全文获取类型
收费全文 | 29657篇 |
免费 | 3563篇 |
国内免费 | 13篇 |
出版年
2016年 | 253篇 |
2015年 | 323篇 |
2014年 | 418篇 |
2013年 | 684篇 |
2012年 | 658篇 |
2011年 | 716篇 |
2010年 | 500篇 |
2009年 | 421篇 |
2008年 | 616篇 |
2007年 | 663篇 |
2006年 | 625篇 |
2005年 | 627篇 |
2004年 | 634篇 |
2003年 | 660篇 |
2002年 | 655篇 |
2001年 | 1874篇 |
2000年 | 1858篇 |
1999年 | 1455篇 |
1998年 | 470篇 |
1997年 | 476篇 |
1996年 | 483篇 |
1995年 | 433篇 |
1994年 | 432篇 |
1993年 | 430篇 |
1992年 | 1081篇 |
1991年 | 1094篇 |
1990年 | 1066篇 |
1989年 | 1042篇 |
1988年 | 966篇 |
1987年 | 907篇 |
1986年 | 779篇 |
1985年 | 759篇 |
1984年 | 603篇 |
1983年 | 514篇 |
1982年 | 377篇 |
1981年 | 348篇 |
1980年 | 336篇 |
1979年 | 573篇 |
1978年 | 452篇 |
1977年 | 420篇 |
1976年 | 344篇 |
1975年 | 466篇 |
1974年 | 484篇 |
1973年 | 420篇 |
1972年 | 397篇 |
1971年 | 312篇 |
1970年 | 260篇 |
1969年 | 241篇 |
1968年 | 232篇 |
1967年 | 205篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Heidi Hyytiä Noora Ristiniemi Päivi Laitinen Timo Lövgren Kim Pettersson 《Analytical biochemistry》2014
Nanoparticles have successfully been employed in immunometric assays that require high sensitivity. Certain analytes, however, require dynamic ranges (DRs) around a predetermined cut-off value. Here, we have studied the effects that antibody orientation and addition of free solid-phase and detection antibodies have on assay sensitivity and DR in traditional sandwich-type immunoassays. D-dimer and cardiac troponin I (cTnI), both routinely used in critical care testing, were applied as model analytes. The assays were performed in microtitration wells with preimmobilized solid-phase antibody. Inherently fluorescent nanoparticles coated with second antibody were used to detect the analyte. The selection of antibody orientation and addition of free solid-phase or detection antibody, with nanoparticles and calibrator, desensitized the assays and extended the DR. With D-dimer the upper limit of the DR was improved from 50 to 10,000 ng/ml, and with cTnI from 25 to 1000 ng/ml. Regression analysis with the Stago STA Liatest D-dimer assay yielded a slope (95% confidence interval) of 0.09 (0.07–0.11) and a y-intercept of −7.79 (−17.87–2.29) ng/L (n = 65, r = 0.906). Thus it is concluded that Europium(III)-chelate-doped nanoparticles can also be employed in immunoassays that require wide DRs around a certain cut-off limit. 相似文献
2.
Elevationally biased avian predation as a contributor to the spatial distribution of geometrid moth outbreaks in sub‐arctic mountain birch forest 下载免费PDF全文
ADAM A. PEPI OLE PETTER L. VINDSTAD MALIN EK JANE U. JEPSEN 《Ecological Entomology》2017,42(4):430-438
1. Population dynamics and interactions that vary over a species' range are of particular importance in the context of latitudinal clines in biological diversity. Winter moth (Operophtera brumata) and autumnal moth (Epirrita autumnata) are two species of eruptive geometrids that vary widely in outbreak tendency over their range, which generally increases from south to north and with elevation. 2. The predation pressure on geometrid larvae and pupae over an elevational gradient was tested. The effects of background larval density and bird occupancy of monitoring nest boxes on predation rates were also tested. Predation on larvae was tested through exclusion treatments at 20 replicate stations over four elevations at one site, while pupae were set out to measure predation at two elevations at three sites. 3. Larval densities were reduced by bird predation at three lower elevations, but not at the highest elevation, and predation rates were 1.9 times higher at the lowest elevation than at the highest elevation. The rate of predation on larvae was not related to background larval density or nest box occupancy, although there were more eggs and chicks at the lowest elevation. There were no consistent differences in predation on pupae by elevation. 4. These results suggest that elevational variation in avian predation pressure on larvae may help to drive elevational differences in outbreak tendency, and that birds may play a more important role in geometrid population dynamics than the focus on invertebrate and soil predators of previous work would suggest. 相似文献
3.
H.U. Schweikert U.W. Tunn U.-F. Habenicht J. Arnold Th. Senge H. Schulze F.H. Schrder J.H.M. Blom O. Ennemoser W. Horniger G. Bartsch 《The Journal of steroid biochemistry and molecular biology》1993,44(4-6):573-576
Sex steroids are thought to play an essential role in the pathogenesis of human benign prostatic hyperplasia (BPH). Since recent studies in animal models and in men have shown that estrogens might be causally linked to the onset and maintenance of BPH, we examined the effect of 1-methyl-androsta-1,4-diene-3,17-dione (Atamestane), a newly developed aromatase inhibitor, in men with BPH. In an open multicenter study 49 men (mean age 70.1 years, range 55 to 84) with obstructive BPH were treated with atamestane (3 × 200 mg/day) for 3 months. Of the 49 patients 44 completed the treatment period; the other patients discontinued the study for reasons unrelated to treatment. With treatment BPH-related symptoms such as daytime voiding frequency, nycturia, peak flow and residual urine improved considerably; however, these parameters did not reach statistical significance. The mean prostatic volume decreased significantly from 74.2 ± 31.7 to 64.0 ± 31 ml (mean ± SD). Serum estrogen levels decreased markedly during treatment. In addition intraprostatic estrogen concentration decreased with treatment as compared to estrogen levels in hyperplastic prostates from untreated patients. The following conclusions can be drawn from this study: first, estrogens appear to have an important supportive role in established BPH, and second, estrogen deprivation improved BPH-related symptoms and reduced significantly prostatic volume. 相似文献
4.
Influence of CpG methylation and target spacing on V(D)J recombination in a transgenic substrate. 总被引:10,自引:2,他引:8 下载免费PDF全文
We have previously described a line of transgenic mice with multiple head-to-tail copies of an artificial V-J recombination substrate and have shown that the methylation of this transgene is under the control of a dominant strain-specific modifier gene, Ssm-1. When the transgene array is highly methylated, no recombination is detectable, but when it is unmethylated, V-J joining is seen in the spleen, bone marrow, lymph nodes, and Peyer's patches but not in the thymus or nonlymphoid tissues, including brain tissue. Strikingly, in mice with partially methylated transgene arrays, rearrangement preferentially occurs in hypomethylated copies. Therefore, V-J recombination is negatively correlated with methylated DNA sequences. In addition, it appears that recombination occurs randomly between any two recombination signal sequences within the transgene array. This lack of target preference in an unselectable array of identical targets rules out simple mechanisms of one-dimensional tracking of a V(D)J recombinase complex. 相似文献
5.
6.
U. Kutschera 《Planta》1991,184(1):61-66
The relationship between growth, change in cell osmotic pressure and accumulation of osmotic solutes was investigated in hypocotyls of sunflower (Helianthus annum L.) seedlings. During growth in darkness the osmotic pressure decreased by 50% between days 2 and 6 after sowing. After irradiation of dark-grown seedlings with continuous white light (WL) an inhibition of hypocotyl growth was measured, but the osmotic pressure of the growing cells was not lower than in the dark-grown control. Growth in darkness and after WL irradiation was accompanied by an increase in the amount of osmotic substances (soluble sugars) which was proportional to the increase in length of the organ. During growth in continuous WL the cell osmotic pressure decreased by 45 % between days 2 and 6 after sowing. The transfer of WL-grown seedlings to darkness (“re-etiolation”) resulted in a rapid acceleration of hypocotyl growth, but the cell osmotic pressure was the same as that of the WL grown control. Growth in continuous WL was accompanied by a corresponding accumulation of osmotic substances (soluble sugars). The transition from WL to darkness resulted in an enhanced accumulation of osmotica and an increase in cell-wall extensibility. The results indicate that the relative maintenance of cell osmotic pressure during rapid hypocotyl growth in darkness is caused by an enhanced accumulation of soluble sugars into the growing cells of the organ. 相似文献
7.
Zafar U. Khan Antonia Gutiérrez Celia P. Miralles Angel L. De Blas 《Neurochemical research》1996,21(2):147-159
Subunit-specific antibodies to all the γ subunit isoforms described in mammalian brain (γ1, γ2S, γL, and γ3) have been made. The proportion of GABAA receptors containing each γ subunit isoform in various brain regions has been determined by quantitative immunoprecipitation.
In all tested regions of the rat brain, the γ1, and γ3 subunits are present in considerable smaller proportion of GABAA receptor than the γ2 subunit. Immunocytochemistry shows that γ1 immunoreactivity concentrates in the stratum oriens and stratum radiatum of the CA1 region of the hippocampus. In the dentate
gyrus, γ1 immunoreactivity concentrates on the outer 2/3 of the molecular layer coinciding with the localization of the axospinous
synapses of the perforant pathway. In contrast, γ3 immunoreactivity concentrates on the basket cells and other GABAergic local circuit neurons of the hilus. These cells are
also rich in γ2S. In the cerebellu, γ1 immunolabeling was localized on the Bergmann glia. The γ2S and γ2L subunits are differentially expressed in various brain regions. Thus the γ2S is highly expressed in the olfactory bulb and hippocampus whereas the γ2L is very abundant in inferior colliculus and cerebellum, particularly in Purkinje cells, as immunocytochemistry, in situ hybridization
and immunoprecipitation techniques have revealed. The γ2S and γ2L coexist in some brain areas and cell types. Moreover, the γ2S and γ2L subunits can coexist in the same GABAA receptor pentamer. We have shown that this is the case in some GABAA receptors expressed in cerebellar granule cells. These GABAA receptors also have α and β subunits forming the pentamer. Immunoblots have shown that the rat γ1, γ2S, γ2L and γ3 subunits are peptides of 47, 45, 47 and 44 kDa respectively. Results also indicate that there are aging-related changes in
the expression of the γ2S and γ2L subunits in various brain regions which suggest the existence of aging-related changes in the subunit composition of the
GABAA receptors which in turn might lead to changes in receptor pharmacology. The results obtained with the various γ subunit isoforms
are discussed in terms of the high molecular and binding heterogeneity of the native GABAA receptors in brain.
Special issue dedicated to Dr. Kinya Kuriyama 相似文献
8.
OBJECTIVES--To compare the effectiveness of penicillin V and amoxycillin with placebo in treatment of adult patients with acute sinusitis. DESIGN--Randomised, double blind, placebo controlled trial. SETTING--Norwegian general practice. SUBJECTS--130 adult patients with a clinical diagnosis of acute sinusitis confirmed by computed tomography. MAIN OUTCOME MEASURES--Subjective status after three and 10 days of treatment, difference in clinical severity score between day 0 and day 10 as evaluated by the general practitioner, difference in score from computed tomography on day 0 and day 10, and duration of sinusitis. RESULTS--Amoxycillin and penicillin V led to significantly faster and better recovery than placebo. By day 10, 71 patients receiving antibiotic treatment- (86%) considered themselves to be recovered or much better compared with 25 (57%) receiving placebo. The mean (95% confidence interval) reductions in clinical severity scores by day 10 were 5.4 (5.0 to 5.8) for penicillin V, 5.5 (4.9 to 6.0 for amoxycillin, and 3.4 (2.8 to 4.0) for placebo. For the antibiotic groups combined the number of patients with the greatest degree of improvement on computed tomography (scale 0-16)-that is, score 5-16 on day 10-was 31/83 (37%) compared with 10/44 (23%) receiving placebo. The median duration of the sinusitis was nine days in the amoxycillin group, 11 days in the penicillin V group, and 17 days in the placebo group. CONCLUSION--Penicillin V and amoxycillin are significantly more effective than placebo in the treatment of acute sinusitis. 相似文献
9.
A focal adhesion factor directly linking intracellularly motile Listeria monocytogenes and Listeria ivanovii to the actin-based cytoskeleton of mammalian cells. 总被引:12,自引:4,他引:8 下载免费PDF全文
T Chakraborty F Ebel E Domann K Niebuhr B Gerstel S Pistor C J Temm-Grove B M Jockusch M Reinhard U Walter et al. 《The EMBO journal》1995,14(7):1314-1321
The surface-bound ActA polypeptide of the intracellular bacterial pathogen Listeria monocytogenes is the sole listerial factor needed for recruitment of host actin filaments by intracellularly motile bacteria. Here we report that following Listeria infection the host vasodilator-stimulated phosphoprotein (VASP), a microfilament- and focal adhesion-associated substrate of both the cAMP- and cGMP-dependent protein kinases, accumulates on the surface of intracytoplasmic bacteria prior to the detection of F-actin 'clouds'. VASP remains associated with the surface of highly motile bacteria, where it is polarly located, juxtaposed between one extremity of the bacterial surface and the front of the actin comet tail. Since actin filament polymerization occurs only at the very front of the tail, VASP exhibits properties of a host protein required to promote actin polymerization. Purified VASP binds directly to the ActA polypeptide in vitro. A ligand-overlay blot using purified radiolabelled VASP enabled us to identify the ActA homologue of the related intracellular motile pathogen, Listeria ivanovii, as a protein with a molecular mass of approximately 150 kDa. VASP also associates with actin filaments recruited by another intracellularly motile bacterial pathogen, Shigella flexneri. Hence, by the simple expedient of expressing surface-bound attractor molecules, bacterial pathogens effectively harness cytoskeletal components to achieve intracellular movement. 相似文献
10.
Lysogenic Acetobacter methanolicus strains carrying the prophage Acm1 were found to be unable to synthesize both the capsutar polysaccharide (CPS) and the O-specific side-chain of lipopolysaccharide (LPS) and to represent rough variants of the host bacterium. A 262 bp DNA fragment of phage Acm1, obviously required for interference with LPS biosynthesis, was cloned and expressed in Escherichia coli Independently of the O-type, transformation of various E. coli strains with the recombinant DNA resulted in a suppression of biosynthesis of the O-specific chains. The DNA fragment of phage Acm1 contained three very short open reading frames of 21, 24, and 36 bp. However, attempts to express phage-encoded peptides were not successful. Instead, the Acm1-derived DNA fragment was shown to code for the synthesis of a trans-acting RNA molecule of 97 nucleotides, designated lbi (L PS b iosynthesis-i nterfering) RNA. This RNA contains sequence complementarity to E. coli target RNA sequences and appears to have the ability to form intracellularly RNA hybrid duplexes with mRNA. The data presented in this study support the hypothesis that the phenotypic effect of conversion to rough-type LPS is accompanied by the expression of an antisense RNA of phage Acm1. 相似文献