全文获取类型
收费全文 | 181篇 |
免费 | 3篇 |
专业分类
系统科学 | 4篇 |
教育与普及 | 1篇 |
理论与方法论 | 17篇 |
现状及发展 | 49篇 |
研究方法 | 30篇 |
综合类 | 83篇 |
出版年
2020年 | 1篇 |
2018年 | 4篇 |
2017年 | 5篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 4篇 |
2013年 | 4篇 |
2012年 | 13篇 |
2011年 | 15篇 |
2010年 | 3篇 |
2009年 | 2篇 |
2008年 | 13篇 |
2007年 | 14篇 |
2006年 | 14篇 |
2005年 | 13篇 |
2004年 | 14篇 |
2003年 | 13篇 |
2002年 | 13篇 |
2001年 | 1篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1996年 | 1篇 |
1995年 | 3篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1968年 | 3篇 |
1967年 | 2篇 |
1966年 | 1篇 |
排序方式: 共有184条查询结果,搜索用时 359 毫秒
71.
The oxylipin pathway generates not only prostaglandin-like jasmonates but also green leaf volatiles (GLVs), which confer characteristic aromas to fruits and vegetables. Although allene oxide synthase (AOS) and hydroperoxide lyase are atypical cytochrome P450 family members involved in the synthesis of jasmonates and GLVs, respectively, it is unknown how these enzymes rearrange their hydroperoxide substrates into different products. Here we present the crystal structures of Arabidopsis thaliana AOS, free and in complex with substrate or intermediate analogues. The structures reveal an unusual active site poised to control the reactivity of an epoxyallylic radical and its cation by means of interactions with an aromatic pi-system. Replacing the amino acid involved in these steps by a non-polar residue markedly reduces AOS activity and, unexpectedly, is both necessary and sufficient for converting AOS into a GLV biosynthetic enzyme. Furthermore, by combining our structural data with bioinformatic and biochemical analyses, we have discovered previously unknown hydroperoxide lyase in plant growth-promoting rhizobacteria, AOS in coral, and epoxyalcohol synthase in amphioxus. These results indicate that oxylipin biosynthetic genes were present in the last common ancestor of plants and animals, but were subsequently lost in all metazoan lineages except Placozoa, Cnidaria and Cephalochordata. 相似文献
72.
Depaepe V Suarez-Gonzalez N Dufour A Passante L Gorski JA Jones KR Ledent C Vanderhaeghen P 《Nature》2005,435(7046):1244-1250
Mechanisms controlling brain size include the regulation of neural progenitor cell proliferation, differentiation, survival and migration. Here we show that ephrin-A/EphA receptor signalling plays a key role in controlling the size of the mouse cerebral cortex by regulating cortical progenitor cell apoptosis. In vivo gain of EphA receptor function, achieved through ectopic expression of ephrin-A5 in early cortical progenitors expressing EphA7, caused a transient wave of neural progenitor cell apoptosis, resulting in premature depletion of progenitors and a subsequent dramatic decrease in cortical size. In vitro treatment with soluble ephrin-A ligands similarly induced the rapid death of cultured dissociated cortical progenitors in a caspase-3-dependent manner, thereby confirming a direct effect of ephrin/Eph signalling on apoptotic cascades. Conversely, in vivo loss of EphA function, achieved through EphA7 gene disruption, caused a reduction in apoptosis occurring normally in forebrain neural progenitors, resulting in an increase in cortical size and, in extreme cases, exencephalic forebrain overgrowth. Together, these results identify ephrin/Eph signalling as a physiological trigger for apoptosis that can alter brain size and shape by regulating the number of neural progenitors. 相似文献
73.
Leroy EM Kumulungui B Pourrut X Rouquet P Hassanin A Yaba P Délicat A Paweska JT Gonzalez JP Swanepoel R 《Nature》2005,438(7068):575-576
The first recorded human outbreak of Ebola virus was in 1976, but the wild reservoir of this virus is still unknown. Here we test for Ebola in more than a thousand small vertebrates that were collected during Ebola outbreaks in humans and great apes between 2001 and 2003 in Gabon and the Republic of the Congo. We find evidence of asymptomatic infection by Ebola virus in three species of fruit bat, indicating that these animals may be acting as a reservoir for this deadly virus. 相似文献
74.
75.
Julie Lecomte Krystel Louis Benoit Detry Silvia Blacher Vincent Lambert Sandrine Bekaert Carine Munaut Jenny Paupert Pierre Blaise Jean-Michel Foidart Jean-Marie Rakic Stephen M. Krane Agn��s Noel 《Cellular and molecular life sciences : CMLS》2011,68(4):677-686
In this study, we evaluate the potential involvement of collagenase-3 (MMP13), a matrix metalloproteinase (MMP) family member, in the exudative form of age-related macular degeneration characterized by a neovascularisation into the choroid. RT-PCR analysis revealed that human neovascular membranes issued from patients with AMD expressed high levels of Mmp13. The contribution of MMP13 in choroidal neovascularization (CNV) formation was explored by using a murine model of laser-induced CNV and applying it to wild-type mice (WT) and Mmp13-deficient mice (Mmp13 ?/? mice). Angiogenic and inflammatory reactions were explored by immunohistochemistry. The implication of bone marrow (BM)-derived cells was determined by BM engraftment into irradiated mice and by injecting mesenchymal stem cells (MSC) isolated from WT BM. The deficiency of Mmp13 impaired CNV formation which was fully restored by WT BM engraftment and partially rescued by several injections of WT MSC. The present study sheds light on a novel function of MMP13 during BM-dependent choroidal vascularization and provides evidence for a role for MSC in the pathogenesis of CNV. 相似文献
76.
77.
Optimal Variable Weighting for Ultrametric and Additive Trees and K-means Partitioning: Methods and Software 总被引:1,自引:0,他引:1
K -means partitioning. We also describe some new features and improvements to the algorithm proposed by De Soete. Monte Carlo
simulations have been conducted using different error conditions. In all cases (i.e., ultrametric or additive trees, or K-means partitioning), the simulation results indicate that the optimal weighting procedure should be used for analyzing data
containing noisy variables that do not contribute relevant information to the classification structure. However, if the data
involve error-perturbed variables that are relevant to the classification or outliers, it seems better to cluster or partition
the entities by using variables with equal weights. A new computer program, OVW, which is available to researchers as freeware,
implements improved algorithms for optimal variable weighting for ultrametric and additive tree clustering, and includes a
new algorithm for optimal variable weighting for K-means partitioning. 相似文献
78.
79.
A new method, TreeOfTrees, is proposed to compare X-tree structures obtained from several sets of aligned gene sequences of
the same taxa. Its aim is to detect genes or sets of genes having different evolutionary histories. The comparison between
sets of trees is based on several tree metrics, leading to a unique tree labelled by the gene trees. The robustness values
of its edges are estimated by bootstrapping and consensus procedures that allow detecting subsets of genes having differently
evolved. Simulations are performed under various evolutionary conditions to test the efficiency of the method and an application
on real data is described. Tests of arboricity and various consensus algorithms are also discussed. A corresponding software
package is available. 相似文献
80.
The anomeric effect is a chemical phenomenon that refers to an observed stabilization of six-membered carbohydrate rings when they contain an electronegative substituent at the C1 position of the ring. This stereoelectronic effect influences the three-dimensional shapes of many biological molecules. It can be manifested not only in this classical manner involving interaction of the endocyclic oxygen atom (O5) found in such sugars with the C1 substituent (endo-anomeric effect) but also through a corresponding interaction of the electronegative exocyclic substituent with O5 (exo-anomeric effect). However, the underlying physical origin(s) of this phenomenon is still not clear. Here we show, using a combination of laser spectroscopy and computational analysis, that a truncated peptide motif can engage the two anomers of an isolated sugar in the gas phase, an environment lacking extraneous factors which could confound the analysis. (Anomers are isomers that differ in the orientation of the substituent at C1.) Complexes formed between the peptide and the α- or β-anomers of d-galactose are nearly identical structurally; however, the strength of the polarization of their interactions with the peptide differs greatly. Natural bond order calculations support this observation, and together they reveal the dominance of the exo- over the endo-anomeric effect. As interactions between oxygen atoms at positions C1 and C2 (O1 and O2, respectively) on the pyranose ring can alter the exo/endo ratio of a carbohydrate, our results suggest that it will be important to re-evaluate the influence, and biological effects, of substituents at position C2 in sugars. 相似文献