全文获取类型
收费全文 | 1855篇 |
免费 | 115篇 |
专业分类
1970篇 |
出版年
2022年 | 10篇 |
2021年 | 25篇 |
2019年 | 14篇 |
2018年 | 17篇 |
2017年 | 16篇 |
2016年 | 38篇 |
2015年 | 74篇 |
2014年 | 73篇 |
2013年 | 111篇 |
2012年 | 136篇 |
2011年 | 129篇 |
2010年 | 68篇 |
2009年 | 79篇 |
2008年 | 86篇 |
2007年 | 84篇 |
2006年 | 82篇 |
2005年 | 92篇 |
2004年 | 73篇 |
2003年 | 70篇 |
2002年 | 90篇 |
2001年 | 21篇 |
2000年 | 26篇 |
1999年 | 28篇 |
1998年 | 25篇 |
1997年 | 16篇 |
1996年 | 18篇 |
1995年 | 19篇 |
1994年 | 21篇 |
1993年 | 23篇 |
1992年 | 15篇 |
1991年 | 15篇 |
1990年 | 26篇 |
1989年 | 21篇 |
1988年 | 16篇 |
1987年 | 19篇 |
1986年 | 9篇 |
1985年 | 18篇 |
1984年 | 14篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 12篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1978年 | 9篇 |
1976年 | 8篇 |
1975年 | 12篇 |
1974年 | 13篇 |
1973年 | 15篇 |
1972年 | 9篇 |
1971年 | 11篇 |
排序方式: 共有1970条查询结果,搜索用时 15 毫秒
81.
Krauss S Obert CA Franks J Walker D Jones K Seiler P Niles L Pryor SP Obenauer JC Naeve CW Widjaja L Webby RJ Webster RG 《PLoS pathogens》2007,3(11):e167
Migratory waterfowl of the world are the natural reservoirs of influenza viruses of all known subtypes. However, it is unknown whether these waterfowl perpetuate highly pathogenic (HP) H5 and H7 avian influenza viruses. Here we report influenza virus surveillance from 2001 to 2006 in wild ducks in Alberta, Canada, and in shorebirds and gulls at Delaware Bay (New Jersey), United States, and examine the frequency of exchange of influenza viruses between the Eurasian and American virus clades, or superfamilies. Influenza viruses belonging to each of the subtypes H1 through H13 and N1 through N9 were detected in these waterfowl, but H14 and H15 were not found. Viruses of the HP Asian H5N1 subtypes were not detected, and serologic studies in adult mallard ducks provided no evidence of their circulation. The recently described H16 subtype of influenza viruses was detected in American shorebirds and gulls but not in ducks. We also found an unusual cluster of H7N3 influenza viruses in shorebirds and gulls that was able to replicate well in chickens and kill chicken embryos. Genetic analysis of 6,767 avian influenza gene segments and 248 complete avian influenza viruses supported the notion that the exchange of entire influenza viruses between the Eurasian and American clades does not occur frequently. Overall, the available evidence does not support the perpetuation of HP H5N1 influenza in migratory birds and suggests that the introduction of HP Asian H5N1 to the Americas by migratory birds is likely to be a rare event. 相似文献
82.
Dimethylsulfoxide reduction by marine sulfate-reducing bacteria 总被引:2,自引:0,他引:2
Henk M. Jonkers Marc J.E.C. van der Maarel Hans van Gemerden Theo A. Hansen 《FEMS microbiology letters》1996,136(3):283-287
Abstract Dimethylsulfoxide (DMSO) reduction occurred in five out of nine strains of sulfate-reducing bacteria from marine or saline environments, but not in three freshwater isolates. DMSO reduction supported growth in all positive strains. In Desulfovibrio desulfuricans strain PA2805, DMSO reduction occurred simultaneously with sulfate reduction and was not effectively inhibited by molybdate, a specific inhibitor of sulfate reduction. The growth yield per mol lactate was 26% higher with DMSO than with sulfate as electron acceptor. In extracts of cells of strain PA2805 grown on sulfate, a low level of DMSO-reducing activity was present (0.013 μmol (mg protein)− min− ); higher levels were found in cells grown on DMSO (0.56 μmol (mg protein)− min− ). In anoxic marine environments DMSO reduction by sulfate-reducing bacteria may lead to enhanced dimethylsulfide emission rates. 相似文献
83.
1. The activation of S-adenosyl-L-methionine decarboxylase (SAM-DC) by putrescine and a series of 1,4-butanediamines with a substituent in position 2 was studied. 2. Kinetic data show the activation of SAM-DC by putrescine is essentially uncompetitive. 3. All 2-substituted 1,4-butanediamines were activators, although not as potent as putrescine itself. 4. At high concentrations of SAM activation of SAM-DC by putrescine and putrescine analogs deviated considerably from uncompetitive activation kinetics. 5. In order to explain the experimental data, especially the non-linearity of the "fractional velocity plots", it was necessary to postulate two independent, but equivalent activator binding sites, for which substrate (SAM) and activator compete. 6. Based on this kinetic model an equation was derived which describes the rate of SAM decarboxylation as a function of substrate and activator concentrations. 7. From the simulated curves, approximate values for equilibrium constants for the binding of activator and substrate to the activator binding sites, and relative rate constants for the product forming steps were calculated. 8. Even a minor change of the structure, such as the substitution of one hydrogen atom by fluorine in the 2-position of putrescine had a very considerable effect on the potency of activation. 9. It is apparent that the structural requirements of an activator of SAM-DC are highly specific. 相似文献
84.
Maciek Godycki-Cwirko Jochen W. L. Cals Nick Francis Theo Verheij Christopher C. Butler Herman Goossens Izabela Zakowska Lech Panasiuk 《PloS one》2014,9(10)
Introduction
General public views and expectations around the use of antibiotics can influence general practitioners'' antibiotic prescribing decisions. We set out to describe the knowledge, attitudes and beliefs about the use of antibiotics for respiratory tract infections in adults in Poland, and explore differences according to where people live in an urban-rural continuum.Material and Methods
Face to face survey among a stratified random sample of adults from the general population.Results
1,210 adults completed the questionnaire (87% response rate); 44.3% were rural; 57.9% were women. 49.4% of rural respondents and 44.4% of urban respondents had used an antibiotic in the last 2 years. Rural participants were less likely to agree with the statement “usually I know when I need an antibiotic,” (53.5% vs. 61.3% respectively; p = 0.015) and reported that they would consult with a physician for a cough with yellow/green phlegm (69.2% vs. 74.9% respectively; p = 0.004), and were more likely to state that they would leave the decision about antibiotic prescribing to their doctor (87.5% vs. 85.6% respectively; p = 0.026). However, rural participants were more likely to believe that antibiotics accelerate recovery from sore throat (45.7% vs. 37.1% respectively; p = 0.017). Use of antibiotic in the last 2 years, level of education, number of children and awareness of the problem of developing antimicrobial resistance predicted accurate knowledge about antibiotic effectiveness.Conclusions
There were no major differences in beliefs about antibiotics between urban and rural responders, although rural responders were slightly less confident in their knowledge about antibiotics and self-reported greater use of antibiotics. Despite differences in the level of education between rural and urban responders, there were no significant differences in their knowledge about antibiotic effectiveness. 相似文献85.
Pardo A Stöcker M Kampmeier F Melmer G Fischer R Thepen T Barth S 《Cancer immunology, immunotherapy : CII》2012,61(10):1617-1626
Purpose
Preclinical in vivo analyses of treatment responses are an important prerequisite to evaluate new therapeutics. Molecular in vivo imaging in the far red (FR)/near infra red (NIR) is a promising method, as it enables measurements at different time points in individual animals, thereby reducing the number of animals required, while increasing statistical significance. Here, we show the establishment of a method to monitor response to treatment using fluorescent cells, expressing the epidermal growth factor receptor (EGFR), a target already used in therapy.Methods
We transfected A-431 tumour cells with the far red–emitting protein Katushka (Kat2), resulting in strong fluorescence allowing for the monitoring of tumour growth when implanted in BALB/c nu/nu mice with a CRi Maestro in vivo imager. We targeted A-431 cells with a previously reported immunotoxin (IT), consisting of the anti-EGFR antibody single-chain variable fragment (scFv) 425, fused to Pseudomonas aeruginosa Exotoxin A’ (ETA’). In addition, EGFR expression was verified using the 425(scFv) conjugated to a NIR dye BG-747 through a SNAP-tag linker.Results
The results show the feasibility to evaluate response to treatment in vivo by FR imaging, while at the same location detecting EGFR expression. Treatment with 425(scFv)-ETA’ resulted in decelerated tumour growth, while not affecting the overall health of the animals. This is in contrast to treatment with Doxorubicin, which, although decreasing the tumour size, resulted in poor health.Conclusions
We developed a novel method to non-invasively determine treatment responses by in vivo imaging of multiple parameters which showed the efficacy of 425(scFv)-ETA’. 相似文献86.
Nuclear Dbf2p-related (NDR) kinases and associated proteins are recognized as a conserved network that regulates eukaryotic cell polarity. NDR kinases require association with MOB adaptor proteins and phosphorylation of two conserved residues in the activation segment and hydrophobic motif for activity and function. We demonstrate that the Neurospora crassa NDR kinase COT1 forms inactive dimers via a conserved N-terminal extension, which is also required for the interaction of the kinase with MOB2 to generate heterocomplexes with basal activity. Basal kinase activity also requires autophosphorylation of the COT1-MOB2 complex in the activation segment, while hydrophobic motif phosphorylation of COT1 by the germinal center kinase POD6 fully activates COT1 through induction of a conformational change. Hydrophobic motif phosphorylation is also required for plasma membrane association of the COT1-MOB2 complex. MOB2 further restricts the membrane-associated kinase complex to the hyphal apex to promote polar cell growth. These data support an integrated mechanism of NDR kinase regulation in vivo, in which kinase activation and cellular localization of COT1 are coordinated by dual phosphorylation and interaction with MOB2. 相似文献
87.
Alize J. Ferrari Rosana E. Norman Greg Freedman Amanda J. Baxter Jane E. Pirkis Meredith G. Harris Andrew Page Emily Carnahan Louisa Degenhardt Theo Vos Harvey A. Whiteford 《PloS one》2014,9(4)
Background
The Global Burden of Disease Study 2010 (GBD 2010) identified mental and substance use disorders as the 5th leading contributor of burden in 2010, measured by disability adjusted life years (DALYs). This estimate was incomplete as it excluded burden resulting from the increased risk of suicide captured elsewhere in GBD 2010''s mutually exclusive list of diseases and injuries. Here, we estimate suicide DALYs attributable to mental and substance use disorders.Methods
Relative-risk estimates of suicide due to mental and substance use disorders and the global prevalence of each disorder were used to estimate population attributable fractions. These were adjusted for global differences in the proportion of suicide due to mental and substance use disorders compared to other causes then multiplied by suicide DALYs reported in GBD 2010 to estimate attributable DALYs (with 95% uncertainty).Results
Mental and substance use disorders were responsible for 22.5 million (14.8–29.8 million) of the 36.2 million (26.5–44.3 million) DALYs allocated to suicide in 2010. Depression was responsible for the largest proportion of suicide DALYs (46.1% (28.0%–60.8%)) and anorexia nervosa the lowest (0.2% (0.02%–0.5%)). DALYs occurred throughout the lifespan, with the largest proportion found in Eastern Europe and Asia, and males aged 20–30 years. The inclusion of attributable suicide DALYs would have increased the overall burden of mental and substance use disorders (assigned to them in GBD 2010 as a direct cause) from 7.4% (6.2%–8.6%) to 8.3% (7.1%–9.6%) of global DALYs, and would have changed the global ranking from 5th to 3rd leading cause of burden.Conclusions
Capturing the suicide burden attributable to mental and substance use disorders allows for more accurate estimates of burden. More consideration needs to be given to interventions targeted to populations with, or at risk for, mental and substance use disorders as an effective strategy for suicide prevention. 相似文献88.
Rezzonico F Braun-Kiewnick A Mann RA Rodoni B Goesmann A Duffy B Smits TH 《Molecular Plant Pathology》2012,13(8):975-984
Comparative genomic analysis revealed differences in the lipopolysaccharide (LPS) biosynthesis gene cluster between the Rubus‐infecting strain ATCC BAA‐2158 and the Spiraeoideae‐infecting strain CFBP 1430 of Erwinia amylovora. These differences corroborate rpoB‐based phylogenetic clustering of E. amylovora into four different groups and enable the discrimination of Spiraeoideae‐ and Rubus‐infecting strains. The structure of the differences between the two groups supports the hypothesis that adaptation to Rubus spp. took place after species separation of E. amylovora and E. pyrifoliae that contrasts with a recently proposed scenario, based on CRISPR data, in which the shift to domesticated apple would have caused an evolutionary bottleneck in the Spiraeoideae‐infecting strains of E. amylovora which would be a much earlier event. In the core region of the LPS biosynthetic gene cluster, Spiraeoideae‐infecting strains encode three glycosyltransferases and an LPS ligase (Spiraeoideae‐type waaL), whereas Rubus‐infecting strains encode two glycosyltransferases and a different LPS ligase (Rubus‐type waaL). These coding domains share little to no homology at the amino acid level between Rubus‐ and Spiraeoideae‐infecting strains, and this genotypic difference was confirmed by polymerase chain reaction analysis of the associated DNA region in 31 Rubus‐ and Spiraeoideae‐infecting strains. The LPS biosynthesis gene cluster may thus be used as a molecular marker to distinguish between Rubus‐ and Spiraeoideae‐infecting strains of E. amylovora using primers designed in this study. 相似文献
89.
Wim van Uden Niesko Pras Esther M. Vossebeld Jos N. M. Mol Theo M. Malingré 《Plant Cell, Tissue and Organ Culture》1990,20(2):81-87
Cell suspension cultures of Linum flavum L., routinely grown on a NAA-containing medium, accumulated low levels of the phenylpropanoid-derived lignan 5-methoxypodophyllotoxin (5-MPT), up to 0.004% on a dry weight basis. Feeding experiments with the precursor L-phenylalanine resulted in a 3–5-fold increase in 5-MPT levels, but caused the levels of PAL activity to fall. Treatment of the cultures with the elicitor Nigeran, either alone or in combination with phenylalanine, caused the 5-MPT production to cease, even though PAL activity was rapidly enhanced by these treatments. Transfer of the cultures to NAA-free medium resulted in a 40–50 fold higher level of 5-MPT accumulation, the PAL activity levels being lowered compared to the routinely grown cells. With these more differentiated cultures, phenylalanine feeding and elicitor treatment, both on its own and in combination with the precursor, had no effect on 5-MPT production, even though the PAL activity levels were higher than in the untreated cells. It can be concluded that in lignan-accumulating cultures of L. flavum, PAL activity is nearly always detectable and seems to show a reciprocal relationship with 5-MPT accumulation.Abbreviations 5-MPT
5-methoxypodophyllotoxin
- PAL
phenylalanine ammonia lyase (EC 4:3:1.5)
- NAA
naphthaleneacetic acid 相似文献
90.
Complexation of M+=Li+, Na+, Ag+ and TI+ by the cryptands 4, 7, 13, 18-tetraoxa-l, 10-diazabicyclo[8.5.5]eicosane (C211) and 4,7,13-trioxa-1,10-diazabicyclo[8.5.5]eicosane (C21C5) to form the cryptates [M.C211]+ and [M.C21C5]+ has been studied in trimethyl phosphate by potentiometric titration and 7Li and 23Na NMR spectroscopy. For [M.C211]+ the logarithm of the apparent stability constants, log K (dm3 mol-1)=6.98±0.05, 5.38±0.05, 9.82±0.02 and 3.95±0.02 for M+ =Li+, Na+, Ag+ and TI+, respectively; and for [M.C21C5]+ log K (dm3 mol-1)=2.40±0.10, 1.90±0.05, 6.04±0.02 and 2.42±0.10 for M+=Li+, Na+, Ag+ and Tl+, respectively. The decomplexation kinetic parameters for [Na.C211]+ are: kd (298.2 K)=6.924±0.50 s-l, ΔHd≠=62.2±0.9 kJ mol-1, and ΔSd≠= -20.3±2.7 J K-1 mol-1; and those for [Li.C21C5]+ are: kd (298.2 K)=23.3±0.4 s-1, ΔHd≠ =61.2±1.1 kJ mol-1, and ΔSd≠= -13.6±3.6 J K-1 mol-1. Metal ion exchange on [Li.C211]+ is in the very slow extreme of the NMR timescale up to 390 K and kd « 4 s-1 at 298.2 K, while in contrast exchange on [Na.C21C5]+ is in the fast extreme of the NMR timescale at 298.2 K (kd≈ 104 s-1). These data are compared with those obtained in other solvents. 相似文献