全文获取类型
收费全文 | 29581篇 |
免费 | 81篇 |
国内免费 | 127篇 |
专业分类
系统科学 | 134篇 |
丛书文集 | 293篇 |
教育与普及 | 74篇 |
理论与方法论 | 109篇 |
现状及发展 | 12634篇 |
研究方法 | 1317篇 |
综合类 | 14773篇 |
自然研究 | 455篇 |
出版年
2013年 | 230篇 |
2012年 | 458篇 |
2011年 | 873篇 |
2010年 | 194篇 |
2008年 | 585篇 |
2007年 | 611篇 |
2006年 | 627篇 |
2005年 | 610篇 |
2004年 | 588篇 |
2003年 | 563篇 |
2002年 | 516篇 |
2001年 | 969篇 |
2000年 | 911篇 |
1999年 | 612篇 |
1994年 | 358篇 |
1992年 | 557篇 |
1991年 | 397篇 |
1990年 | 500篇 |
1989年 | 440篇 |
1988年 | 425篇 |
1987年 | 468篇 |
1986年 | 460篇 |
1985年 | 621篇 |
1984年 | 464篇 |
1983年 | 382篇 |
1982年 | 376篇 |
1981年 | 341篇 |
1980年 | 358篇 |
1979年 | 931篇 |
1978年 | 689篇 |
1977年 | 642篇 |
1976年 | 579篇 |
1975年 | 614篇 |
1974年 | 756篇 |
1973年 | 692篇 |
1972年 | 688篇 |
1971年 | 781篇 |
1970年 | 1039篇 |
1969年 | 823篇 |
1968年 | 861篇 |
1967年 | 793篇 |
1966年 | 716篇 |
1965年 | 528篇 |
1959年 | 290篇 |
1958年 | 502篇 |
1957年 | 326篇 |
1956年 | 321篇 |
1955年 | 270篇 |
1954年 | 304篇 |
1948年 | 247篇 |
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
931.
Tyrosine phosphorylation of vav proto-oncogene product containing SH2 domain and transcription factor motifs. 总被引:30,自引:0,他引:30
B Margolis P Hu S Katzav W Li J M Oliver A Ullrich A Weiss J Schlessinger 《Nature》1992,356(6364):71-74
932.
933.
Mitochondria contain a complex machinery for the import of nuclear-encoded proteins. Receptor proteins exposed on the outer membrane surface are required for the specific binding of precursor proteins to mitochondria, either by binding of cytosolic signal recognition factors or by direct recognition of the precursor polypeptides. Subsequently, the precursors are inserted into the outer membrane at the general insertion site GIP (general insertion protein). Here we report the analysis of receptors and GIP by crosslinking of translocation intermediates and by coimmunoprecipitation. Surface-accumulated precursors were crosslinked to the receptors MOM19 and MOM72, suggesting a direct interaction of preproteins with surface receptors. We identified three novel mitochondrial outer membrane proteins, MOM7, MOM8, and MOM30 that, together with the previously identified MOM38, seem to form the GIP site and are present in the mitochondrial receptor complex. 相似文献
934.
Identification of the genes orchestrating neurogenesis would greatly enhance our understanding of this process. Genes have been identified that specify neuron type (for example cut and numb in Drosophila and mec-3 in Caenorhabditis elegans) and process guidance (for example, unc-5, unc-6 and unc-40 in C. elegans and the fas-1 gene of Drosophila). We sought genes defining synaptic specificity by identifying mutations that alter synaptic connectivity in the motor circuitry in the nematode C. elegans. We used electron microscopy of serial sections to reconstruct the ventral nerve-cords of uncoordinated (unc) mutants that have distinctive locomotory choreographies. Here we describe the phenotype of mutations in the unc-4 gene in which a locomotory defect is correlated with specific changes in synaptic input to a subset of the excitatory VA motor neurons, normally used in reverse locomotion. The circuitry alterations do not arise because of the inaccessibility of the appropriate synaptic partners, but are a consequence of changes in synaptic specificity. The VA motor neurons with altered synaptic inputs are all lineal sisters of VB motor neurons; the VA motor neurons without VB sisters have essentially the same synaptic inputs as in wild-type animals. The normal function of the wild-type allele of unc-4 may thus be to invoke the appropriate synaptic specificities to VA motor neurons produced in particular developmental contexts. 相似文献
935.
Close linkage of glucokinase locus on chromosome 7p to early-onset non-insulin-dependent diabetes mellitus. 总被引:22,自引:0,他引:22
P Froguel M Vaxillaire F Sun G Velho H Zouali M O Butel S Lesage N Vionnet K Clément F Fougerousse 《Nature》1992,356(6365):162-164
Non-insulin-dependent diabetes mellitus (NIDDM) is a major health problem, affecting 5% of the world population. Genetic factors are important in NIDDM, but the mechanisms leading to glucose intolerance are unknown. Genetic linkage has been investigated in multigeneration families to localize, and ultimately identify, the gene(s) predisposing to NIDDM. Here we report linkage between the glucokinase locus on chromosome 7p and diabetes in 16 French families with maturity-onset diabetes of the young, a form of NIDDM characterized by monogenic autosomal dominant transmission and early age of onset. Statistical evidence of genetic heterogeneity was significant, with an estimated 45-95% of the 16 families showing linkage to glucokinase. Because glucokinase is a key enzyme of blood glucose homeostasis, these results are evidence that a gene involved in glucose metabolism could be implicated in the pathogenesis of NIDDM. 相似文献
936.
A human recombinant haemoglobin designed for use as a blood substitute. 总被引:19,自引:0,他引:19
D Looker D Abbott-Brown P Cozart S Durfee S Hoffman A J Mathews J Miller-Roehrich S Shoemaker S Trimble G Fermi 《Nature》1992,356(6366):258-260
The need to develop a blood substitute is now urgent because of the increasing concern over blood-transmitted viral and bacterial pathogens. Cell-free haemoglobin solutions and human haemoglobin synthesized in Escherichia coli and Saccharomyces cerevisiae have been investigated as potential oxygen-carrying substitutes for red blood cells. But these haemoglobins cannot be used as a blood substitute because (1) the oxygen affinity in the absence of 2,3-bisphosphoglycerate is too high to allow unloading of enough oxygen in the tissues, and (2) they dissociate into alpha beta dimers that are cleared rapidly by renal filtration, which can result in long-term kidney damage. We have produced a human haemoglobin using an expression vector containing one gene encoding a mutant beta-globin with decreased oxygen affinity and one duplicated, tandemly fused alpha-globin gene. Fusion of the two alpha-globin subunits increases the half-life of this haemoglobin molecule in vivo by preventing its dissociation into alpha beta dimers and therefore also eliminates renal toxicity. 相似文献
937.
Normal dystrophin transcripts detected in Duchenne muscular dystrophy patients after myoblast transplantation. 总被引:26,自引:0,他引:26
E Gussoni G K Pavlath A M Lanctot K R Sharma R G Miller L Steinman H M Blau 《Nature》1992,356(6368):435-438
938.
Tumour necrosis factor alpha restores granulomas and induces parasite egg-laying in schistosome-infected SCID mice. 总被引:38,自引:0,他引:38
P Amiri R M Locksley T G Parslow M Sadick E Rector D Ritter J H McKerrow 《Nature》1992,356(6370):604-607
Schistosomiasis (bilharzia) is a parasitic disease caused by several species of schistosome worms (blood flukes). The key pathogenic event in this disease is the formation of granulomas around schistosome eggs trapped in portal venules of the liver. Granulomas are a distinctive form of chronic inflammation characterized by localized aggregation of activated macrophages around an inciting stimulus. Each granuloma evolves to form a fibrous scar; in schistosomiasis, the result is widespread hepatic fibrosis and portal hypertension. To identify the specific immune signal molecules necessary for granuloma formation, we studied schistosome infections in severe combined immunodeficient (SCID) mice, which have normal macrophages but lack functional B or T lymphocytes. Here we report that the immunoregulatory cytokine tumour necrosis factor alpha is necessary and sufficient to reconstitute granuloma formation in schistosome-infected SCID mice. Moreover, we find that the parasitic worms require tumour necrosis factor alpha for egg-laying and for excretion of eggs from the host. The implication of this latter result is that the parasite has adapted so successfully to its host that it uses a host-derived immunoregulatory protein as a signal for replication and transmission. 相似文献
939.
Superantigen implicated in dependence of HIV-1 replication in T cells on TCR V beta expression. 总被引:3,自引:0,他引:3
In the pathogenesis of AIDS it is not yet understood whether the small fraction of CD4+ T cells (approximately 1%) infected with the human immunodeficiency virus (HIV) are randomly targeted or not. Here we present evidence that human CD4 T-cell lines expressing selected T-cell antigen receptor V beta gene products can all be infected in vitro with HIV-1, but give markedly different titres of HIV-1 virion production. For example, V beta 12 T-cell lines from several unrelated donors reproducibly yielded up to 100-fold more gag gene product (p24gag antigen) than V beta 6.7a lines. This is consistent with a superantigen effect, because the V beta selectivity was observed with several divergent HIV-1 isolates, was dependent on antigen-presenting cells and on major histocompatibility complex (MHC) class II but was not MHC class II-restricted. The in vivo significance of these findings is supported by the preferential stimulation of V beta 12+ T cells by freshly obtained irradiated antigen-presenting cells from some HIV-1-seropositive but not HIV-1-negative donors. Moreover, cells from patients positive for viral antigen (gp120) were enriched in the V beta 12 subpopulation. V beta 12+ T cells were not deleted in AIDS patients, however, raising the possibility that a variety of mechanisms contribute to T-cell depletion. Our results indicate that a superantigen targets a subpopulation of CD4+ cells for viral replication. 相似文献
940.
本文给出了一个在传统转速电流双闭环基础上增加锁相环路构成的直流调速系统。该系统具有稳定性好,稳态精度高,调速范围宽的优点。文中给出了系统的稳定性分析及参数设计方法。 相似文献