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   842篇
  免费   41篇
  883篇
  2024年   2篇
  2022年   3篇
  2021年   13篇
  2020年   6篇
  2019年   8篇
  2018年   16篇
  2017年   6篇
  2016年   19篇
  2015年   25篇
  2014年   46篇
  2013年   59篇
  2012年   73篇
  2011年   75篇
  2010年   46篇
  2009年   34篇
  2008年   55篇
  2007年   49篇
  2006年   46篇
  2005年   40篇
  2004年   48篇
  2003年   38篇
  2002年   40篇
  2001年   12篇
  2000年   5篇
  1999年   11篇
  1998年   10篇
  1997年   2篇
  1996年   8篇
  1995年   8篇
  1994年   6篇
  1993年   4篇
  1992年   9篇
  1991年   4篇
  1989年   4篇
  1988年   4篇
  1987年   2篇
  1986年   8篇
  1985年   2篇
  1984年   7篇
  1983年   5篇
  1981年   4篇
  1980年   3篇
  1979年   2篇
  1976年   2篇
  1974年   1篇
  1973年   4篇
  1972年   1篇
  1971年   2篇
  1970年   1篇
  1969年   1篇
排序方式: 共有883条查询结果,搜索用时 15 毫秒
171.
172.
173.
We analyze the network of relations between parliament members according to their voting behavior. In particular, we examine the emergent community structure with respect to political coalitions and government alliances. We rely on tools developed in the Complex Network literature to explore the core of these communities and use their topological features to develop new metrics for party polarization, internal coalition cohesiveness and government strength. As a case study, we focus on the Chamber of Deputies of the Italian Parliament, for which we are able to characterize the heterogeneity of the ruling coalition as well as parties specific contributions to the stability of the government over time. We find sharp contrast in the political debate which surprisingly does not imply a relevant structure based on established parties. We take a closer look to changes in the community structure after parties split up and their effect on the position of single deputies within communities. Finally, we introduce a way to track the stability of the government coalition over time that is able to discern the contribution of each member along with the impact of its possible defection. While our case study relies on the Italian parliament, whose relevance has come into the international spotlight in the present economic downturn, the methods developed here are entirely general and can therefore be applied to a multitude of other scenarios.  相似文献   
174.
Protein interaction domain families that modulate the formation of macromolecular complexes recognize specific sequence or structural motifs. For instance SH3 and WW domains bind to polyproline peptides while SH2 and FHA domains bind to peptides phosphorylated in Tyr and Thr respectively. Within each family, variations in the chemical characteristics of the domain binding pocket modulate a finer peptide recognition specificity and, as a consequence, determine the selection of functional protein partners in vivo. In the proteomic era there is the need for reliable inference methods to help restricting the sequence space of the putative targets to be confirmed experimentally by more laborious experimental approaches. Here we will review the published data about the peptide recognition specificity of the SH3 domain family and we will propose a classification of SH3 domains into eight classes. Finally, we will discuss whether the available information is sufficient to infer the recognition specificity of any uncharacterized SH3 domain.  相似文献   
175.
In oxidosqualene cyclases (OSCs), an enzyme which has been extensively studied as a target for hypocholesterolemic or antifungal drugs, a lipophilic channel connects the surface of the protein with the active site cavity. Active site and channel are separated by a narrow constriction operating as a mobile gate for the substrate passage. In Saccharomyces cerevisiae OSC, two aminoacidic residues of the channel/constriction apparatus, Ala525 and Glu526, were previously showed as critical for maintaining the enzyme functionality. In this work sixteen novel mutants, each bearing a substitution at or around the channel constrictions, were tested for their enzymatic activity. Modelling studies showed that the most functionality-lowering substitutions deeply alter the H-bond network involving the channel/constriction apparatus. A rotation of Tyr239 is proposed as part of the mechanism permitting the access of the substrate to the active site. The inhibition of OSC by squalene was used as a tool for understanding whether the residues under study are involved in a pre-catalytic selection and docking of the substrate oxidosqualene.  相似文献   
176.
Polynucleotide phosphorylase is a prokaryotic enzyme that catalyzes phosphorolysis of polynucleotides with release of nucleotide diphosphates. By taking advantage of this property, we developed a photometric assay for inorganic phosphate. In the presence of polyadenylic acid, phosphate is converted into adenosine 5'-diphosphate (ADP) by this enzyme. ADP then reacts with phosphoenolpyruvate in a pyruvate kinase-catalyzed reaction, thus giving rise to adenosine 5'-triphosphate and pyruvate. Finally, pyruvate oxidizes reduced nicotinamide adenine dinucleotide (NADH) through the action of L-lactate dehydrogenase, with concomitant decrease in absorbance at 340 nm. As expected, in this detection system 1 mol of NADH was oxidized per mole of phosphate. The assay showed an excellent reproducibility, as the standard deviations never exceeded 5%. It also was shown to be unaffected by several compounds that are regarded as major interferents of the traditional colorimetric assays. Absence of interference was also demonstrated when determining phosphate content in different biological samples, such as human serum and perchloric acid extracts from Escherichia coli, yeast, and bovine liver. An E. coli strain overexpressing His-tagged polynucleotide phosphorylase developed in our laboratories allowed quick and straightforward purification of enzyme, making the assay feasible and convenient. Since all other reagents required are inexpensive, the assay represents a cheaper alternative to commercially available phosphate assay kits.  相似文献   
177.
ARGoS: a modular, parallel, multi-engine simulator for multi-robot systems   总被引:1,自引:0,他引:1  
We present a novel multi-robot simulator named ARGoS. ARGoS is designed to simulate complex experiments involving large swarms of robots of different types. ARGoS is the first multi-robot simulator that is at the same time both efficient (fast performance with many robots) and flexible (highly customizable for specific experiments). Novel design choices in ARGoS have enabled this breakthrough. First, in ARGoS, it is possible to partition the simulated space into multiple sub-spaces, managed by different physics engines running in parallel. Second, ARGoS?? architecture is multi-threaded, thus designed to optimize the usage of modern multi-core CPUs. Finally, the architecture of ARGoS is highly modular, enabling easy addition of custom features and appropriate allocation of computational resources. We assess the efficiency of ARGoS and showcase its flexibility with targeted experiments. Experimental results demonstrate that simulation run-time increases linearly with the number of robots. A 2D-dynamics simulation of 10,000 e-puck robots can be performed in 60?% of the time taken by the corresponding real-world experiment. We show how ARGoS can be extended to suit the needs of an experiment in which custom functionality is necessary to achieve sufficient simulation accuracy. ARGoS is open source software licensed under GPL3 and is downloadable free of charge.  相似文献   
178.
Rhamnus glaucophylla Sommier is an endemic small tree or shrub belonging to sect. Eurhamnus. It grows on cliffs of the Apuan Alps and of the Lucca Apennine in Tuscany (Italy). ISSRs were used to detect the genetic diversity within and among five populations representative of the species distribution range. Moderate levels of genetic diversity were revealed as percentage of polymorphic band (PPB) both at the population (PPB?=?22.63%, H S?=?0.0801, Sh?=?0.1193) and species level (PPB?=?48.68%, H T?=?0.1155, Sh?=?0.1837). The correlation between genetic and geographic distances was not significant (Mantel test, r?=?0.17; P?<?0.26). The information on the pattern of genetic variation obtained in this work bears important implications for conservation management, and in particular for ex situ conservation programmes involving long-term seed storage in seed banks.  相似文献   
179.
Italian vascular flora is highly representative of the Euro-Mediterranean area, because the region includes high mountain territories, temperate areas, and regions dominated by the Mediterranean climate. Chromosome number information about the Italian flora stored in the online database Chrobase.it includes 6,756 records, referable to 3,539 cytotypes and 2,785 accepted species and subspecies (approx. 35% of the national flora). Appropriate queries to Chrobase.it enabled us to map chromosome numbers, at order rank, in a robust phylogenetic framework, derived from APGIII and other recent phylogenetic studies on vascular plants. Similar work was conducted for selected families and genera. Chromosome number data were available for 41 out of 80 vascular plant orders (51%) currently recognized world-wide and 107 out of 428 families (25%), represented by 661 genera (4.5%). The large number of records enabled us to compute the mean chromosome number for each taxon, and to highlight significant differences among all orders and among subsets of families and genera. For each taxon, we analysed the variability in chromosome number by use of common statistical methods, and computed the frequency of chromosome numbers, the coefficient of variation of chromosome number (CVCN), the frequency of B-chromosomes (fB), and that of odd chromosome numbers (fOCN). The phylogenetic relevance of our results is discussed and the usefulness of basic karyological data, often neglected in current phylogenetic studies, is stressed.  相似文献   
180.
The SHP-2 tyrosine phosphatase plays key regulatory roles in the modulation of the cell response to growth factors and cytokines. Over the past decade, the integration of genetic, biochemical, and structural data has helped in interpreting the pathological consequences of altered SHP-2 function. Using complementary approaches, we provide evidence here that endogenous SHP-2 can dimerize through the formation of disulfide bonds that may also involve the catalytic cysteine. We show that the fraction of dimeric SHP-2 is modulated by growth factor stimulation and by the cell redox state. Comparison of the phosphatase activities of the monomeric self-inhibited and dimeric forms indicated that the latter is 3-fold less active, thus pointing to the dimerization process as an additional mechanism for controlling SHP-2 activity. Remarkably, dimers formed by different SHP-2 mutants displaying diverse biochemical properties were found to respond differently to epidermal growth factor (EGF) stimulation. Although this differential behavior cannot be rationalized mechanistically yet, these findings suggest a possible regulatory role of dimerization in SHP-2 function.  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