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   2545篇
  免费   212篇
  国内免费   1篇
  2023年   13篇
  2022年   34篇
  2021年   73篇
  2020年   31篇
  2019年   43篇
  2018年   51篇
  2017年   53篇
  2016年   89篇
  2015年   162篇
  2014年   154篇
  2013年   176篇
  2012年   249篇
  2011年   229篇
  2010年   130篇
  2009年   118篇
  2008年   154篇
  2007年   159篇
  2006年   158篇
  2005年   136篇
  2004年   129篇
  2003年   123篇
  2002年   108篇
  2001年   11篇
  2000年   9篇
  1999年   16篇
  1998年   16篇
  1997年   12篇
  1996年   12篇
  1995年   12篇
  1994年   14篇
  1993年   10篇
  1992年   5篇
  1991年   8篇
  1990年   5篇
  1989年   4篇
  1988年   8篇
  1987年   4篇
  1986年   2篇
  1985年   2篇
  1984年   7篇
  1983年   3篇
  1982年   3篇
  1981年   4篇
  1980年   3篇
  1979年   4篇
  1978年   3篇
  1974年   2篇
  1973年   3篇
  1971年   1篇
  1969年   1篇
排序方式: 共有2758条查询结果,搜索用时 296 毫秒
91.
Innovations are central for the economic growth; however, the use of new technologies needs to be widely accepted in the general public and the society as a whole. Biotechnology in general, and the use of genetic engineering in food production in particular are seen critically by the European public and perceived as "risky", and a transatlantic divide between European and US citizens has been observed. This review investigates the reasons for those differing perceptions and proposes new strategies to communicate the benefits of biotechnology in agriculture to a broader public. When analyzing the dialogue process that has taken place between public, scientists, governmental organizations and industry, questions arise on what has been done differently in Europe, in order to propose new, more successful and efficient communication strategies for the future.  相似文献   
92.
The effect of minocycline on nerve regeneration was studied in a rat model of acute sciatic nerve injury, in which the injury was caused by resection and reimplantation of the right sciatic nerve. Immunohistochemical and molecular biological methods, as well as morphometric and electron microscopic techniques, were used. Compared with uninjured and PBS-treated injured nerves, the minocycline-treated injured nerve showed: (i) a decrease in macrophage recruitment and activation, probably resulting from inhibition of blood-brain-barrier break-down via reduced MMP2 and MMP9 induction, inhibition of revascularization via additional reduction of VEGF induction, and inhibition of inducible NO synthase (iNOS) induction; (ii) reduced activation of phagocytic Schwann cells, probably by inhibition of iNOS, MMP2 and MMP9 expression; (iii) a slowed Wallerian degeneration; and subsequently, (iv) a diminished nerve regeneration. Macrophages, especially their function in the removal of cellular debris and formation of a microenvironment beneficial for nerve regeneration, are strongly implicated in constructive events after nerve injuries. Therefore, we suggest that additional research into optimizing minocycline intervention for treatment of neurodegenerative diseases is needed before further clinical trials are performed.  相似文献   
93.
The timing of sexual maturation in non-avian dinosaurs is not known. In extant squamates and crocodilians it occurs in conjunction with the initial slowing of growth rates as adult size is approached. In birds (living dinosaurs) on the other hand, reproductive activity begins well after somatic maturity. Here we used growth line counts and spacing in all of the known brooding non-avian dinosaurs to determine the stages of development when they perished. It was revealed that sexual maturation occurred well before full adult size was reached-the primitive reptilian condition. In this sense, the life history and physiology of non-avian dinosaurs was not like that of modern birds. Palaeobiological ramifications of these findings include the potential to deduce reproductive lifespan, fecundity and reproductive population sizes in non-avian dinosaurs, as well as aid in the identification of secondary sexual characteristics.  相似文献   
94.
Dimethylamine [DMA, (CH(3))(2)NH)] is abundantly present in human urine. Main sources of urinary DMA have been reported to include trimethylamine N-oxide, a common food component, and asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis. ADMA is excreted in the urine in part unmetabolized and in part after hydrolysis to DMA by dimethylarginine dimethylaminohydrolase (DDAH). Here we describe a GC-MS method for the accurate and rapid quantification of DMA in human urine. The method involves use of (CD(3))(2)NH as internal standard, simultaneous derivatization with pentafluorobenzoyl chloride and extraction in toluene, and selected-ion monitoring of m/z 239 for DMA and m/z 245 for (CD(3))(2)NH in the electron ionization mode. GC-MS analysis of urine samples from 10 healthy volunteers revealed a DMA concentration of 264+/-173 microM equivalent to 10.1+/-1.64 micromol/mmol creatinine. GC-tandem MS analysis of the same urine samples revealed an ADMA concentration of 27.3+/-15.3 microM corresponding to 1.35+/-1.2 micromol/mmol creatinine. In these volunteers, a positive correlation (R=0.83919, P=0.0024) was found between urinary DMA and ADMA, with the DMA/ADMA molar ratio being 10.8+/-6.2. Elevated excretion rates of DMA (52.9+/-18.5 micromol/mmol creatinine) and ADMA (3.85+/-1.65 micromol/mmol creatinine) were found by the method in 49 patients suffering from coronary artery disease, with the DMA/ADMA molar ratio also being elevated (16.8+/-12.8). In 12 patients suffering from end-stage liver disease, excretion rates of DMA (47.8+/-19.7 micromol/mmol creatinine) and ADMA (5.6+/-1.5 micromol/mmol creatinine) were found to be elevated, with the DMA/ADMA molar ratio (9.17+/-4.2) being insignificantly lower (P=0.46). Between urinary DMA and ADMA there was a positive correlation (R=0.6655, P<0.0001) in coronary artery disease, but no correlation (R=0.27339) was found in end-stage liver disease.  相似文献   
95.
iTRAQ compatibility of peptide immobilized pH gradient isoelectric focusing   总被引:9,自引:0,他引:9  
Lengqvist J  Uhlén K  Lehtiö J 《Proteomics》2007,7(11):1746-1752
Immobilized pH gradient isoelectric focusing (IPG-IEF) has emerged as a highly promising alternative to strong-cation exchange fractionation as the first dimension in shot-gun proteomics. Herein is shown the compatibility of this method with iTRAQ isotope labeling for relative quantitation and validation of sequence matches from database searching.  相似文献   
96.
We have previously identified and characterized two novel nuclear RNA binding proteins, p34 and p37, which have been shown to bind 5S rRNA in Trypanosoma brucei. These two proteins are nearly identical, with one major difference, an 18-amino-acid insert in the N-terminal region of p37, as well as three minor single-amino-acid differences. Homologues to p34 and p37 have been found only in other trypanosomatids, suggesting that these proteins are unique to this ancient family. We have employed RNA interference (RNAi) studies in order to gain further insight into the interaction between p34 and p37 with 5S rRNA in T. brucei. In our p34/p37 RNAi cells, decreased expression of the p34 and p37 proteins led to morphological alterations, including loss of cell shape and vacuolation, as well as to growth arrest and ultimately to cell death. Disruption of a higher-molecular-weight complex containing 5S rRNA occurs as well as a dramatic decrease in 5S rRNA levels, suggesting that p34 and p37 serve to stabilize 5S rRNA. In addition, an accumulation of 60S ribosomal subunits was observed, accompanied by a significant decrease in overall protein synthesis within p34/p37 RNAi cells. Thus, the loss of the trypanosomatid-specific proteins p34 and p37 correlates with a diminution in 5S rRNA levels as well as a decrease in ribosome activity and an alteration in ribosome biogenesis.  相似文献   
97.
Arthropod crop pests are responsible for 20% of global annual crop losses, a figure predicted to increase in a changing climate where the ranges of numerous species are projected to expand. At the same time, many insect species are beneficial, acting as pollinators and predators of pest species. For thousands of years, humans have used increasingly sophisticated chemical formulations to control insect pests but, as the scale of agriculture expanded to meet the needs of the global population, concerns about the negative impacts of agricultural practices on biodiversity have grown. While biological solutions, such as biological control agents and pheromones, have previously had relatively minor roles in pest management, biotechnology has opened the door to numerous new approaches for controlling insect pests. In this review, we look at how advances in synthetic biology and biotechnology are providing new options for pest control. We discuss emerging technologies for engineering resistant crops and insect populations and examine advances in biomanufacturing that are enabling the production of new products for pest control.  相似文献   
98.
99.
100.
It is an important goal of computational biology to correctly predict the association state of a protein based on its amino acid sequence and the structures of known homologues. We have pursued this goal on the example of anthranilate phosphoribosyltransferase (AnPRT), an enzyme that is involved in the biosynthesis of the amino acid tryptophan. Firstly, known crystal structures of naturally occurring homodimeric AnPRTs were analyzed using the Protein Interfaces, Surfaces, and Assemblies (PISA) service of the European Bioinformatics Institute (EBI). This led to the identification of two hydrophobic “hot spot” amino acids in the protein-protein interface that were predicted to be essential for self-association. Next, in a comprehensive multiple sequence alignment (MSA), naturally occurring AnPRT variants with hydrophilic or charged amino acids in place of hydrophobic residues in the two hot spot positions were identified. Representative variants were characterized in terms of thermal stability, enzymatic activity, and quaternary structure. We found that AnPRT variants with charged residues in both hot spot positions exist exclusively as monomers in solution. Variants with hydrophilic amino acids in one hot spot position occur in both forms, monomer and dimer. The results of the present study provide a detailed characterization of the determinants of the AnPRT monomer-dimer equilibrium and show that analysis of hot spots in combination with MSAs can be a valuable tool in prediction of protein quaternary structures.  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