首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   36537篇
  免费   129篇
  国内免费   121篇
系统科学   190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   16037篇
研究方法   1503篇
综合类   17528篇
自然研究   433篇
  2013年   211篇
  2012年   477篇
  2011年   962篇
  2010年   217篇
  2009年   180篇
  2008年   637篇
  2007年   650篇
  2006年   676篇
  2005年   672篇
  2004年   707篇
  2003年   649篇
  2002年   648篇
  2001年   1116篇
  2000年   1040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   571篇
  1983年   461篇
  1982年   404篇
  1981年   436篇
  1980年   529篇
  1979年   1136篇
  1978年   998篇
  1977年   974篇
  1976年   674篇
  1975年   758篇
  1974年   1075篇
  1973年   912篇
  1972年   950篇
  1971年   1156篇
  1970年   1524篇
  1969年   1158篇
  1968年   1086篇
  1967年   1148篇
  1966年   966篇
  1965年   728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
951.
Sex determination compared in Drosophila and Caenorhabditis   总被引:20,自引:0,他引:20  
J Hodgkin 《Nature》1990,344(6268):721-728
Fruitflies and nematodes show many similarities in the general organization of the gene networks that control sexual dimorphism and dosage compensation. In contrast, the underlying molecular mechanisms appear to be very different in these two species. Developmental processes such as sex determination need not be strongly conserved in evolution.  相似文献   
952.
A binding site for the T-cell co-receptor CD8 on the alpha 3 domain of HLA-A2   总被引:23,自引:0,他引:23  
Adhesion measurements between CD8 and 48 point mutants of HLA-A2.1 show that the CD8 alpha-chain binds to the alpha 3 domain of HLA-A2.1. Three clusters of alpha 3 residues contribute to the binding, with an exposed, negatively charged loop (residues 223-229) playing a dominant role. CD8 binding correlates with cytotoxic T-cell recognition and sensitivity to inhibition by anti-CD8 antibodies. Impaired alloreactive T-cell recognition of an HLA-A2.1 mutant with reduced affinity for CD8 is not restored by functional CD8 binding sites on an antigenically irrelevant class I molecule. Therefore, complexes of CD8 and the T-cell receptor bound to the same class I major histocompatibility complex molecule seem to be necessary for T-cell activation.  相似文献   
953.
954.
D Pilbeam  M D Rose  J C Barry  S M Shah 《Nature》1990,348(6298):237-239
New humeri of two species of the Miocene hominoid Sivapithecus are described from near Chinji in Pakistan from between approximately 9 and 11 Myr ago. Sivapithecus, a middle and late Miocene hominoid from Turkey and Indo-Pakistan, is overall unlike any living hominoid, although facial-palatal similarities to the extant orangoutan, Pongo, have been used to support a hypothesis of close relationship. Living hominoids have postcranial similarities assumed to be shared derived, among them features of the proximal humerus. However, the new Sivapithecus proximal humeri differ from those of living hominoids, supporting an alternative hypothesis in which Sivapithecus and Pongo are not closely related. It is not clear how to choose between these incompatible hypotheses.  相似文献   
955.
956.
957.
M A Félix  J C Labbé  M Dorée  T Hunt  E Karsenti 《Nature》1990,346(6282):379-382
The cell cycles of early Xenopus embryos consist of a rapid succession of alternating S and M phases. These cycles are controlled by the activity of a protein kinase complex (cdc2 kinase) which contains two subunits. One subunit is encoded by the frog homologue of the fission yeast cdc2+ gene, p34cdc2 and the other is a cyclin. The concentration of cyclins follows a sawtooth oscillation because they accumulate in interphase and are destroyed abruptly during mitosis. The association of cyclin and p34cdc2 is not sufficient for activation of cdc2 kinase, however; dephosphorylation of key tyrosine and threonine residues of p34cdc2 is necessary to turn on its kinase activity. The activity of cdc2 kinase is thus regulated by a combination of translational and post-translational mechanisms. The loss of cdc2 kinase activity at the end of mitosis depends on the destruction of the cyclin subunits. It has been suggested that this destruction is induced by cdc2 kinase itself, thereby providing a negative feedback loop to terminate mitosis. Here we report direct experimental evidence for this idea by showing that cyclin proteolysis can be triggered by adding cdc2 kinase to a cell-free extract of interphase Xenopus eggs.  相似文献   
958.
A Ghosh  A Antonini  S K McConnell  C J Shatz 《Nature》1990,347(6289):179-181
The neurons of layer 4 in the adult cerebral cortex receive their major ascending inputs from the thalamus. In development, however, thalamic axons arrive at the appropriate cortical area long before their target layer 4 neurons have migrated into the cortical plate. The axons accumulate and wait in the zone below the cortical plate, the subplate, for several weeks before invading the cortical plate. The subplate is a transient zone that contains the first postmitotic neurons of the telencephalon. These neurons mature well before other cortical neurons, and disappear by cell death after the thalamic axons have grown into the overlying cortical plate. The close proximity of growing thalamocortical axons and subplate neurons suggests that they might be involved in interactions important for normal thalamocortical development. Here we show that early in development the deletion of subplate neurons located beneath visual cortex prevents axons from the lateral geniculate nucleus of the thalamus from recognizing and innervating visual cortex, their normal target. In the absence of subplate neurons, lateral geniculate nucleus axons continue to grow in the white matter past visual cortex despite the presence of their target layer 4 neurons. Thus the transient subplate neurons are necessary for appropriate cortical target selection by thalamocortical axons.  相似文献   
959.
Detection and characterization of a folding intermediate in barnase by NMR   总被引:17,自引:0,他引:17  
Protein engineering is being developed for mapping the energetics and pathway of protein folding. From kinetic studies on wild-type and mutant proteins, the sequence and energetics of formation of tertiary interactions of side chains can be mapped and the formation of secondary structure inferred. Here we cross-check and complement results from this approach by using a recently developed procedure that traps and characterizes secondary structure in intermediate states using 1H NMR. The refolding of barnase is shown to be a multiphasic process in which the secondary structure in alpha-helices and beta-sheets and some turns is formed more rapidly than is the overall folding.  相似文献   
960.
Acute leukaemia in bcr/abl transgenic mice   总被引:38,自引:0,他引:38  
  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号