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   7431篇
  免费   599篇
  国内免费   18篇
  8048篇
  2024年   9篇
  2023年   74篇
  2022年   151篇
  2021年   247篇
  2020年   124篇
  2019年   180篇
  2018年   212篇
  2017年   210篇
  2016年   267篇
  2015年   384篇
  2014年   383篇
  2013年   496篇
  2012年   589篇
  2011年   563篇
  2010年   351篇
  2009年   281篇
  2008年   372篇
  2007年   383篇
  2006年   320篇
  2005年   315篇
  2004年   233篇
  2003年   209篇
  2002年   245篇
  2001年   160篇
  2000年   204篇
  1999年   149篇
  1998年   58篇
  1997年   34篇
  1996年   36篇
  1995年   55篇
  1994年   44篇
  1993年   34篇
  1992年   73篇
  1991年   71篇
  1990年   65篇
  1989年   65篇
  1988年   50篇
  1987年   41篇
  1986年   47篇
  1985年   53篇
  1984年   31篇
  1983年   29篇
  1982年   14篇
  1981年   20篇
  1980年   10篇
  1979年   22篇
  1978年   16篇
  1977年   15篇
  1976年   11篇
  1972年   10篇
排序方式: 共有8048条查询结果,搜索用时 15 毫秒
21.
3-Hydroxypropionic acid (3-HP) is a platform molecule whose biological production was carried out by the bacterium Limosilactobacillus reuteri according to a two-step process: first, a growth phase in batch mode on glucose, then a glycerol bioconversion into 3-HP in fed-batch mode. With the objective of improving 3-HP bioproduction, this study aimed at defining the operating conditions during the bioconversion phase that increases the bioproduction performance. A central composite rotatable design allowed testing various pH levels and specific glycerol feeding rates. By establishing response surfaces, optimal conditions have been identified that were different depending on the considered output variable (final 3-HP quantity, 3-HP production yield and production rate). Of them, 3-HP final quantity and 3-HP production yield were maximized at pH 6.0 and at specific glycerol feeding rates of 60 and 55 mggly gCDW−1 h−1, respectively. The specific 3-HP production rate was the highest at the upper limit of the specific substrate feeding rate (80 mggly gCDW−1 h−1) but was not affected by the pH. An additional experiment was carried out at pH 6.0 and a specific glycerol feeding rate of 80 mggly gCDW−1 h−1 to validate the previous observations. In conclusion, the results showed a significant improvement of 3-HP concentration by 13%, of specific production rate by 34% and of 3-HP volumetric productivity by 39%, as compared to the initial values.  相似文献   
22.
M. Nguyen  A. Alfonso  C. D. Johnson    J. B. Rand 《Genetics》1995,140(2):527-535
We characterized 18 genes from Caenorhabditis elegans that, when mutated, confer recessive resistance to inhibitors of acetylcholinesterase. These include previously described genes as well as newly identified genes; they encode essential as well as nonessential functions. In the absence of acetylcholinesterase inhibitors, the different mutants display a wide range of behavioral deficits, from mild uncoordination to almost complete paralysis. Measurements of acetylcholine levels in these mutants suggest that some of the genes are involved in presynaptic functions.  相似文献   
23.
Protein interactions during coronavirus assembly.   总被引:14,自引:9,他引:5       下载免费PDF全文
Coronaviruses assemble and obtain their envelope at membranes of the intermediate compartment between the endoplasmic reticulum and Golgi complex. Like other enveloped viruses, coronavirus assembly is presumably dependent on protein localization and protein-protein as well as protein-RNA interactions. We have used the bovine coronavirus (BCV) as a model to study interactions between the viral proteins in virus-infected cells that are important for coronavirus assembly. BCV is a prototype for the coronaviruses that express an additional major structural protein, the hemagglutinin esterase (HE), in addition to the spike (S) glycoprotein, membrane (M) glycoprotein, and nucleocapsid (N) protein. Complexes consisting of the M, S, and HE proteins were detected in virus-infected cells by coimmunoprecipitations. Kinetic analyses demonstrated that S protein and HE each quickly formed a complex with M protein after synthesis, whereas heterocomplexes consisting of all three proteins formed more slowly. The kinetics of HE biosynthesis revealed that the half-life of oligomerization was approximately 30 min, which correlated with the appearance of complexes consisting of M, HE, and S proteins, suggesting that oligomerization and/or conformational changes may be important for the S-M-HE protein complexes to form. Only HE dimers were found associated with the heterocomplexes consisting of all three proteins. S-M-HE protein complexes were detected prior to processing of the oligosaccharide chains on HE, indicating that these protein complexes formed in a premedial Golgi compartment before trimming of sugar chains. Transient coexpressions and double-labeling immunofluorescence demonstrated that HE and S proteins colocalized with M protein. This was further supported by coimmunoprecipitation of specific HE-M and S-M protein complexes from transfected cells, indicating that these proteins can form complexes in the absence of other viral proteins.  相似文献   
24.
25.
Nuclear DNA amounts were determined by cytofluorometry for twelve species and subspecies of the genus Xenopus. Absolute values, in pg per nucleus, were obtained by direct comparison with human lymphocyte nuclei. The lowest DNA amount (3.55 pg) was found in X. tropicalis, which possess only 20 chromosomes, and the highest (16.25 pg), in the hexaploid X. ruwenzoriensis, with 108 chromosomes. The two recently discovered tetraploid species, X. sp.n. and X. vestitus have, respectively, 12.57 and 12.83 pg of DNA. Among the species and subspecies with 36 chromosomes, the DNA content ranges from 6.35 to 8.45 pg.  相似文献   
26.
27.
Transabdominal fine needle aspiration biopsy of a solitary space-occupying lesion in the liver produced smears containing irregular bundles of smooth muscle cells with granular or fibrillary cytoplasm and slightly pleomorphic nuclei. In a few bundles, aggregates of mature fat cells were present, which is characteristic for an angiomyolipoma. Histologic examination of the resected mass showed it to be a solitary angiomyolipoma of the liver. The diagnosis was further confirmed by immunohistochemical and electron microscopic studies.  相似文献   
28.
29.
In order to correlate and compare the antigenic relationships observed between nine yeast strains, we have studied their major antigenic determinant represented by the mannans. This material is a constituent of their cell.The compounds that we have isolated are, in reality, glycopeptides. All their polysaccharide fractions consist of mannose only — except for those of the compounds that we have isolated of R. glutinis and C. lipolytica. All these polysaccharides have very close ramified structures; the only one that seems to be very different is that of R. glutinis, and this is in agreement with the antigenic properties of that yeast.The peptidic fractions of these glycopeptides are rich in threonine, serine and aspartic acid and it is very likely that these amino acids make the link with the polysaccharide fraction. The quantity of these amino acids seem to be affected by the conditions of culture of the yeasts, but not their presence or absence.  相似文献   
30.
Research in human-associated microbiomes often involves the analysis of taxonomic count tables generated via high-throughput sequencing. It is difficult to apply statistical tools as the data is high-dimensional, sparse, and compositional. An approachable way to alleviate high-dimensionality and sparsity is to aggregate variables into pre-defined sets. Set-based analysis is ubiquitous in the genomics literature and has demonstrable impact on improving interpretability and power of downstream analysis. Unfortunately, there is a lack of sophisticated set-based analysis methods specific to microbiome taxonomic data, where current practice often employs abundance summation as a technique for aggregation. This approach prevents comparison across sets of different sizes, does not preserve inter-sample distances, and amplifies protocol bias. Here, we attempt to fill this gap with a new single-sample taxon enrichment method that uses a novel log-ratio formulation based on the competitive null hypothesis commonly used in the enrichment analysis literature. Our approach, titled competitive balances for taxonomic enrichment analysis (CBEA), generates sample-specific enrichment scores as the scaled log-ratio of the subcomposition defined by taxa within a set and the subcomposition defined by its complement. We provide sample-level significance testing by estimating an empirical null distribution of our test statistic with valid p-values. Herein, we demonstrate, using both real data applications and simulations, that CBEA controls for type I error, even under high sparsity and high inter-taxa correlation scenarios. Additionally, CBEA provides informative scores that can be inputs to downstream analyses such as prediction tasks.  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