass monocultures. Realized legume proportion in stands and the relative N gain in mixture (Ngainmix/Ntot in mixture) were most severely impaired by minimum site temperature (R = 0.70, P = 0.003 for legume proportion; R = 0.64, P = 0.010 for Ngainmix/Ntot in mixture). Nevertheless, the relative N gain in mixture was not correlated to site productivity (P = 0.500), suggesting that, within climatic restrictions, balanced grass–legume mixtures can benefit from comparable relative gains in N yield across largely differing productivity levels. We conclude that the use of grass–legume mixtures can substantially contribute to resource‐efficient agricultural grassland systems over a wide range of productivity levels, implying important savings in N fertilizers and thus greenhouse gas emissions and a considerable potential for climate change mitigation. 相似文献
123.
The 2-methallyl complex [(η5-C9H7)Ru(η3-2-MeC3H4)(PPh3)] (3), prepared from [(η5-C9H7)Ru(PPh3)2Cl] (2) and 2-MeC3H4MgCl, reacts with HX (X = Cl, CF3CO2) in the presence of ethene to give the chiral-at-metal compounds [(η5-C9H7)Ru(C2H4)(PPh3)X] (4, 5) in nearly quantitative yields. Treatment of 2 with AgPF6 and ethene affords [(η5-C9H7)Ru(C2H4)(PPh3)2]PF6 (6), which reacts with acetone to give the substitution product [(η5-C9H7)Ru(OCMe2)(PPh3)2]PF6 (7). The molecular structure of 7 has been determined crystallographically. Whereas treatment of 4 with CH(CO2Et)N2 yields the olefin complex [(η5-C9H7)Ru{η2-(Z)-C2H2(CO2Et)2}(PPh3)Cl] (8), the reactions of 4 and 5 with Ph2CN2, PhCHN2 and (Me3Si)CHN2 lead to the formation of the carbeneruthenium(II) derivatives [(η5-C9H7)Ru(CRR′)(PPh3)Cl] (9-11) and [(η5-C9H7)Ru(CRR′)(PPh3)(κ1-O2CCF3)] (12-14), respectively. Treatment of 9 (R = R′ = Ph), 10 (R = H, R′ = Ph) and 11 (R = H, R′ = SiMe3) with MeLi produces the hydrido(olefin) complexes [(η5-C9H7)RuH(η2-CH2CPh2)(PPh3)] (15), [(η5-C9H7)RuH(η2-CH2CHPh)(PPh3)] (18a,b) and [(η5-C9H7)RuH(η2-CH2CHSiMe3)(PPh3)] (19) via C-C coupling and β-hydride shift. The analogous reactions of 11 with PhLi gives the η3-benzyl compound [(η5-C9H7)Ru{η3-(Me3Si)CHC6H5}(PPh3)] (20). The η3-allyl complex [(η5-C9H7)Ru(η3-1-PhC3H4)(PPh3)] (17) was prepared from 10 and CH2CHMgBr by nucleophilic attack. 相似文献
124.
125.
A single amino acid change in rabies virus glycoprotein increases virus spread and enhances virus pathogenicity 总被引:10,自引:0,他引:10 下载免费PDF全文
Faber M Faber ML Papaneri A Bette M Weihe E Dietzschold B Schnell MJ 《Journal of virology》2005,79(22):14141-14148
Several rabies virus (RV) vaccine strains containing an aspartic acid (Asp) or glutamic acid (Glu) instead of an arginine (Arg) at position 333 of the RV glycoprotein (G) are apathogenic for immunocompetent mice even after intracranial inoculation. However, we previously showed that the nonpathogenic phenotype of the highly attenuated RV strain SPBNGA, which contains a Glu at position 333 of G, is unstable when this virus is passaged in newborn mice. While the Glu(333) remained unchanged after five mouse passages, an Asn(194)-->Lys(194) mutation occurred in RV G. This mutation was associated with increased pathogenicity for adult mice. Using site-directed mutagenesis to exchange Asn(194) with Lys(194) in the G protein of SPBNGA, resulting in SPBNGA-K, we show here that this mutation is solely responsible for the increase in pathogenicity and that the Asn(194)-->Lys(194) mutation does not arise when Asn(194) is exchanged with Ser(194) (SPBNGA-S). Our data presented indicate that the increased pathogenicity of SPBNGA-K is due to increased viral spread in vivo and in vitro, faster internalization of the pathogenic virus into cells, and a shift in the pH threshold for membrane fusion. These results are consistent with the notion that the RV G protein is a major contributor to RV pathogenesis and that the more pathogenic RVs escape the host responses by a faster spread than that of less pathogenic RVs. 相似文献
126.
Disruption of the mouse mTOR gene leads to early postimplantation lethality and prohibits embryonic stem cell development 总被引:10,自引:0,他引:10 下载免费PDF全文
Gangloff YG Mueller M Dann SG Svoboda P Sticker M Spetz JF Um SH Brown EJ Cereghini S Thomas G Kozma SC 《Molecular and cellular biology》2004,24(21):9508-9516
The mammalian target of rapamycin (mTOR) is a key component of a signaling pathway which integrates inputs from nutrients and growth factors to regulate cell growth. Recent studies demonstrated that mice harboring an ethylnitrosourea-induced mutation in the gene encoding mTOR die at embryonic day 12.5 (E12.5). However, others have shown that the treatment of E4.5 blastocysts with rapamycin blocks trophoblast outgrowth, suggesting that the absence of mTOR should lead to embryonic lethality at an earlier stage. To resolve this discrepancy, we set out to disrupt the mTOR gene and analyze the outcome in both heterozygous and homozygous settings. Heterozygous mTOR (mTOR(+/-)) mice do not display any overt phenotype, although mouse embryonic fibroblasts derived from these mice show a 50% reduction in mTOR protein levels and phosphorylation of S6 kinase 1 T389, a site whose phosphorylation is directly mediated by mTOR. However, S6 phosphorylation, raptor levels, cell size, and cell cycle transit times are not diminished in these cells. In contrast to the situation in mTOR(+/-) mice, embryonic development of homozygous mTOR(-/-) mice appears to be arrested at E5.5; such embryos are severely runted and display an aberrant developmental phenotype. The ability of these embryos to implant corresponds to a limited level of trophoblast outgrowth in vitro, reflecting a maternal mRNA contribution, which has been shown to persist during preimplantation development. Moreover, mTOR(-/-) embryos display a lesion in inner cell mass proliferation, consistent with the inability to establish embryonic stem cells from mTOR(-/-) embryos. 相似文献
127.
The enterobacterium Erwinia amylovora causes fire blight on members of the family Rosaceae, with economic importance on apple and pear. During pathogenesis, the bacterium is exposed to a variety of plant-borne antimicrobial compounds. In plants of Rosaceae, many constitutively synthesized isoflavonoids affecting microorganisms were identified. Bacterial multidrug efflux transporters which mediate resistance toward structurally unrelated compounds might confer tolerance to these phytoalexins. To prove this hypothesis, we cloned the acrAB locus from E. amylovora encoding a resistance nodulation division-type transport system. In Escherichia coli, AcrAB of E. amylovora conferred resistance to hydrophobic and amphiphilic toxins. An acrB-deficient E. amylovora mutant was impaired in virulence on apple rootstock MM 106. Furthermore, it was susceptible toward extracts of leaves of MM 106 as well as to the apple phytoalexins phloretin, naringenin, quercetin, and (+)-catechin. The expression of acrAB was determined using the promoterless reporter gene egfp. The acrAB operon was up-regulated in vitro by the addition of phloretin and naringenin. The promoter activity of acrR, encoding a regulatory protein involved in acrAB expression, was increased by naringenin. In planta, an induction of acrAB was proved by confocal laser scanning microscopy. Our results strongly suggest that the AcrAB transport system plays an important role as a protein complex required for virulence of E. amylovora in resistance toward apple phytoalexins and that it is required for successful colonization of a host plant. 相似文献
128.
Krishna Saxena Ulrich Schieborr Oliver Anderka Elke Duchardt-Ferner Bettina Elshorst Santosh Lakshmi Gande Julia Janzon Denis Kudlinzki Sridhar Sreeramulu Matthias K. Dreyer K. Ulrich Wendt Corentin Herbert Philippe Duchaussoy Marc Bianciotto Pierre-Alexandre Driguez Gilbert Lassalle Pierre Savi Moosa Mohammadi Fran?oise Bono Harald Schwalbe 《The Journal of biological chemistry》2010,285(34):26628-26640
Fibroblast growth factor (FGF) signaling regulates mammalian development and metabolism, and its dysregulation is implicated in many inherited and acquired diseases, including cancer. Heparan sulfate glycosaminoglycans (HSGAGs) are essential for FGF signaling as they promote FGF·FGF receptor (FGFR) binding and dimerization. Using novel organic synthesis protocols to prepare homogeneously sulfated heparin mimetics (HM), including hexasaccharide (HM6), octasaccharide (HM8), and decasaccharide (HM10), we tested the ability of these HM to support FGF1 and FGF2 signaling through FGFR4. Biological assays show that both HM8 and HM10 are significantly more potent than HM6 in promoting FGF2-mediated FGFR4 signaling. In contrast, all three HM have comparable activity in promoting FGF1·FGFR4 signaling. To understand the molecular basis for these differential activities in FGF1/2·FGFR4 signaling, we used NMR spectroscopy, isothermal titration calorimetry, and size-exclusion chromatography to characterize binding interactions of FGF1/2 with the isolated Ig-domain 2 (D2) of FGFR4 in the presence of HM, and binary interactions of FGFs and D2 with HM. Our data confirm the existence of both a secondary FGF1·FGFR4 interaction site and a direct FGFR4·FGFR4 interaction site thus supporting the formation of the symmetric mode of FGF·FGFR dimerization in solution. Moreover, our results show that the observed higher activity of HM8 relative to HM6 in stimulating FGF2·FGFR4 signaling correlates with the higher affinity of HM8 to bind and dimerize FGF2. Notably FGF2·HM8 exhibits pronounced positive binding cooperativity. Based on our findings we propose a refined symmetric FGF·FGFR dimerization model, which incorporates the differential ability of HM to dimerize FGFs. 相似文献
129.
Enhanced cytotoxicity without internuclear spread of adenovirus upon cell fusion by measles virus glycoproteins 下载免费PDF全文
Horn GP Vongpunsawad S Kornmann E Fritz B Dittmer DP Cattaneo R Dobbelstein M 《Journal of virology》2005,79(3):1911-1917
The efficiency of viruses in cancer therapy is enhanced by proteins that mediate the fusion of infected cells with their neighbors. It was reported that replication-competent adenovirus particles can spread between nuclei within fusion-generated syncytia. To assess this conjecture, we generated fusogenic adenoviruses that express a balanced ratio of the F and H glycoproteins of measles virus. The viruses displayed enhanced cytotoxicity but largely unchanged replication efficiencies compared to a nonfusogenic virus. Most notably, the virus genomes did not spread through fusion-generated multinuclear cells. Hence, adenovirus replication in syncytia remains largely restricted to initially transduced nuclei. 相似文献
130.
SUMMARY: ProfDist is a user-friendly software package using the profile-neighbor-joining method (PNJ) in inferring phylogenies based on profile distances on DNA or RNA sequences. It is a tool for reconstructing and visualizing large phylogenetic trees providing new and standard features with a special focus on time efficency, robustness and accuracy. AVAILABILITY: A Windows version of ProfDist comes with a graphical user interface and is freely available at http://profdist.bioapps.biozentrum.uni-wuerzburg.de 相似文献