全文获取类型
收费全文 | 40601篇 |
免费 | 122篇 |
国内免费 | 216篇 |
专业分类
系统科学 | 199篇 |
丛书文集 | 600篇 |
教育与普及 | 82篇 |
理论与方法论 | 135篇 |
现状及发展 | 18777篇 |
研究方法 | 1605篇 |
综合类 | 18997篇 |
自然研究 | 544篇 |
出版年
2013年 | 361篇 |
2012年 | 560篇 |
2011年 | 1097篇 |
2010年 | 240篇 |
2008年 | 729篇 |
2007年 | 804篇 |
2006年 | 812篇 |
2005年 | 760篇 |
2004年 | 743篇 |
2003年 | 726篇 |
2002年 | 691篇 |
2001年 | 1277篇 |
2000年 | 1198篇 |
1999年 | 798篇 |
1994年 | 387篇 |
1992年 | 781篇 |
1991年 | 572篇 |
1990年 | 694篇 |
1989年 | 651篇 |
1988年 | 618篇 |
1987年 | 715篇 |
1986年 | 649篇 |
1985年 | 824篇 |
1984年 | 649篇 |
1983年 | 542篇 |
1982年 | 510篇 |
1981年 | 519篇 |
1980年 | 601篇 |
1979年 | 1308篇 |
1978年 | 1057篇 |
1977年 | 1030篇 |
1976年 | 851篇 |
1975年 | 887篇 |
1974年 | 1171篇 |
1973年 | 1044篇 |
1972年 | 1053篇 |
1971年 | 1146篇 |
1970年 | 1502篇 |
1969年 | 1139篇 |
1968年 | 1143篇 |
1967年 | 1105篇 |
1966年 | 972篇 |
1965年 | 692篇 |
1959年 | 375篇 |
1958年 | 645篇 |
1957年 | 417篇 |
1956年 | 385篇 |
1955年 | 365篇 |
1954年 | 357篇 |
1948年 | 257篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Expression and characterization of the cystic fibrosis transmembrane conductance regulator. 总被引:36,自引:0,他引:36
R J Gregory S H Cheng D P Rich J Marshall S Paul K Hehir L Ostedgaard K W Klinger M J Welsh A E Smith 《Nature》1990,347(6291):382-386
Cystic fibrosis (CF) is a common lethal genetic disease that manifests itself in airway and other epithelial cells as defective chloride ion absorption and secretion, resulting at least in part from a defect in a cyclic AMP-regulated, outwardly-rectifying Cl- channel in the apical surface. The gene responsible for CF has been identified and predicted to encode a membrane protein termed the CF transmembrane conductance regulator (CFTR). Identification of a cryptic bacterial promoter within the CFTR coding sequence led us to construct a complementary DNA in a low-copy-number plasmid, thereby avoiding the deleterious effects of CFTR expression on Escherischia coli. We have used this cDNA to express CFTR in vitro and in vivo. Here we demonstrate that CFTR is a membrane-associated glycoprotein that can be phosporylated in vitro by cAMP-dependent protein kinase. Polyclonal and monoclonal antibodies directed against distinct domains of the protein immunoprecipitated recombinant CFTR as well as the endogenous CFTR in nonrecombinant T84 cells. Partial proteolysis fingerprinting showed that the recombinant and non-recombinant proteins are indistinguishable. These data, which establish several characteristics of the protein responsible for CF, will now enable CFTR function to be studied and will provide a basis for diagnosis and therapy. 相似文献
992.
993.
Different voltage-dependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex 总被引:25,自引:0,他引:25
In the hippocampus and neocortex, high-frequency (tetanic) stimulation of an afferent pathway leads to long-term potentiation (LTP) of synaptic transmission. In the hippocampus it has recently been shown that long-term depression (LTD) of excitatory transmission can also be induced by certain combinations of synaptic activation. In most hippocampal and all neocortical pathways studied so far, the induction of LTP requires the activation of N-methyl-D-aspartate (NMDA) receptor-gated conductances. Here we report that LTD can occur in neurons of slices of the rat visual cortex and that the same tetanic stimulation can induce either LTP or LTD depending on the level of depolarization of the postsynaptic neuron. By applying intracellular current injections or pharmacological disinhibition to modify the depolarizing response of the postsynaptic neuron to tetanic stimulation, we show that the mechanisms of induction of LTD and LTP are both postsynaptic. LTD is obtained if postsynaptic depolarization exceeds a critical level but remains below a threshold related to NMDA receptor-gated conductances, whereas LTP is induced if this second threshold is reached. 相似文献
994.
995.
(Mg-ATP)-dependent self-assembly of molecular chaperone GroEL 总被引:7,自引:0,他引:7
The important Escherichia coli heat-shock protein GroEL of relative molecular mass 57,259 is a typical molecular chaperone. It possesses ATPase activity and interacts in ATP-driven reactions with non-folded proteins to stimulate their correct folding and/or assembly by preventing the formation of improper protein structures or aggregates. As GroEL is isolated and functions as a 20-25S tetradecameric particle (GroELp), the question arises--what is the mechanism of its own assembly? Here we show the (Mg-ATP)-dependent self-stimulation ('self-chaperoning') in vitro of GroELp reassembly from its monomeric state. 相似文献
996.
Localization of VP4 neutralization sites in rotavirus by three-dimensional cryo-electron microscopy 总被引:35,自引:0,他引:35
Three-dimensional structures of several spherical viruses have been determined by electron microscopy and X-ray crystallography. We report here the first three-dimensional structure of the complex between an intact virus and Fab fragments of a neutralizing monoclonal antibody. The antibody is against VP4, one of the two outer capsid proteins of rotaviruses. These large icosahedral viruses cause gastroenteritis in children and young animals and account for over a million human deaths annually. VP4 in these viruses has been implicated in several important functions such as cell penetration, haemagglutination, neutralization and virulence. Here we demonstrate that the surface spikes on rotavirus particles are made up of VP4. Antigenic sites are located near the distal ends of the spikes and two Fab fragments bind to each of the sixty spikes. The mass of the spike indicates that it is a dimer of VP4. The bilobed structure at the distal end of the spike may be involved in both the attachment to the cell and in viral penetration. A novel feature in the virus-Fab complex is the structural difference between the two chemically equivalent Fab fragments on each spike, which could be indicative of variations in the Fab elbow angles. 相似文献
997.
Expression of a candidate sex-determining gene during mouse testis differentiation 总被引:39,自引:0,他引:39
The development of a eutherian mammal as a male is a consequence of testis formation in the embryo, which is thought to be initiated by a gene on the Y chromosome. In the absence of this gene, ovaries are formed and female characteristics develop. Sex determination therefore hinges on the action of this testis-determining gene, known as Tdy in mice and TDF in humans. In the past, several genes proposed as candidates for Tdy/TDF have subsequently been dismissed on the grounds of inappropriate location or expression. We have recently described a candidate for Tdy, which maps to the minimum sex-determining region of the mouse Y chromosome. To examine further the involvement of this gene, Sry, in testis development, we have studied its expression in detail. Fetal expression of Sry is limited to the period in which testes begin to form. This expression is confined to gonadal tissue and does not require the presence of germ cells. Our observations strongly support a primary role for Sry in mouse sex determination. 相似文献
998.
Host-parasitoid associations in patchy environments 总被引:2,自引:0,他引:2
Studies of insect host-parasitoid interactions have contributed much to the consensus that spatial patchiness is important in the regulation of natural populations. A variety of theoretical models predict that host and parasitoid populations, although unstable in the absence of environmental heterogeneity, may persist at roughly steady overall densities in a patchy environment owing to variation in levels of parasitism from patch to patch. Observed patterns of parasitism, however, have a variety of forms (with variation in attack rates among patches depending directly or indirectly on host density, or showing variation uncorrelated with host density). There is some confusion about the dynamical consequences of these different forms. Here we first show how the dynamical effects of all these forms of environmental heterogeneity can be assessed by a common criterion. This 'CV2 greater than 1 rule' states that the overall population densities will remain roughly steady from generation to generation if the coefficient of variation squared (CV2) of the density of searching parasitoids in the vicinity of each host exceeds approximately unity. By partitioning CV2 into components, we show that both direct and inverse patterns of dependence on host density, and density-independent patterns, all contribute to population regulation in the same way. Second, we show how a maximum-likelihood method can be applied to the kind of field data that are usually available (that is, percentage parasitism versus local host density) to estimate the components of CV2. This analysis indicates that heterogeneity is large enough to stabilize dynamics in 9 of 34 published studies, and that density-independent heterogeneity is the main factor in most cases. 相似文献
999.
Molecular cloning of the microtubule-associated mechanochemical enzyme dynamin reveals homology with a new family of GTP-binding proteins 总被引:59,自引:0,他引:59
A complementary DNA encoding the D100 polypeptide of rat brain dynamin--a force-producing, microtubule-activated nucleotide triphosphatase--has been cloned and sequenced. The predicted amino acid sequence includes a guanine nucleotide-binding domain that is homologous with those of a family of antiviral factors, inducible by interferon and known as Mx proteins, and with the product of the essential yeast vacuolar protein sorting gene VPS1. These relationships imply the existence of a new family of GTPases with physiological roles that may include microtubule-based motility and protein sorting. 相似文献
1000.
本文将文献[3]的引理1推广到s-单式环上,并用迭代技术给出文献[4]的定理2一个简易的证明,将若干有1环的交换性定理推广到s-单式环上。 相似文献