全文获取类型
收费全文 | 56249篇 |
免费 | 4703篇 |
国内免费 | 48篇 |
专业分类
61000篇 |
出版年
2023年 | 200篇 |
2022年 | 584篇 |
2021年 | 1003篇 |
2020年 | 556篇 |
2019年 | 741篇 |
2018年 | 1133篇 |
2017年 | 889篇 |
2016年 | 1566篇 |
2015年 | 2585篇 |
2014年 | 2875篇 |
2013年 | 3368篇 |
2012年 | 4342篇 |
2011年 | 4153篇 |
2010年 | 2637篇 |
2009年 | 2319篇 |
2008年 | 3337篇 |
2007年 | 3100篇 |
2006年 | 2833篇 |
2005年 | 2558篇 |
2004年 | 2502篇 |
2003年 | 2226篇 |
2002年 | 1897篇 |
2001年 | 1645篇 |
2000年 | 1536篇 |
1999年 | 1218篇 |
1998年 | 528篇 |
1997年 | 468篇 |
1996年 | 401篇 |
1995年 | 393篇 |
1994年 | 305篇 |
1993年 | 298篇 |
1992年 | 639篇 |
1991年 | 515篇 |
1990年 | 474篇 |
1989年 | 479篇 |
1988年 | 405篇 |
1987年 | 390篇 |
1986年 | 318篇 |
1985年 | 329篇 |
1984年 | 270篇 |
1983年 | 224篇 |
1982年 | 189篇 |
1981年 | 162篇 |
1980年 | 160篇 |
1979年 | 220篇 |
1978年 | 197篇 |
1977年 | 179篇 |
1976年 | 170篇 |
1974年 | 196篇 |
1972年 | 155篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
The genes that encode the transfer properties of plasmid F, the fertility factor of Escherichia coli K12, are known to be clustered over a large, 33.3-kb segment of F DNA. As the central segment of the transfer region has not previously been well characterized, we constructed a detailed restriction map of the large F EcoRI DNA fragment, fl, and isolated a series of plasmid derivatives that carry various overlapping segments of this F tra operon DNA. We also analyzed the protein products of those clones that carried DNA segments extending over the region between traF and traH. This region was known to include traQ, a gene required for efficient conversion of the direct product of traA to the 7000-Da pilin polypeptide. We identified the traQ product as a polypeptide that migrates as a 12,500-Da protein on sodium dodecyl sulfate-polyacrylamide gels. We also detected the products of two other new genes that we have named trbA and trbB. These polypeptides migrate with apparent molecular weights of 14,200 and 18,400, respectively. Analysis of plasmid deletion derivatives that we constructed in vitro shows that these genes map in the order traF trbA traQ trbB traH. The presence of a plasmid carrying a small 0.43-kb fragment that expressed only the 12,500 traQ product caused the traA product of a co-resident compatible plasmid to be converted to the 7000-Da pilin polypeptide, demonstrating that TraQ is the only tra operon product required for this step of F-pilin biosynthesis. 相似文献
962.
Flavio I. Bachini Danilo Pereira Ruan Santos Matheus Hausen Glauber Pereira Camila Vieira Lee Taylor Marcelle Pegurier 《Biology of sport / Institute of Sport》2022,39(3):745
Capillary dried blood spot (DBS) samples facilitate field-based collection without venipuncture. This pilot study aims to evaluate the viability of creatine (Cr) and creatinine (Crt) quantification using fresh capillary serum (CrS/CrtS) and DBS samples (CrDBS/CrtDBS), using Flow Injection Analysis Mass Spectrometry (FIA – MS). Nine Olympic Athletes provided a capillary blood sample to assess CrS/CrtS and CrDBS/CrtDBS quantified by FIA – MS. No difference between CrtS (mean ± SD: 813.6 ± 102.4 μmol/L) and CrtDBS (812.4 ± 108.1 μmol/L) was observed with acceptable variance [SEM 88.7; CV 10.7%; ICC 0.57 (CI 95% 0.06 – 0.84)] and agreement [very strong (Spearman: r = 0.77; p < 0.01) or strong (Pearson: r = 0.56; p = 0.04); Bland Altman: lower (-193) and upper (+196) limits of agreement]. CrS (mean ± SD: 691.8 ± 165.2 μmol/L) was significantly different to CrDBS (2911 ± 571.4 μmol/L) with unacceptable variance [SEM 171.6; CV 27%; ICC 0.002 (CI 95% -0.02 – 0.07)] and ‘weak’ agreement [Spearman: r = 0.21, p = 0.47 and Pearson: r = 0.06, p = 0.84; Bland Altman lower (-3367) and upper (-1072) limits of agreement]. Crt quantification is viable using both CrtS and CrtDBS (but not for Cr and CrS/CrDBS), with the DBS tissue handling technique offering several methodological and practice facing advantages. Future work should expand upon the sample size, explore sport/discipline relevant analytes across a full competitive season, including key training, recovery and performance blocks of their periodized performance plan. 相似文献
963.
Genome-Wide Association Studies in an Isolated Founder Population from the Pacific Island of Kosrae 总被引:1,自引:0,他引:1
Jennifer K. Lowe Julian B. Maller Itsik Pe'er Benjamin M. Neale Jacqueline Salit Eimear E. Kenny Jessica L. Shea Ralph Burkhardt J. Gustav Smith Weizhen Ji Martha Noel Jia Nee Foo Maude L. Blundell Vita Skilling Laura Garcia Marcia L. Sullivan Heather E. Lee Anna Labek Hope Ferdowsian Steven B. Auerbach Richard P. Lifton Christopher Newton-Cheh Jan L. Breslow Markus Stoffel Mark J. Daly David M. Altshuler Jeffrey M. Friedman 《PLoS genetics》2009,5(2)
It has been argued that the limited genetic diversity and reduced allelic heterogeneity observed in isolated founder populations facilitates discovery of loci contributing to both Mendelian and complex disease. A strong founder effect, severe isolation, and substantial inbreeding have dramatically reduced genetic diversity in natives from the island of Kosrae, Federated States of Micronesia, who exhibit a high prevalence of obesity and other metabolic disorders. We hypothesized that genetic drift and possibly natural selection on Kosrae might have increased the frequency of previously rare genetic variants with relatively large effects, making these alleles readily detectable in genome-wide association analysis. However, mapping in large, inbred cohorts introduces analytic challenges, as extensive relatedness between subjects violates the assumptions of independence upon which traditional association test statistics are based. We performed genome-wide association analysis for 15 quantitative traits in 2,906 members of the Kosrae population, using novel approaches to manage the extreme relatedness in the sample. As positive controls, we observe association to known loci for plasma cholesterol, triglycerides, and C-reactive protein and to a compelling candidate loci for thyroid stimulating hormone and fasting plasma glucose. We show that our study is well powered to detect common alleles explaining ≥5% phenotypic variance. However, no such large effects were observed with genome-wide significance, arguing that even in such a severely inbred population, common alleles typically have modest effects. Finally, we show that a majority of common variants discovered in Caucasians have indistinguishable effect sizes on Kosrae, despite the major differences in population genetics and environment. 相似文献
964.
The aim of the present study was to understand the effects of abiotic conditions on seasonal feeding activity of diverse herbivores on the same oak tree species in two different forests. We tracked changes in herbivore feeding activities on an oak tree species (Quercus serrata) in two localities: a low elevation small hillock forest patch (Muan, MN) and a middle elevation mountain forest patch (Mt. Jirisan, JR). A total of five sites were selected in each of two forest localities. Data for leaf expansion, leaf chemical qualities, leaf damage ratio, and numbers of lepidopteran caterpillars were collected during spring (May) and summer (July to August), 2012. Leaf expansion rate was higher at the low hillock forest than the mid‐mountain forest from spring to summer. Nitrogen and carbon content decreased seasonally at both localities. Lepidopteran larval diversity was high in the mid‐mountain forest, and two‐way ANOVA showed that species richness of lepidopteran larvae was significantly affected by the interaction between season and locality. Leaf damage by all herbivores was higher in the low hillock forest than the mid‐mountain forest in spring, but was higher in the mid‐mountain forest in summer. Relative proportion of general herbivores increased from spring to summer in the mid‐mountain forest, but not in the low hillock forest. Canonical Correspondence Analysis (CCA) ordination showed that altitude‐ and season‐related variables were significant species and environment interaction factors. Our data indicate that locality and temperature disproportionally affected the feeding activities of diverse herbivores in two different temperate forests. 相似文献
965.
966.
Use of Tetrahymena thermophila To Study the Role of Protozoa in Inactivation of Viruses in Water 总被引:1,自引:0,他引:1 下载免费PDF全文
Marcel D. O. Pinheiro Mary E. Power Barbara J. Butler Vivian R. Dayeh Robin Slawson Lucy E. J. Lee Denis H. Lynn Niels C. Bols 《Applied microbiology》2007,73(2):643-649
The ability of a ciliate to inactivate bacteriophage was studied because these viruses are known to influence the size and diversity of bacterial populations, which affect nutrient cycling in natural waters and effluent quality in sewage treatment, and because ciliates are ubiquitous in aquatic environments, including sewage treatment plants. Tetrahymena thermophila was used as a representative ciliate; T4 was used as a model bacteriophage. The T4 titer was monitored on Escherichia coli B in a double-agar overlay assay. T4 and the ciliate were incubated together under different conditions and for various times, after which the mixture was centrifuged through a step gradient, producing a top layer free of ciliates. The T4 titer in this layer decreased as coincubation time increased, but no decrease was seen if phage were incubated with formalin-fixed Tetrahymena. The T4 titer associated with the pellet of living ciliates was very low, suggesting that removal of the phage by Tetrahymena inactivated T4. When Tetrahymena cells were incubated with SYBR gold-labeled phage, fluorescence was localized in structures that had the shape and position of food vacuoles. Incubation of the phage and ciliate with cytochalasin B or at 4°C impaired T4 inactivation. These results suggest the active removal of T4 bacteriophage from fluid by macropinocytosis, followed by digestion in food vacuoles. Such ciliate virophagy may be a mechanism occurring in natural waters and sewage treatment, and the methods described here could be used to study the factors influencing inactivation and possibly water quality. 相似文献
967.
Characterization of highly pathogenic H5N1 avian influenza A viruses isolated from South Korea 总被引:23,自引:0,他引:23 下载免费PDF全文
Lee CW Suarez DL Tumpey TM Sung HW Kwon YK Lee YJ Choi JG Joh SJ Kim MC Lee EK Park JM Lu X Katz JM Spackman E Swayne DE Kim JH 《Journal of virology》2005,79(6):3692-3702
An unprecedented outbreak of H5N1 highly pathogenic avian influenza (HPAI) has been reported for poultry in eight different Asian countries, including South Korea, since December 2003. A phylogenetic analysis of the eight viral genes showed that the H5N1 poultry isolates from South Korea were of avian origin and contained the hemagglutinin and neuraminidase genes of the A/goose/Guangdong/1/96 (Gs/Gd) lineage. The current H5N1 strains in Asia, including the Korean isolates, share a gene constellation similar to that of the Penfold Park, Hong Kong, isolates from late 2002 and contain some molecular markers that seem to have been fixed in the Gs/Gd lineage virus since 2001. However, despite genetic similarities among recent H5N1 isolates, the topology of the phylogenetic tree clearly differentiates the Korean isolates from the Vietnamese and Thai isolates which have been reported to infect humans. A representative Korean isolate was inoculated into mice, with no mortality and no virus being isolated from the brain, although high titers of virus were observed in the lungs. The same isolate, however, caused systemic infections in chickens and quail and killed all of the birds within 2 and 4 days of intranasal inoculation, respectively. This isolate also replicated in multiple organs and tissues of ducks and caused some mortality. However, lower virus titers were observed in all corresponding tissues of ducks than in chicken and quail tissues, and the histological lesions were restricted to the respiratory tract. This study characterizes the molecular and biological properties of the H5N1 HPAI viruses from South Korea and emphasizes the need for comparative analyses of the H5N1 isolates from different countries to help elucidate the risk of a human pandemic from the strains of H5N1 HPAI currently circulating in Asia. 相似文献
968.
Jeonghan Lee Michael T. Nelson Kirstin E. Rose Slobodan M. Todorovic 《Molecular neurobiology》2013,48(2):274-280
T-type calcium channels in the dorsal root ganglia (DRG) have a central function in tuning neuronal excitability and are implicated in sensory processing including pain. Previous studies have implicated redox agents in control of T-channel activity; however, the mechanisms involved are not completely understood. Here, we recorded T-type calcium currents from acutely dissociated DRG neurons from young rats and investigated the mechanisms of CaV3.2 T-type channel modulation by S-nitrosothiols (SNOs). We found that extracellular application of S-nitrosoglutathione (GSNO) and S-nitroso-N-acetyl-penicillamine rapidly reduced T-type current amplitudes. GSNO did not affect voltage dependence of steady-state inactivation and macroscopic current kinetics of T-type channels. The effects of GSNO were abolished by pretreatment of the cells with N-ethylmaleimide, an irreversible alkylating agent, but not by pretreatment with 1H-(1,2,4) oxadiazolo (4,3-a) quinoxalin-1-one, a specific soluble guanylyl cyclase inhibitor, suggesting a potential effect of GSNO on putative extracellular thiol residues on T-type channels. Expression of wild-type CaV3.2 channels or a quadruple Cys-Ala mutant in human embryonic kidney cells revealed that Cys residues in repeats I and II on the extracellular face of the channel were required for channel inhibition by GSNO. We propose that SNO-related molecules in vivo may lead to alterations of T-type channel-dependent neuronal excitability in sensory neurons and in the central nervous system in both physiological and pathological conditions such as neuronal ischemia/hypoxia. 相似文献
969.
Use of glucose biosensors to measure extracellular glucose exudation by intertidal microphytobenthos in southern Tasmania 下载免费PDF全文
Micro glucose biosensors were used to measure net extracellular glucose produced by natural microphytobenthos and three diatom cultures (Amphora coffeaeformis, Navicula menisculus, Nitzschia longissima) from southern Tasmania, Australia. They were exposed to a light gradient in either nutrient‐replete or nutrient‐limiting conditions. Glucose exudation in the natural communities increased with increased light but the response in the cultures was variable. Similarly, nutrient‐replete conditions elicited lower rates of glucose exudation in the natural communities but produced variable species‐specific responses in the cultures. Increased glucose exudation mostly correlated with a reduction in maximum quantum yield (Fv/Fm). The same trend was observed in the natural communities for relative maximum electron transfer rates (rETRmax) but responses in the cultures were again variable and species‐specific. Responses of the three species to increased light and nutrient deficiency were variable, although glucose exudation, Fv/Fm and rETRmax was mostly lower in the nutrient‐limited media. In a second set of experiments species/communities were treated with/without antibiotics. In the dark, glucose concentrations in treatments with antibiotics remained unchanged, while in those with bacteria, it fell rapidly. In the sediment communities, glucose consumption in the dark was ~25% the rate of exudation at the highest light level. In culture, exudation rates were up to 100% greater than those with active bacteria. Rates of glucose consumption in the dark in the antibiotic–treated samples were negligible and up to 104 times lower than those with active bacteria. These results demonstrate the important role extracellular glucose exudation has on maintaining an active microbial loop. 相似文献