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   877篇
  免费   64篇
  941篇
  2023年   4篇
  2022年   9篇
  2021年   27篇
  2020年   6篇
  2019年   12篇
  2018年   9篇
  2017年   16篇
  2016年   26篇
  2015年   37篇
  2014年   43篇
  2013年   81篇
  2012年   89篇
  2011年   79篇
  2010年   52篇
  2009年   37篇
  2008年   66篇
  2007年   60篇
  2006年   49篇
  2005年   53篇
  2004年   49篇
  2003年   47篇
  2002年   38篇
  2001年   5篇
  2000年   1篇
  1999年   6篇
  1998年   8篇
  1997年   7篇
  1996年   2篇
  1995年   4篇
  1994年   2篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1990年   3篇
  1989年   2篇
  1988年   1篇
  1985年   1篇
  1984年   1篇
  1981年   3篇
  1976年   1篇
  1968年   1篇
排序方式: 共有941条查询结果,搜索用时 10 毫秒
131.
Recombinant expression of the large alternansucrase (2057 amino acids) was hindered in E. coli due to poor enzyme solubility and protein degradation. The effects of deletions of the alternansucrase C-terminal CW-like and APY repeated motifs on enzyme solubility and specificity were investigated. A truncated variant deleted of the APY repeats but harboring four C-terminal CW-like repeats displayed a high specific activity and the same specificity of product synthesis as the native enzyme. It is more soluble and suffers less degradation than full length alternansucrase. Hence this truncated variant is a promising tool for the further structural and kinetic study of this interesting enzyme.  相似文献   
132.
Alterations in DNA repair lead to genomic instability and higher risk of cancer. DNA base excision repair (BER) corrects damaged bases, apurinic sites, and single-strand DNA breaks. Here, a regulatory mechanism for DNA polymerase beta (Pol beta) is described. Pol beta was found to form a complex with the protein arginine methyltransferase 6 (PRMT6) and was specifically methylated in vitro and in vivo. Methylation of Pol beta by PRMT6 strongly stimulated DNA polymerase activity by enhancing DNA binding and processivity, while single nucleotide insertion and dRP-lyase activity were not affected. Two residues, R83 and R152, were identified in Pol beta as the sites of methylation by PRMT6. Genetic complementation of Pol beta knockout cells with R83/152K mutant revealed the importance of these residues for the cellular resistance to DNA alkylating agent. Based on our findings, we propose that PRMT6 plays a role as a regulator of BER.  相似文献   
133.
134.
Is Pneumocystis pneumonia (PcP) a transmissible fungal disease? Does nosocomial PcP occur? Is there Pneumocystis transmission in the community? These questions, which could not be tackled before the 2000s, may at present be approached using either noninvasive detection methods or experimental transmission models. Represented by a unique entity (P.?carinii) for almost one century, the Pneumocystis genus was shown to contain several species, being P.?jirovecii the sole species identified in humans hitherto. Molecular methods combined with cross infection experiments revealed strong host specificity that precludes Pneumocystis inter-species transmission. In contrast, respiratory transmission between mammals of a same species is usually highly active, even between immunocompetent hosts. Other transmission ways could also exist. New data show that human being is the unique P.?jirovecii reservoir; it would constitute the sole infection source in both hospital and community.  相似文献   
135.
To advance the understanding of the molecular mechanisms controlling microbial activities involved in carbon cycling and mitigation of environmental pollution in freshwaters, the influence of heavy metals and natural as well as xenobiotic organic compounds on laccase gene expression was quantified using quantitative real-time PCR (qRT-PCR) in an exclusively aquatic fungus (the aquatic hyphomycete Clavariopsis aquatica) for the first time. Five putative laccase genes (lcc1 to lcc5) identified in C. aquatica were differentially expressed in response to the fungal growth stage and potential laccase inducers, with certain genes being upregulated by, e.g., the lignocellulose breakdown product vanillic acid, the endocrine disruptor technical nonylphenol, manganese, and zinc. lcc4 is inducible by vanillic acid and most likely encodes an extracellular laccase already excreted during the trophophase of the organism, suggesting a function during fungal substrate colonization. Surprisingly, unlike many laccases of terrestrial fungi, none of the C. aquatica laccase genes was found to be upregulated by copper. However, copper strongly increases extracellular laccase activity in C. aquatica, possibly due to stabilization of the copper-containing catalytic center of the enzyme. Copper was found to half-saturate laccase activity already at about 1.8 μM, in favor of a fungal adaptation to low copper concentrations of aquatic habitats.  相似文献   
136.
Weissella confusa is a rod-shaped heterofermentative lactic acid bacterium from the family of Leuconostocaceae. Here we report the draft genome sequence of the strain W. confusa LBAE C39-2 isolated from a traditional French wheat sourdough.  相似文献   
137.
138.
139.
Bee samples were collected in French apiaries that displayed severe losses and mortality during the winter (from November 2007 to March 2008). They were screened for the presence of Israeli acute paralysis virus (IAPV) by using RT-PCR. Five out of 35 surveyed apiaries, located in two different geographical areas, were found positive. This represents the first reported detection of IAPV in France. The specificity of the PCR products was checked by sequencing. The phylogenetic analysis showed that French isolates of IAPV were closely related to a cluster including American and Australian isolates. Nevertheless, most of American isolates previously reported to be associated to Colony Collapse Disorder (CCD) and an Israeli isolate first isolated in 2004 from dead bees were included in another cluster. Since IAPV was detected in only 14% of the affected apiaries, it was not possible to establish a causal link between IAPV and the severe winter losses that occurred.  相似文献   
140.
The development of the axial transmission technique now enables in vivo evaluation of cortical bone quality, which plays an important role in bone fragility. Cortical bone is a complex multiscale material, which may be made of different types of microstructure. The interaction between ultrasound and cortical bone remains unclear and most studies have been confined to wave speed analysis. The first aim of this study is to investigate the dependence of the frequency-dependent attenuation on the type of bone microstructure. The second goal is to determine whether broadband ultrasonic attenuation (BUA) is related to volumetric bone mineral density (vBMD) and mass density. Parallelepipedic samples of bovine cortical bone were cut from three specimens and tested in the axial, radial and tangential directions using an ultrasonic transmission device. BUA was evaluated over a 1-MHz wide bandwidth around 4MHz. In addition, the microstructure of each sample was determined using an optical microscope. BUA values measured in porotic microstructure are significantly higher than in Haversian microstructure. The lowest BUA values are obtained for plexiform microstructure. For all structures, BUA in the axial direction is significantly smaller than in the radial and tangential directions. Moreover, BUA is correlated with both vBMD and density (determination coefficient (R2) equal to 0.44 and 0.65, respectively, in the axial direction). BUA variations can be explained by scattering and viscoelastic mechanisms. This study suggests that BUA measurements have the potential to discriminate among different cortical bone microstructures in addition to providing material properties.  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