全文获取类型
收费全文 | 36584篇 |
免费 | 130篇 |
国内免费 | 121篇 |
专业分类
系统科学 | 192篇 |
丛书文集 | 792篇 |
教育与普及 | 90篇 |
理论与方法论 | 214篇 |
现状及发展 | 16040篇 |
研究方法 | 1501篇 |
综合类 | 17570篇 |
自然研究 | 436篇 |
出版年
2013年 | 211篇 |
2012年 | 478篇 |
2011年 | 961篇 |
2010年 | 217篇 |
2009年 | 180篇 |
2008年 | 636篇 |
2007年 | 649篇 |
2006年 | 675篇 |
2005年 | 673篇 |
2004年 | 706篇 |
2003年 | 646篇 |
2002年 | 648篇 |
2001年 | 1116篇 |
2000年 | 1041篇 |
1999年 | 719篇 |
1992年 | 637篇 |
1991年 | 537篇 |
1990年 | 551篇 |
1989年 | 553篇 |
1988年 | 550篇 |
1987年 | 569篇 |
1986年 | 571篇 |
1985年 | 702篇 |
1984年 | 572篇 |
1983年 | 461篇 |
1982年 | 404篇 |
1981年 | 435篇 |
1980年 | 529篇 |
1979年 | 1134篇 |
1978年 | 998篇 |
1977年 | 974篇 |
1976年 | 673篇 |
1975年 | 758篇 |
1974年 | 1074篇 |
1973年 | 911篇 |
1972年 | 952篇 |
1971年 | 1156篇 |
1970年 | 1524篇 |
1969年 | 1160篇 |
1968年 | 1090篇 |
1967年 | 1148篇 |
1966年 | 965篇 |
1965年 | 727篇 |
1959年 | 400篇 |
1958年 | 601篇 |
1957年 | 504篇 |
1956年 | 423篇 |
1955年 | 358篇 |
1954年 | 424篇 |
1948年 | 232篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
House mice (Mus musculus domesticus) form communal nests and appear to nurse each other's pups indiscriminately. Communal nesting probably functions to reduce infanticide, but it also makes females vulnerable to exploitation if nursing partners fail to provide their fair share of care. Kinship theory predicts that females will preferentially form communal nests with relatives to minimize exploitation and further increase inclusive fitness. Here we provide evidence from seminatural populations that females prefer communal nesting partners that share allelic forms of major histocompatibility complex genes. Such behaviour would lead to the selection of close relatives as communal nesting partners. Although criteria for the demonstration of kin recognition are currently embroiled in controversy, this is the first vertebrate study to meet Grafen's restrictive requirements: discrimination is based on genetic similarity at highly polymorphic loci, incidental correlations due to relatedness are experimentally controlled, and strong reasons exist for expecting the assayed behaviour to be kin-selected. 相似文献
993.
Mutation of the beta-amyloid precursor protein in familial Alzheimer's disease increases beta-protein production. 总被引:41,自引:0,他引:41
M Citron T Oltersdorf C Haass L McConlogue A Y Hung P Seubert C Vigo-Pelfrey I Lieberburg D J Selkoe 《Nature》1992,360(6405):672-674
Progressive cerebral deposition of the 39-43-amino-acid amyloid beta-protein (A beta) is an invariant feature of Alzheimer's disease which precedes symptoms of dementia by years or decades. The only specific molecular defects that cause Alzheimer's disease which have been identified so far are missense mutations in the gene encoding the beta-amyloid precursor protein (beta-APP) in certain families with an autosomal dominant form of the disease (familial Alzheimer's disease, or FAD). These mutations are located within or immediately flanking the A beta region of beta-APP, but the mechanism by which they cause the pathological phenotype of early and accelerated A beta deposition is unknown. Here we report that cultured cells which express a beta-APP complementary DNA bearing a double mutation (Lys to Asn at residue 595 plus Met to Leu at position 596) found in a Swedish FAD family produce approximately 6-8-fold more A beta than cells expressing normal beta-APP. The Met 596 to Leu mutation is principally responsible for the increase. These data establish a direct link between a FAD genotype and the clinicopathological phenotype. Further, they confirm the relevance of the continuous A beta production by cultured cells for elucidating the fundamental mechanism of Alzheimer's disease. 相似文献
994.
The thymus positively selects thymocytes that bear T-cell receptors which recognize antigen presented by self major histocompatibility complex (MHC) proteins. Positive selection is usually driven by MHC products on radiation-resistant cortical epithelial cells. It is unknown whether positive selection is mediated by all thymic epithelial cells or by some specialized subsets. Here we introduce an H-2b-expressing thymic epithelial cell line into the thymuses of lethally irradiated H-2k animals reconstituted with H-2b/k F1 BM or fetal liver cells. I-Ab-restricted T cells are found in these animals, demonstrating that selection occurs on the introduced epithelial cells. 相似文献
995.
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
996.
997.
Escherichia coli divides by forming a septum across the middle of the cell. The biochemical mechanism underlying this process is unknown. Genetic evidence suggests that of all the fts (filamentation temperature sensitive) genes involved in E. coli cell division, ftsZ plays a central role at the earliest known step of septation. Here we show that FtsZ protein binds GTP in vitro using unusual sequence elements. In contrast, such binding to the product of the conditional-lethal ftsZ84 allele is impaired. Purified FtsZ displays a Mg(2+)-dependent GTPase activity which is markedly reduced in the FtsZ84 protein. FtsZ copurifies with near stoichiometric amounts of noncovalently-bound GDP, implying the presence of a GTPase cycle in vivo, similar to that known for signal-transducing GTP-binding proteins. We also show that a small fraction of FtsZ exists as a distinct membrane-associated species that binds GTP. The membrane association of FtsZ and the known ability of GTPases to act as molecular switches implicate FtsZ in a GTP-activated signal transduction pathway that may regulate the start of septation in E. coli. 相似文献
998.
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
999.
1000.
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