全文获取类型
收费全文 | 1046篇 |
免费 | 77篇 |
出版年
2021年 | 7篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 7篇 |
2017年 | 8篇 |
2016年 | 21篇 |
2015年 | 40篇 |
2014年 | 32篇 |
2013年 | 29篇 |
2012年 | 56篇 |
2011年 | 61篇 |
2010年 | 34篇 |
2009年 | 32篇 |
2008年 | 58篇 |
2007年 | 55篇 |
2006年 | 50篇 |
2005年 | 54篇 |
2004年 | 64篇 |
2003年 | 63篇 |
2002年 | 55篇 |
2001年 | 23篇 |
2000年 | 21篇 |
1999年 | 19篇 |
1998年 | 18篇 |
1997年 | 20篇 |
1996年 | 12篇 |
1995年 | 23篇 |
1994年 | 10篇 |
1993年 | 13篇 |
1992年 | 21篇 |
1991年 | 11篇 |
1990年 | 22篇 |
1989年 | 16篇 |
1988年 | 11篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1983年 | 14篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 7篇 |
1974年 | 7篇 |
1972年 | 5篇 |
1970年 | 3篇 |
排序方式: 共有1123条查询结果,搜索用时 296 毫秒
151.
152.
The [4Fe-4S] cluster of quinolinate synthase from Escherichia coli: investigation of cluster ligands
Nicotinamide adenine dinucleotide (NAD) derives from quinolinic acid which is synthesized in Escherichia coli from l-aspartate and dihydroxyacetone phosphate through the concerted action of l-aspartate oxidase and the [4Fe-4S] quinolinate synthase (NadA). Here, we addressed the question of the identity of the cluster ligands. We performed in vivo complementation experiments as well as enzymatic, spectroscopic and structural in vitro studies using wild-type vs. Cys-to-Ala mutated NadA proteins. These studies reveal that only three cysteine residues, the conserved Cys113, Cys200 and Cys297, are ligands of the cluster. This result is in contrast to the previous proposal that pointed the three cysteines of the C(291)XXC(294)XXC(297) motif. Interestingly, we demonstrated that Cys291 and Cys294 form a disulfide bridge and are important for activity. 相似文献
153.
Foley S Stolarczyk E Mouni F Brassart C Vidal O Aïssi E Bouquelet S Krzewinski F 《Archives of microbiology》2008,189(2):157-167
Bifidobacterium bifidum, in contrast to other bifidobacterial species, is auxotrophic for N-acetylglucosamine. Growth experiments revealed assimilation of radiolabelled N-acetylglucosamine in bacterial cell walls and in acetate, an end-product of central metabolism via the bifidobacterial d-fructose-6-phosphate shunt. While supplementation with fructose led to reduced N-acetylglucosamine assimilation via the d-fructose-6-phosphate shunt, no significant difference was observed in levels of radiolabelled N-acetylglucosamine incorporated into cell walls. Considering the central role played by glutamine fructose-6-phosphate transaminase
(GlmS) in linking the biosynthetic pathway for N-acetylglucosamine to hexose metabolism, the GlmS of Bifidobacterium was characterized. The genes encoding the putative GlmS of B. longum DSM20219 and B. bifidum DSM20082 were cloned and sequenced. Bioinformatic analyses of the predicted proteins revealed 43% amino acid identity with
the Escherichia coli GlmS, with conservation of key amino acids in the catalytic domain. The B. longum GlmS was over-produced as a histidine-tagged fusion protein. The purified C-terminal His-tagged GlmS possessed glutamine
fructose-6-phosphate amidotransferase activity as demonstrated by synthesis of glucosamine-6-phosphate from fructose-6-phosphate
and glutamine. It also possesses an independent glutaminase activity, converting glutamine to glutamate in the absence of
fructose-6-phosphate. This is of interest considering the apparently reduced coding potential in bifidobacteria for enzymes
associated with glutamine metabolism.
S. Foley and E. Stolarczyk contributed equally to this work 相似文献
154.
Our knowledge of heredity has recently undergone major upheaval. Heredity transmits considerably more than just genetic elements. First, the oocyte is full of maternal cytoplasmic components that subsequently are present in each new cell. Second, maternal cells can pass to the progeny, where they remain active into adult life (microchimerism). Here, we examine the notion that the transmission of characters involves at least two processes in addition to that of mendelian heredity, long considered to be the only hereditary mechanism. These processes all involve epigenetic processes, including the transmission of macromolecules, subcellular organelles, and living cells solely from the mother to her offspring, whether female or male, during pregnancy and lactation. We postulate that cytoplasmic heredity and maternal transmission of cells leading to a long-term state of microchimerism in progeny are two good examples of matrilineal, nonmendelian heredity. A mother's important contribution to the development and health of her progeny seems to possess many uncharted depths. 相似文献
155.
Laetitia Plaisance Vincent Rousset Serge Morand D. Timothy J. Littlewood 《Journal of Biogeography》2008,35(1):76-87
Aim To investigate the phylogeographical patterns of two poorly dispersing but widely distributed monogenean species, Haliotrema aurigae and Euryhaliotrematoides grandis, gill parasites of coral reef fishes from the family Chaetodontidae. Location South Pacific Ocean (SPO). Methods Sequence data from the mitochondrial cytochrome oxidase subunit I (COI) gene were obtained from samples from five localities of the SPO (Heron Island, Lizard Island, Moorea, Palau and Wallis) for the two parasite species. Phylogenetic and genetic diversity analyses were used to reconstruct phylogeographical patterns, and dates of cladogenetic events were estimated. Results Overall, 50 individuals of 17 Haliotrema aurigae and 33 of Euryhaliotrematoides grandis were sequenced from five localities of the SPO for COI mtDNA (798 bp). Our results revealed a deep phylogeographical structure in the species Euryhaliotrematoides grandis. The molecular divergence between individuals from Moorea and individuals from the remaining localities (7.7%) may be related to Pleistocene sea‐level fluctuations. In contrast, Haliotrema aurigae shows no phylogeographical patterns with the presence of most of the mitochondrial haplotypes in every locality sampled. Main conclusions Our study shows contrasting phylogeographical patterns of the two monogenean parasite species studied, despite many shared characteristics. Both parasites are found on the same host family, share the same geographical range and ecology, and are phylogenetically close. We propose two hypotheses that may help explain the diparity: the hypotheses involve differences in the evolutionary age of the parasite species and their dispersal capabilities. Additionally, the lack of phylogeographical structure in Haliotrema aurigae contrasts with its apparently restricted dispersion, which is likely to occur during the egg stage of the life cycle, inducing a passive dispersal mechanism in butterflyfish monogeneans. 相似文献
156.
Bulckaen H Prévost G Boulanger E Robitaille G Roquet V Gaxatte C Garçon G Corman B Gosset P Shirali P Creusy C Puisieux F 《American journal of physiology. Heart and circulatory physiology》2008,294(4):H1562-H1570
The age-related impairment of endothelium-dependent vasodilatation contributes to increased cardiovascular risk in the elderly. For primary and secondary prevention, aspirin can reduce the incidence of cardiovascular events in this patient population. The present work evaluated the effect of low-dose aspirin on age-related endothelial dysfunction in C57B/J6 aging mice and investigated its protective antioxidative effect. Age-related endothelial dysfunction was assessed by the response to acetylcholine of phenylephrine-induced precontracted aortic segments isolated from 12-, 36-, 60-, and 84-wk-old mice. The effect of low-dose aspirin was examined in mice presenting a decrease in endothelial-dependent relaxation (EDR). The effects of age and aspirin treatment on structural changes were determined in mouse aortic sections. The effect of aspirin on the oxidative stress markers malondialdehyde and 8-hydroxy-2'-deoxyguanosine (8-OhdG) was also quantified. Compared with that of 12-wk-old mice, the EDR was significantly reduced in 60- and 84-wk-old mice (P < 0.05); 68-wk-old mice treated with aspirin displayed a higher EDR compared with control mice of the same age (83.9 +/- 4 vs. 66.3 +/- 5%; P < 0.05). Aspirin treatment decreased 8-OHdG levels (P < 0.05), but no significant effect on intima/media thickness ratio was observed. The protective effect of aspirin was not observed when treatment was initiated in older mice (96 wk of age). It was found that low-dose aspirin is able to prevent age-related endothelial dysfunction in aging mice. However, the absence of this effect in the older age groups demonstrates that treatment should be initiated early on. The underlying mechanism may involve the protective effect of aspirin against oxidative stress. 相似文献
157.
PARTIAL MANTEL TESTS: REPLY TO CASTELLANO AND BALLETTO 总被引:5,自引:0,他引:5
158.
Although the involvement of the nonvisual arrestins in the agonist-induced internalization of the human lutropin receptor (hLHR) has been documented previously with the use of dominant-negative mutants, a physical association of the nonvisual arrestins with the hLHR in intact cells has not been established. In the studies presented herein, we used a cross-linking/coimmunoprecipitation/immunoblotting approach as well as confocal microscopy to document the association of the hLHR with the nonvisual arrestins in co-transfected 293 cells. We also used this approach to examine the relative importance of receptor activation and receptor phosphorylation in the formation of this complex. Using hLHR mutants that impair phosphorylation, activation, or both, we show that the formation of the hLHR-nonvisual arrestin complex depends mostly on the agonist-induced activation of the hLHR rather than on the phosphorylation of the hLHR. These results stand in contrast to those obtained with several other G protein-coupled receptors (i.e. the beta2-adrenergic receptor, the m2 muscarinic receptor, rhodopsin, and the type 1A angiotensin receptor) where arrestin binding depends mostly on receptor phosphorylation rather than on receptor activation. We have also examined the association of the nonvisual arrestins with naturally occurring gain-of-function mutations of the hLHR found in boys with Leydig cell hyperplasia or Leydig cell adenomas. Our results show that these mutants associate with the nonvisual arrestins in an agonist-independent fashion. 相似文献
159.
Dubois Michel Gerard Jean-François Sampaio Elineuza de Faria Galvão Olavo Guilhem Colette 《International journal of primatology》2001,22(6):993-1006
Activities ordinarily performed by an animal in a given place may influence the way it behaves towards an object newly found there. We examined the use of probing tools in a group of 4 wedge-capped capuchins (Cebus olivaceus), in the home cage into which we simultaneously introduced two identical apparatuses, one at a site ordinarily used to manipulate objects, and the other at a site mainly used for visual monitoring. We expected that spatial facilitation would occur, i.e., the subjects would be more efficient at the site where manipulative behaviors were more frequently performed. Two monkeys used probing tools to extract syrup from the openings of the apparatuses. Tool use efficiency was higher with the apparatus located at the site where they had performed more manipulative and combinative behaviors. These results are consistent with the hypothesis of spatial facilitation and illustrate the fact that spatial context can be an overwhelming variable that should not be neglected in behavioral research dealing with instrumental tasks. 相似文献
160.
T-DNA tagging in the model legume Medicago truncatula allows efficient gene discovery 总被引:3,自引:0,他引:3
Scholte Marije d'Erfurth Isabelle Rippa Sonia Mondy Samuel Cosson Viviane Durand Patricia Breda Colette Trinh Hanh Rodriguez-Llorente Ignacio Kondorosi Eva Schultze Michael Kondorosi Adam Ratet Pascal 《Molecular breeding : new strategies in plant improvement》2002,10(4):203-215
The annual legume Medicago truncatula has been proposed as a model plant to study various aspects of legume biology including rhizobial and mycorrhizal symbiosis because it is well suited for the genetic analysis of these processes . To facilitate the characterization of M. truncatula genes participating in various developmental processes we have initiated an insertion mutagenesis program in this plant using three different T-DNAs as tags. To investigate which type of vector is the most suitable for mutagenesis we compared the behavior of these T-DNAs. One T-DNA vector was a derivative of pBin19 and plant selection was based on kanamycin resistance. The two other vectors carried T-DNA conferring Basta resistance in the transgenic plants. For each T-DNA type, we determined the copy number in the transgenic lines, the structure of the T-DNA loci and the sequences of the integration sites. The T-DNA derived from pBin19 generated complex T-DNA insertion patterns. The two others generally gave single copy T-DNA inserts that could result in gene fusions for the pGKB5 T-DNA. Analysis of the T-DNA borders revealed that several M. truncatula genes were tagged in these transgenic lines and in vivo gus fusions were also obtained. These results demonstrate that T-DNA tagging can efficiently be used in M. truncatula for gene discovery. 相似文献