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   10916篇
  免费   857篇
  国内免费   6篇
  2023年   72篇
  2022年   187篇
  2021年   280篇
  2020年   155篇
  2019年   283篇
  2018年   310篇
  2017年   277篇
  2016年   399篇
  2015年   624篇
  2014年   634篇
  2013年   829篇
  2012年   956篇
  2011年   900篇
  2010年   556篇
  2009年   539篇
  2008年   651篇
  2007年   661篇
  2006年   564篇
  2005年   502篇
  2004年   444篇
  2003年   400篇
  2002年   369篇
  2001年   94篇
  2000年   62篇
  1999年   83篇
  1998年   94篇
  1997年   67篇
  1996年   63篇
  1995年   67篇
  1994年   66篇
  1993年   44篇
  1992年   56篇
  1991年   43篇
  1990年   31篇
  1989年   43篇
  1988年   24篇
  1987年   28篇
  1986年   30篇
  1985年   31篇
  1984年   20篇
  1983年   28篇
  1982年   14篇
  1981年   12篇
  1980年   18篇
  1979年   23篇
  1978年   17篇
  1977年   16篇
  1974年   15篇
  1972年   11篇
  1968年   9篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
271.
Abnormal methylation is related to cancer development. Since DNMT3B is an enzyme that modulates genomic methylation, we hypothesized that genetic variants of the promoter DNMT3B may be associated with an increased risk of developing cervical cancer. Our aim was to investigate the association between ?579GT and 46359CT polymorphisms of DNMT3B and cervical cancer, high-grade squamous intraepithelial lesions (HSIL), and low-grade squamous intraepithelial lesions (LSIL). Samples from 200 healthy women and 130 women with squamous intraepithelial lesions (70 with cervical cancer, 30 with HSIL, and 30 with LSIL) were analyzed. Polymorphism genotyping was performed using PCR and restriction fragment length polymorphism. The ?579GT polymorphism was not associated with cervical cancer, HSIL, or LSIL. The CT genotype of 46359CT polymorphism was significantly associated with cervical cancer risk (OR 8.75, CI 1.27–374.1), whereas the TT genotype was associated with a significantly decreased risk of HSIL (OR 0.66, CI 0.01–0.32) and LSIL (OR 0.11, CI 0.026–0.45). Our results suggest that genotyping the 46359CT polymorphism in DNMT3B may help identify women who are genetically susceptible to cervical cancer development. Additional studies with larger sample sizes are necessary to confirm our findings.  相似文献   
272.
Eight genes encoding cellulolytic enzymes were obtained by direct PCR amplification of genomic DNA recovered from woodland soil samples. The direct amplifications were carried out by using primers designed from available online cellulase nucleotide sequences. The isolated genes were all different from each other and homologous to endo-β-1,4-glucanases of Bacillus subtilis. The cellulases were functionally expressed in Escherichia coli and tested on soluble substrate at 37 and 60 °C, showing different cellulolytic activities. Among these, the enzyme renamed CelWS6 exhibited good activity at higher temperatures. Further analysis of CelWS6 showed a high performance in acid environments (between pH 4.0 and 6.0) and at elevated temperatures with its maximum activity at pH 5.0 and 50 °C. At the optimum pH, it was very stable since more than 80 % of its original activity was maintained after an incubation of 120 min at 60 °C. Because the cellulases had different cellulolytic activities, but similar amino acid sequences, it was possible to assess the relationship between sequence and protein function.  相似文献   
273.
Galactomannan (GM) was recently included in consensus guidelines as an indirect mycological criterion for the diagnosis of invasive aspergillosis. Currently, there is an enzyme immunoassay available to detect GM in biological samples, the Platelia? Aspergillus EIA. In this study, the reproducibility of positive results obtained using this assay was evaluated using serum samples from neutropenic patients. A trend toward lower values was observed, and 55 %(27/49) of positive results were negative after retesting. A low reproducibility of positive results for the detection of GM in serum was observed.  相似文献   
274.
As the only mammalian Argonaute protein capable of directly cleaving mRNAs in a small RNA-guided manner, Argonaute-2 (Ago2) is a keyplayer in RNA interference (RNAi) silencing via small interfering (si) or short hairpin (sh) RNAs. It is also a rate-limiting factor whose saturation by si/shRNAs limits RNAi efficiency and causes numerous adverse side effects. Here, we report a set of versatile tools and widely applicable strategies for transient or stable Ago2 co-expression, which overcome these concerns. Specifically, we engineered plasmids and viral vectors to co-encode a codon-optimized human Ago2 cDNA along with custom shRNAs. Furthermore, we stably integrated this Ago2 cDNA into a panel of standard human cell lines via plasmid transfection or lentiviral transduction. Using various endo- or exogenous targets, we demonstrate the potential of all three strategies to boost mRNA silencing efficiencies in cell culture by up to 10-fold, and to facilitate combinatorial knockdowns. Importantly, these robust improvements were reflected by augmented RNAi phenotypes and accompanied by reduced off-targeting effects. We moreover show that Ago2/shRNA-co-encoding vectors can enhance and prolong transgene silencing in livers of adult mice, while concurrently alleviating hepatotoxicity. Our customizable reagents and avenues should broadly improve future in vitro and in vivo RNAi experiments in mammalian systems.  相似文献   
275.
276.
The male sand gobies calls and spawns inside cavities beneath stones, shells and other submerged objects (including artificial shelters) which he covers by piling sand on them. A previous study showed fish shelters likely act as impedance-matching devices enhancing sound frequencies below 500 Hz. This study examines the effect of the sand pile on sound amplification by shelters commonly used by Mediterranean sand gobies as nest sites in the field (bivalve shells, pebbles), or within aquarium tanks (tunnel-shaped plastic tiles, halves of clay flower-pots). Shelters were acoustically stimulated with white noise and artificial pulse-trains emitted by a small underwater buzzer placed inside the cavity. Results showed the sand pile increased the low-frequency gain by up to 12 dB. Conclusions were verified by examining the role of natural sand builds in sound amplification using data from a previous laboratory study on sound production in the male sand goby P. minutus. Implications for acoustic communication in sand gobies are discussed.  相似文献   
277.
Airborne particles (pollens and fungal spores) are recognized as important causes of allergies and many other pathologies whose main symptoms are usually associated with respiratory problems. In addition, these particles seem to be responsible for clinical symptoms of oculorhinitis and bronchial asthma. Many authors showed how pollen and spore concentrations are critically linked to meteorological conditions, while other studies investigated the possibility to estimate these concentrations through meteorological parameters. So, many different approaches have been proposed, and one of the most sophisticated is based on the use of a complex artificial neural network architecture. Once the neural device is calibrated using simultaneous time series of observed meteorological parameters and airborne biological particles, it is straightforward to use the Neural Network to predict spore concentrations using operational Limited Area Meteorological Model. In a previous work, it has been shown that the MM5 meteorological model developed by National Center for Atmospheric Research and Pennsylvania State University can be coupled with the above-cited neural predictor to provide a good prediction of Alternaria and Pleospora spore in the location of L’Aquila (Central Italy). Following the same approach, this work aims to provide the mapping of spore concentration over a wide area covered by high-resolution meteorological prediction in Central Italy. The complex patterns of fungal spore concentrations in selected areas will be described, and the high temporal variability of such fields will be discussed as well. The possibility to infer useful information from the predicted pattern of spore concentrations is discussed, as an example it appears that for people suffering from allergy to fungal spores is more comfortable to spend summertime close to the east coast of Italian Peninsula respect to the west coast. A further step of this work may easily lead to an operational use of the model for supporting the clinical management of allergies and for establishing a preventive strategy in agriculture to avoid unsafe and useless pollution of atmosphere, crops and fields.  相似文献   
278.
The sweet protein brazzein, a member of the Csβα fold family, contains four disulfide bonds that lend a high degree of thermal and pH stability to its structure. Nevertheless, a variable temperature study has revealed that the protein undergoes a local, reversible conformational change between 37 and 3°C with a midpoint about 27°C that changes the orientations and side‐chain hydrogen bond partners of Tyr8 and Tyr11. To test the functional significance of this effect, we used NMR saturation transfer to investigate the interaction between brazzein and the amino terminal domain of the sweet receptor subunit T1R2; the results showed a stronger interaction at 7°C than at 37°C. Thus the low temperature conformation, which alters the orientations of two loops known to be critical for the sweetness of brazzein, may represent the bound state of brazzein in the complex with the human sweet receptor. Proteins 2013; © 2012 Wiley Periodicals, Inc.  相似文献   
279.
280.
A coarse-grained model for simulation of interfacial phenomena in aqueous systems has been developed. The model captures the hydrophobic effect by only considering the structure and cohesiveness of water. Monte Carlo (MC) simulations of water-oil mixtures show that low concentrations of oil are solvated with little perturbation of the hydrogen bonding network structure of the water, while high concentrations of oil are excluded altogether. Analysis of the water structure in the simulations indicates that the water molecules maintain close to four coordination in the presence of solutes and the distribution of bond angles is not markedly affected by the presence of solutes. MC simulations of an alkane oligomer in water and a poly(ethylene oxide) (PEO) oligomer in water indicate that the chains are quite flexible and also do not perturb the network structure of the water phase.  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