首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2911篇
  免费   162篇
  国内免费   2篇
  3075篇
  2023年   4篇
  2022年   28篇
  2021年   41篇
  2020年   20篇
  2019年   28篇
  2018年   47篇
  2017年   42篇
  2016年   70篇
  2015年   118篇
  2014年   128篇
  2013年   206篇
  2012年   198篇
  2011年   207篇
  2010年   146篇
  2009年   110篇
  2008年   202篇
  2007年   184篇
  2006年   181篇
  2005年   201篇
  2004年   192篇
  2003年   182篇
  2002年   180篇
  2001年   24篇
  2000年   14篇
  1999年   23篇
  1998年   41篇
  1997年   24篇
  1996年   25篇
  1995年   13篇
  1994年   21篇
  1993年   21篇
  1992年   14篇
  1991年   9篇
  1990年   7篇
  1989年   7篇
  1988年   10篇
  1986年   6篇
  1985年   10篇
  1984年   15篇
  1983年   8篇
  1982年   14篇
  1981年   8篇
  1980年   8篇
  1979年   7篇
  1978年   3篇
  1977年   6篇
  1976年   4篇
  1975年   3篇
  1974年   3篇
  1973年   3篇
排序方式: 共有3075条查询结果,搜索用时 15 毫秒
61.
62.
In this paper, we present a method of fabricating a rigid antibody-immobilized surface using electric activation of a glutaraldehyde (GA)-modified aminopropylsilyl surface for stable antibody-modified field effect transistors (FETs). Electric activation of the GA-modified gate surface of the FET reduces Schiff bases, which are easily hydrolyzed and collapsed, formed between GA and 3-aminopropyltriethoxysilane, resulting in preventing the immobilized antibodies from desorbing from the surface. The lack of Raman peaks that could be assigned to a Schiff base after the electrical activation of the GA-modified surface indicated that the electric activation had reduced the Schiff base. The use of the antibody-modified FETs has three advantages for the detection of antigens: increased sensitivity, distinct recognition ability, and improved reproducibility. A tumor marker, alpha-fetoprotein (AFP), was quantitatively detected up to a concentration of 10 ng/mL using the antibody-modified FET. The detection ability of the FET accomplished a cutoff value of hepatic cancer. The quantitative detection of AFP in a solution with contaminating proteins was also demonstrated. This electric activation method is applicable to other antibody-modified FETs.  相似文献   
63.
Hen egg lysozyme (HEL) is one of the sweet-tasting proteins. To understand why lysozyme is sweet, the enzyme was synthesized at high yields by a recombinant method. The mature HEL gene was cloned from a Taq polymerase-amplified PCR product into the Pichia pastoris expression and secretion vector pPIC6alpha. This expression vector contains both the Saccharomyces cerevisiae pre-pro alpha-mating factor secretion signal and the blasticidin resistance gene (bsd) for selection of transformants in bacteria and yeast. Expression of HEL was carried out in fermenter cultures. Culture supernatants were concentrated by ultrafiltration and purified by CM-ion exchange chromatography. Approximately 400 mgL-1 of recombinant HEL was obtained. The high yield of recombinant lysozyme enabled us to perform a sensory analysis in humans. The purified recombinant lysozyme elicited as a sweet taste sensation as does the lysozyme purified directly from egg white, and showed full lytic activity against cells of Micrococcus luteus. These results demonstrate that the P. pastoris expression system with the blasticidin S selection system is useful in producing recombinant sweet-tasting protein in active form at a high yield.  相似文献   
64.
Phylogenetic relationships of tapeworms of the genus Moniezia Blanchard, 1891 (Cestoda, Anoplocephalidae) parasitizing the Eurasian elk Alces alces, the moose A. americanus and the reindeer/caribou Rangifer tarandus (Cervidae) were studied using DNA sequences of two mitochondrial genes (cox1 and nad1). Several isolates from domestic ruminants, representing Moniezia expansa (Rudolphi, 1810) sensu lato and M. benedeni (Moniez, 1879) sensu lato, and one unidentified isolate from an African antelope, were also included in the analysis.Both genes identified the same six species of Moniezia, but interspecific phylogenetic relationships were better resolved by the nad1 data. The six species of Moniezia comprised two main clades: clade 1 that originates in bovids, with subsequent colonization of northern cervids in Eurasia, and clade 2 that originates in northern cervids, with subsequent specific divergence within these hosts. Clade 2 has a Holarctic distribution.None of the Moniezia specimens in Alces and Rangifer was conspecific with the species in domestic ruminants, suggesting that the custom of identifying Moniezia spp. in northern cervids either as M. expansa or M. benedeni is incorrect. At least two of the species parasitizing Alces and Rangifer have not been previously recognized. These findings challenge the results of all previous studies concerning the diversity and ecology of Moniezia spp. in northern cervids.The traditional classification into three subgenera (Moniezia Blanchard, 1891, Blanchariezia Skrjabin & Schultz, 1937 and Baeriezia Skrjabin & Schultz, 1937), based on the presence and type of interproglottidal glands, conflicts with the currently observed molecular phylogenetic relationships within the genus Moniezia.  相似文献   
65.
The synapse number and the related dendritic spine number in the cerebral cortex of primates shows a rapid increase after birth. Depending on the brain region and species, the number of synapses reaches a peak before adulthood, and pruning takes place after this peak (overshoot-type synaptic formation). Human mental disorders, such as autism and schizophrenia, are hypothesized to be a result of either too weak or excessive pruning after the peak is reached. Thus, it is important to study the molecular mechanisms underlying overshoot-type synaptic formation, particularly the pruning phase.  相似文献   
66.
Rhodoliths are free-living coralline algae (Rhodophyta, Corallinales) that are ecologically important for the functioning of marine environments. They form extensive beds distributed worldwide, providing a habitat and nursery for benthic organisms and space for fisheries, and are an important source of calcium carbonate. The Abrolhos Bank, off eastern Brazil, harbors the world''s largest continuous rhodolith bed (of ∼21 000 km2) and has one of the largest marine CaCO3 deposits (producing 25 megatons of CaCO3 per year). Nevertheless, there is a lack of information about the microbial diversity, photosynthetic potential and ecological interactions within the rhodolith holobiont. Herein, we performed an ecophysiologic and metagenomic analysis of the Abrolhos rhodoliths to understand their microbial composition and functional components. Rhodoliths contained a specific microbiome that displayed a significant enrichment in aerobic ammonia-oxidizing betaproteobacteria and dissimilative sulfate-reducing deltaproteobacteria. We also observed a significant contribution of bacterial guilds (that is, photolithoautotrophs, anaerobic heterotrophs, sulfide oxidizers, anoxygenic phototrophs and methanogens) in the rhodolith metagenome, suggested to have important roles in biomineralization. The increased hits in aromatic compounds, fatty acid and secondary metabolism subsystems hint at an important chemically mediated interaction in which a functional job partition among eukaryal, archaeal and bacterial groups allows the rhodolith holobiont to thrive in the global ocean. High rates of photosynthesis were measured for Abrolhos rhodoliths (52.16 μmol carbon m−2 s−1), allowing the entire Abrolhos rhodolith bed to produce 5.65 × 105 tons C per day. This estimate illustrates the great importance of the Abrolhos rhodolith beds for dissolved carbon production in the South Atlantic Ocean.  相似文献   
67.

Objectives

The Great East Japan Earthquake caused a gigantic tsunami which devastated coastal areas of northern Japan on 11 March 2011. Despite the large number of ‘resident survivors’ who continued to reside in their damaged houses on the second or upper floors, research on the mental health of these individuals has been limited. This study explored the prevalence of depressive reaction and risk factors for depressive reaction among these resident survivors.

Methods

A cross-sectional household health support needs screening was conducted for resident survivors in Higashi-Matsushima city, Miyagi prefecture, two to four months after the tsunami. The health interview that was conducted including mental status, assessed by the Patient Health Questionnaire-2 (PHQ-2).

Results

Of 5,454 respondents, 8.1% had depressive reaction. After adjustment by the number of weeks from the tsunami and the mortality rate at each respondent''s place of residence, depressive reaction was significantly associated with house flooding below or above the ground floor (odds ratios of 1.92, 2.36, respectively), the unavailability of gas supply (odds ratio, 1.67), being female (odds ratio, 1.47), middle aged or elderly (odds ratios of 2.41, 2.42, respectively), regular intake of psychotropic medicine(s) since before the tsunami (odds ratio, 2.53) and the presence of one to five or more than six cohabiters (odds ratios of 0.61, 0.52, respectively).

Conclusions

The results suggest a considerable psychological burden (depressive reaction) following the tsunami among resident survivors. Special supports for families with psychiatric problems need to be considered among resident survivors. Restoration of lifeline utilities and the strengthening of social ties of persons living alone may help prevent depressive reaction among resident survivors after a tsunami.  相似文献   
68.
The purpose of this study was to determine absolute protein expression levels of transporters in rat choroid plexus, that is, the blood–cerebrospinal fluid barrier, and to compare them with the levels in the human choroid plexus. Plasma membrane fractions were prepared from pooled, freshly isolated choroid plexuses of 30 male Wistar rats and from frozen choroid plexus of one male human donor. Protein expression levels of 54 rat and 121 human molecules were measured, using a quantitative targeted absolute proteomics technique. In rat, oatp1a5 showed the most abundant protein expression (30.3 fmol/μg protein), and its expression level was 3.1‐, 4.5‐, 5.5‐, 8.4‐, 9.0‐, 9.9‐, 22‐, 91‐, and 95‐fold greater than those of glut1, oatp1c1, mrp1, mct1, oat3, pept2, mrp4, bcrp, and mdr1a, respectively. OATP1A2 (a possible homolog of rat oatp1a5), OATP1C1 and PEPT2 were not detected in human choroid plexus. MRP1, OAT3, and MRP4 showed 4.0‐, 1.8‐, and 1.7‐fold smaller expression levels in human than rat, respectively. MATE1 was detected in human, but not rat, and its expression level (8.61 fmol/μg protein) was the highest among the xenobiotic transporters examined in human choroid plexus. These findings should be useful for understanding rat blood–cerebrospinal fluid barrier function and its differences from that in human.

  相似文献   

69.
Neurovascular degeneration contributes to the pathogenesis of Alzheimer's disease (AD). Because erythropoietin (EPO) promotes endothelial regeneration, we investigated the therapeutic effects of EPO in animal models of AD. In aged Tg2576 mice, EPO receptors (EPORs) were expressed in the cortex and hippocampus. Tg2576 mice were treated with daily injection of EPO (5000 IU/kg/day) for 5 days. At 14 days, EPO improved contextual memory as measured by fear-conditioning test. EPO enhanced endothelial proliferation and the level of synaptophysin expression in the brain. EPO also increased capillary density, and decreased the level of the receptor for advanced glycation endproducts (RAGE) in the brain, while decreasing in the amount of amyloid plaque and amyloid-β (Aβ). In cultured human endothelial cells, EPO enhanced angiogenesis and suppressed the expression of the RAGE. These results show that EPO improves memory and ameliorates endothelial degeneration induced by Aβ in AD models. This pre-clinical evidence suggests that EPO may be useful for the treatment of AD.  相似文献   
70.
Hotta T  Kong Z  Ho CM  Zeng CJ  Horio T  Fong S  Vuong T  Lee YR  Liu B 《The Plant cell》2012,24(4):1494-1509
Plant cells assemble the bipolar spindle and phragmoplast microtubule (MT) arrays in the absence of the centrosome structure. Our recent findings in Arabidopsis thaliana indicated that AUGMIN subunit3 (AUG3), a homolog of animal dim γ-tubulin 3, plays a critical role in γ-tubulin-dependent MT nucleation and amplification during mitosis. Here, we report the isolation of the entire plant augmin complex that contains eight subunits. Among them, AUG1 to AUG6 share low sequence similarity with their animal counterparts, but AUG7 and AUG8 share homology only with proteins of plant origin. Genetic analyses indicate that the AUG1, AUG2, AUG4, and AUG5 genes are essential, as stable mutations in these genes could only be transmitted to heterozygous plants. The sterile aug7-1 homozygous mutant in which AUG7 expression is significantly reduced exhibited pleiotropic phenotypes of seriously retarded vegetative and reproductive growth. The aug7-1 mutation caused delocalization of γ-tubulin in the mitotic spindle and phragmoplast. Consequently, spindles were abnormally elongated, and their poles failed to converge, as MTs were splayed to discrete positions rendering deformed arrays. In addition, the mutant phragmoplasts often had disorganized MT bundles with uneven edges. We conclude that assembly of MT arrays during plant mitosis depends on the augmin complex, which includes two plant-specific subunits.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号