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   7181篇
  免费   406篇
  国内免费   21篇
  7608篇
  2024年   12篇
  2023年   98篇
  2022年   259篇
  2021年   377篇
  2020年   355篇
  2019年   503篇
  2018年   385篇
  2017年   269篇
  2016年   368篇
  2015年   393篇
  2014年   485篇
  2013年   634篇
  2012年   579篇
  2011年   579篇
  2010年   298篇
  2009年   262篇
  2008年   291篇
  2007年   294篇
  2006年   249篇
  2005年   216篇
  2004年   153篇
  2003年   121篇
  2002年   104篇
  2001年   21篇
  2000年   18篇
  1999年   23篇
  1998年   22篇
  1997年   18篇
  1996年   18篇
  1995年   7篇
  1994年   9篇
  1993年   12篇
  1992年   9篇
  1991年   15篇
  1990年   7篇
  1989年   11篇
  1988年   7篇
  1987年   13篇
  1986年   14篇
  1985年   17篇
  1984年   15篇
  1983年   9篇
  1982年   7篇
  1981年   6篇
  1980年   7篇
  1979年   4篇
  1976年   4篇
  1974年   8篇
  1973年   4篇
  1962年   2篇
排序方式: 共有7608条查询结果,搜索用时 0 毫秒
951.
Molecular Biology Reports - Olanzapine (OLA), is prescribed as an anti-psychotic medicine in schizophrenia patients. In this study, the protective effect of OLA against genotoxicity and...  相似文献   
952.
In the current study, midgut α-amylase from Sunn pest ( Eurygaster integriceps Puton) (Hemiptera: Scutelleridae), one of the most serious pests of wheat and barley in the wide area of the Near and Middle East, West Asia, and many of the new independent states of central Asia, were purified and characterized. Amylase activity was detected in the midgut of the insects which were collected from both over-wintering sites during winter and feeding insects during spring. Amylase activities in the midgut of over-wintering and feeding insects were 5.71 and 3.43 U/mg protein, respectively. Initially, a native electrophoretic analysis of E. integriceps crude midgut extract showed that there are two major amylase forms in the midgut. Through the sequence of ammonium sulfate precipitation, first by gel filtration chromatography (Sephadex G-75), anion exchange chromatography (diethylaminoethylcellulose) and second by gel filtration chromatography, specific activity of α-amylase of E. integriceps increased 44-fold from approximately 3 to 133 U/mg protein. Analysis of purified amylases by sodium dodecylsulfate polyacrylamide gel electrophoresis showed that these proteins had estimated molecular masses of 49 and 52 kDa. Optimum temperature was determined to be 30–40°C. The optimum pH value was 6.5 and the K mapp for soluble starch was 0.54%.  相似文献   
953.
Ectomycorrhizal (ECM) fungi are important for efficient nutrient uptake of several widespread arctic plant species. Knowledge of temporal variation of ECM fungi, and the relationship of these patterns to environmental variables, is essential to understand energy and nutrient cycling in Arctic ecosystems. We sampled roots of Bistorta vivipara ten times over two years; three times during the growing‐season (June, July and September) and twice during winter (November and April) of both years. We found 668 ECM OTUs belonging to 25 different ECM lineages, whereof 157 OTUs persisted throughout all sampling time‐points. Overall, ECM fungal richness peaked in winter and species belonging to Cortinarius, Serendipita and Sebacina were more frequent in winter than during summer. Structure of ECM fungal communities was primarily affected by spatial factors. However, after accounting for spatial effects, significant seasonal variation was evident revealing correspondence with seasonal changes in environmental conditions. We demonstrate that arctic ECM richness and community structure differ between summer (growing‐season) and winter, possibly due to reduced activity of the core community, and addition of fungi adapted for winter conditions forming a winter‐active fungal community. Significant month × year interactions were observed both for fungal richness and community composition, indicating unpredictable between‐year variation. Our study indicates that addressing seasonal changes requires replication over several years.  相似文献   
954.
Although CD8+ cytotoxic T lymphocyte (CTL) epitope-based DNA vaccination is valuable experience on vaccine research but many attempts are still continued to achieve acceptable protective response. To study the role of full length antigen in CTL epitope immunization, we evaluated cellular immunity of diverse patterns of complete Herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) and the immunodominant CTL epitope (498–505) DNA injection in C57BL/6 mice. Optimal immune response was observed in the group immunized with the full length of gB in the first injection and CTL epitope in the second and third vaccination as assessed by lymphocyte proliferation assay (MTT), cytokine assay (ELISA) and CTL assay. B cell and spatially CD4+ T cell epitopes in full length protein might be important for appropriate priming of CTL immune response. These findings may have important implication for the improvement of CTL epitope based DNA vaccine against HSV and other pathogens.  相似文献   
955.
Present study focuses on ameliorative potential of Typha elephantina leave’s aqueous (TE.AQ) extract against Paracetamol (PCM) induced toxicity in rabbits. We fed the male rabbits with 300 mg PCM in alone and in combination with TE.AQ at different doses i.e. (100, 200 and 300 mg/kg body weight) or silymarin (100 mg/kg) daily for 21 days. PCM in alone significantly (P < 0.5) increased serum urea, uric acid, creatinine, total protein, albumin, globulin and blood urea nitrogen. Serum sodium, potassium and magnesium level were high. The glutathione, radical scavenging activity and Thiobarbituric acid reactive substances were significantly reduced. Treatment with TE.AQ at dose rate 300 mg/kg body weight and Silymarin significantly ameliorated all the parameters when compared with PCM administered group. The 100 and 200 mg of TE.AQ showed no significant effects. The histopathological examination confirmed the therapeutic potential of TE.AQ. These results established the presence of natural antioxidants in Typha elephantina leaves.  相似文献   
956.
Zaire ebolavirus (ZEBOV), a highly pathogenic zoonotic virus, poses serious public health, ecological and potential bioterrorism threats. Currently no specific therapy or vaccine is available. Virus entry is an attractive target for therapeutic intervention. However, current knowledge of the ZEBOV entry mechanism is limited. While it is known that ZEBOV enters cells through endocytosis, which of the cellular endocytic mechanisms used remains unclear. Previous studies have produced differing outcomes, indicating potential involvement of multiple routes but many of these studies were performed using noninfectious surrogate systems such as pseudotyped retroviral particles, which may not accurately recapitulate the entry characteristics of the morphologically distinct wild type virus. Here we used replication-competent infectious ZEBOV as well as morphologically similar virus-like particles in specific infection and entry assays to demonstrate that in HEK293T and Vero cells internalization of ZEBOV is independent of clathrin, caveolae, and dynamin. Instead the uptake mechanism has features of macropinocytosis. The binding of virus to cells appears to directly stimulate fluid phase uptake as well as localized actin polymerization. Inhibition of key regulators of macropinocytosis including Pak1 and CtBP/BARS as well as treatment with the drug EIPA, which affects macropinosome formation, resulted in significant reduction in ZEBOV entry and infection. It is also shown that following internalization, the virus enters the endolysosomal pathway and is trafficked through early and late endosomes, but the exact site of membrane fusion and nucleocapsid penetration in the cytoplasm remains unclear. This study identifies the route for ZEBOV entry and identifies the key cellular factors required for the uptake of this filamentous virus. The findings greatly expand our understanding of the ZEBOV entry mechanism that can be applied to development of new therapeutics as well as provide potential insight into the trafficking and entry mechanism of other filoviruses.  相似文献   
957.
Solute transport phenomena mediate many aspects of the physiology and contrast agent-based clinical imaging of articular cartilage. Temperatures up to 10°C below standard body temperature (37°C) are common in articulating joints during normal activities and clinically (e.g. cold treatment of injuries). Therefore it is of interest to characterize the effects of temperature changes on solute transport parameters in cartilage. A range of fluorescent solutes including fluorescein isothiocyanate, 4 and 40kDa dextrans, myoglobin, insulin and chondroitin sulfate were prepared and used in assays of solute effective partition coefficient and effective diffusivity in bovine intermediate zone articular cartilage explants maintained at 10, 22 or 37°C. Trends for increasing partition coefficient with increasing temperature were evident for all solutes except chondroitin sulfate, with significant changes between 22 and 37°C for 4kDa dextran, insulin and myoglobin. Diffusivities of most solutes tested also tended to increase with increasing temperature, with significant changes between 10 and 22°C for FITC, 40kDa dextran and myoglobin. Oddly, insulin diffusivity decreased significantly as temperature increased from 22 to 37°C while chondroitin sulfate diffusivity exhibited no clear temperature dependence. These results highlight solute-specific temperature dependences of transport phenomena which may depend upon molecular weight, chemical structure, molecular conformation, and solute-matrix and solute-solute interactions. The articular cartilage explants themselves exhibited small but significant changes in water and glycosaminoglycan contents during experiments, underscoring the importance of solute-matrix interactions. Solute transport parameters in cartilage and their temperature dependences are therefore not easily predicted, and case-by-case experimental determination may be essential.  相似文献   
958.
Microsatellite DNA marker analysis was carried out to assess the population genetic structure of an endangered carp, Labeo calbasu, collected from three different stocks; the Jamuna River, the Halda River and a Hatchery. Four heterologous microsatellite loci (Lr12, Lr14b, Lr21 and Lr24) identified from rohu (Labeo rohita) were analyzed to test the genetic variability of the target kalibaus stocks. The maximum number of alleles observed in loci Lr12, Lr14b, Lr21 and Lr24 were 10, 7, 8 and 6, respectively. The loci were found to be polymorphic (<P 95) in all the populations. The average number of allele was highest in the Jamuna population (6.75) followed by that of the Halda (5.50) and the Hatchery population (4.25). The observed average heterozygosity (Ho) value was almost similar in all three populations. Except locus Lr12 in the Halda population, significant deviations from the Hardy-Weinberg Equilibrium were detected in all cases due to excess heterozygosity. The population differentiation values (F ST ) between all the population pairs were significant. The highest genetic distance value (D = 0.295) was measured between the Halda and the Hatchery populations. A recent bottleneck was observed in the Halda and the Hatchery population.  相似文献   
959.
The present study was designed to assess the influence of geographical factors on essential oil (EO) composition, along with antiradical potential and phytochemical contents of Ferulago angulata (Schltdl .) Boiss (Apiaceae) extracts for the first time. The aerial parts were hydrodistilled by Clevenger apparatus and subjected to gas chromatography coupled with flame ionization detector (GC/FID) and mass spectroscopy (GC/MS). The EO yields were significantly different from populations ‘Mongar’ (south‐slope, 3000 m) with 1.34±0.06 % and ‘Male‐Amiri’ (north slope, 2600 m) with 0.18±0.05 % of total oil. Thirty‐nine compounds were identified from the EOs of nine populations. α‐Pinene was the predominant component ranging from 20.84 to 49.06 % in ‘Gandomkar’ (north‐slope, 2500 m) and ‘Mongar’ (3000 m), respectively. The methanolic extract of ‘Mongar’ (north‐slope at 2500 m) possessed the highest total phenolic contents. Also, this population logically exhibited potent antiradical activity using both 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) and oxygen radical absorbance capacity (ORAC) assays with EC50 of 42.07±4.12 μg/mL and 8.34±0.21 mmol Trolox® equivalents/g, respectively. Due to its moderate free‐radical scavenging potential and high α‐pinene content, the population ‘Mongar’ might be considered as a perspective raw material in food and phytopharmaceutical industries.  相似文献   
960.
Our aim was to comprehensively analyze promoter hypermethylation of a panel of novel and known methylation markers for thyroid neoplasms and to establish their relationship with BRAF mutation and clinicopathologic parameters of thyroid cancer. A cohort of thyroid tumors, consisting of 44 cancers and 44 benign thyroid lesions, as well as 15 samples of adjacent normal thyroid tissue, was evaluated for BRAF mutation and promoter hypermethylation. Genes for quantitative methylation specific PCR (QMSP) were selected by a candidate gene approach. Twenty-two genes were tested: TSHR, RASSF1A, RARβ2, DAPK, hMLH1, ATM, S100, p16, CTNNB1, GSTP1, CALCA, TIMP3, TGFßR2, THBS1, MINT1, CTNNB1, MT1G, PAK3, NISCH, DCC, AIM1 and KIF1A. The PCR-based “mutector assay” was used to detect BRAF mutation. All p values reported are two sided. Considerable overlap was seen in the methylation markers among the different tissue groups. Significantly higher methylation frequency and level were observed for KIF1A and RARß2 in cancer samples compared with benign tumors. A negative correlation between BRAF mutation and RASSF1A methylation, and a positive correlation with RARß2 methylation were observed in accordance with previous results. In addition, positive correlation with TIMP3 and a marginal correlation with DCC methylation were observed. The present study constitutes a comprehensive promoter methylation profile of thyroid neoplasia and shows that results must be analyzed in a tissue-specific manner to identify clinically useful methylation markers. Integration of genetic and epigenetic changes in thyroid cancer will help identify relevant biologic pathways that drive its development.  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