全文获取类型
收费全文 | 12205篇 |
免费 | 122篇 |
国内免费 | 200篇 |
专业分类
系统科学 | 214篇 |
丛书文集 | 500篇 |
教育与普及 | 468篇 |
理论与方法论 | 39篇 |
现状及发展 | 1139篇 |
研究方法 | 1540篇 |
综合类 | 8608篇 |
自然研究 | 19篇 |
出版年
2018年 | 25篇 |
2017年 | 43篇 |
2016年 | 31篇 |
2015年 | 33篇 |
2014年 | 73篇 |
2013年 | 54篇 |
2012年 | 767篇 |
2011年 | 972篇 |
2010年 | 266篇 |
2009年 | 136篇 |
2008年 | 912篇 |
2007年 | 950篇 |
2006年 | 1002篇 |
2005年 | 1043篇 |
2004年 | 791篇 |
2003年 | 779篇 |
2002年 | 695篇 |
2001年 | 559篇 |
2000年 | 843篇 |
1999年 | 274篇 |
1998年 | 68篇 |
1997年 | 47篇 |
1996年 | 33篇 |
1995年 | 26篇 |
1994年 | 37篇 |
1993年 | 49篇 |
1992年 | 38篇 |
1991年 | 50篇 |
1990年 | 71篇 |
1989年 | 58篇 |
1988年 | 38篇 |
1987年 | 53篇 |
1986年 | 59篇 |
1985年 | 52篇 |
1984年 | 47篇 |
1983年 | 45篇 |
1982年 | 51篇 |
1981年 | 40篇 |
1980年 | 23篇 |
1979年 | 30篇 |
1971年 | 27篇 |
1970年 | 51篇 |
1966年 | 32篇 |
1959年 | 149篇 |
1958年 | 273篇 |
1957年 | 174篇 |
1956年 | 154篇 |
1955年 | 161篇 |
1954年 | 163篇 |
1948年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
Transformation: a tool for studying fungal pathogens of plants 总被引:18,自引:0,他引:18
Plant diseases caused by plant pathogenic fungi continuously threaten the sustainability of global crop production. An effective
way to study the disease-causing mechanisms of these organisms is to disrupt their genes, in both a targeted and random manner,
so as to isolate mutants exhibiting altered virulence. Although a number of techniques have been employed for such an analysis,
those based on transformation are by far the most commonly used. In filamentous fungi, the introduction of DNA by transformation
typically results in either the heterologous (illegitimate) integration or the homologous integration of the transforming
DNA into the target genome. Homologous integration permits a targeted gene disruption by replacing the wild-type allele on
the genome with a mutant allele on transforming DNA. This process has been widely used to determine the role of newly isolated
fungal genes in pathogenicity. The heterologous integration of transforming DNA causes a random process of gene disruption
(insertional mutagenesis) and has led to the isolation of many fungal mutants defective in pathogenicity. A big advantage
of insertional mutagenesis over the more traditional chemical or radiation mutagenesis procedures is that the mutated gene
is tagged by transforming DNA and can subsequently be cloned using the transforming DNA. The application of various transformation-based
techniques for fungal gene manipulation and how they have increased our understanding and appreciation of some of the most
serious plant pathogenic fungi are discussed.
Received 9 May 2001; received after revision 2 July 2001; accepted 3 July 2001 相似文献
995.
Horvat A Nikezić G Petrović S Kanazir DT 《Cellular and molecular life sciences : CMLS》2001,58(4):636-644
The subsynaptosomal distribution and specific binding of 17beta-estradiol in vitro to mitochondria isolated from presynaptic nerve endings of female rat brain were examined. 17Beta-estradiol is (i) distributed unequally in synaptosomes and mitochondria posses the highest capacity to bind estradiol with respect to the available amount of the hormone. (ii) Estradiol binds specifically to isolated synaptosomal mitochondria. A Michaelis-Menten plot of specific binding was sigmoidal within a concentration range of 0.1-5 nM of added estradiol, with a saturation plateau at 3 nM. Binding of higher estradiol concentrations demonstrated an exponential Michaelis-Menten plot, indicating non-specific binding to mitochondria. Vmax and Km for the sigmoidal-shape range were estimated as 46 +/- 6 fmol of estradiol/mg of mitochondrial proteins and 0.46 +/- 0.07 nM free estradiol respectively. (iii) Estradiol binding is not affected by the removal of ovaries. The results show that inhibition of Na-dependent Ca2+ efflux from mitochondria by estradiol occurs according to an affinity change of the translocator for Na+, at the same estradiol concentrations that show specific binding to mitochondrial membranes. These data imply that physiological concentrations of estradiol, acting on mitochondrial membrane properties, extragenomically modulate the mitochondrial, and consequently the synaptosomal content of Ca2+, and in that way exert a significant change in nerve cell homeostasis. 相似文献
996.
Analysis of nuclear apoptotic process in a cell-free system 总被引:2,自引:0,他引:2
We report an analysis of the apoptotic process of mouse liver nuclei induced in a cell-free carrot cytosol system by cytochrome
c. Typical characteristics of apoptosis were observed, such as chromatin condensation, margination, apoptotic bodies and DNA
ladders. Furthermore, transmission and scanning electron microscope analysis of the apoptotic nuclei detected chromatin-free
nuclear vesicles before apoptotic bodies appeared at a comparatively late phase. When AC-YVAD-CHO, an inhibitor of caspase
6, was introduced into the system, these vesicles and apoptotic bodies disappeared completely within our study sections. We
confirmed the results using whole-mount electron microscopy, and found that although the nuclear lamina was destroyed early,
the nuclear matrix largely remained intact during the course of apoptosis. The nuclear matrix played an important role in
maintaining the integrity of apoptotic cells and connecting the apoptotic bodies and apoptotic nucleus.
Received 29 September 2000; revised 10 December 2000; accepted 13 December 2000 相似文献
997.
Sinnarajah S Dessauer CW Srikumar D Chen J Yuen J Yilma S Dennis JC Morrison EE Vodyanoy V Kehrl JH 《Nature》2001,409(6823):1051-1055
The heterotrimeric G-protein Gs couples cell-surface receptors to the activation of adenylyl cyclases and cyclic AMP production (reviewed in refs 1, 2). RGS proteins, which act as GTPase-activating proteins (GAPs) for the G-protein alpha-subunits alpha(i) and alpha(q), lack such activity for alpha(s) (refs 3-6). But several RGS proteins inhibit cAMP production by Gs-linked receptors. Here we report that RGS2 reduces cAMP production by odorant-stimulated olfactory epithelium membranes, in which the alpha(s) family member alpha(olf) links odorant receptors to adenylyl cyclase activation. Unexpectedly, RGS2 reduces odorant-elicited cAMP production, not by acting on alpha(olf) but by inhibiting the activity of adenylyl cyclase type III, the predominant adenylyl cyclase isoform in olfactory neurons. Furthermore, whole-cell voltage clamp recordings of odorant-stimulated olfactory neurons indicate that endogenous RGS2 negatively regulates odorant-evoked intracellular signalling. These results reveal a mechanism for controlling the activities of adenylyl cyclases, which probably contributes to the ability of olfactory neurons to discriminate odours. 相似文献
998.
Recognition of haemagglutinins on virus-infected cells by NKp46 activates lysis by human NK cells 总被引:41,自引:0,他引:41
Mandelboim O Lieberman N Lev M Paul L Arnon TI Bushkin Y Davis DM Strominger JL Yewdell JW Porgador A 《Nature》2001,409(6823):1055-1060
Natural killer (NK) cells destroy virus-infected and tumour cells, apparently without the need for previous antigen stimulation. In part, target cells are recognized by their diminished expression of major histocompatibility complex (MHC) class I molecules, which normally interact with inhibitory receptors on the NK cell surface. NK cells also express triggering receptors that are specific for non-MHC ligands; but the nature of the ligands recognized on target cells is undefined. NKp46 is thought to be the main activating receptor for human NK cells. Here we show that a soluble NKp46-immunoglobulin fusion protein binds to both the haemagglutinin of influenza virus and the haemagglutinin-neuraminidase of parainfluenza virus. In a substantial subset of NK cells, recognition by NKp46 is required to lyse cells expressing the corresponding viral glycoproteins. The binding requires the sialylation of NKp46 oligosaccharides, which is consistent with the known sialic binding capacity of the viral glycoproteins. These findings indicate how NKp46-expressing NK cells may recognize target cells infected by influenza or parainfluenza without the decreased expression of target-cell MHC class I protein. 相似文献
999.
Opposing effects of Ets and Id proteins on p16INK4a expression during cellular senescence 总被引:46,自引:0,他引:46
Ohtani N Zebedee Z Huot TJ Stinson JA Sugimoto M Ohashi Y Sharrocks AD Peters G Hara E 《Nature》2001,409(6823):1067-1070
1000.