全文获取类型
收费全文 | 303篇 |
免费 | 0篇 |
专业分类
系统科学 | 4篇 |
教育与普及 | 1篇 |
理论与方法论 | 1篇 |
现状及发展 | 89篇 |
研究方法 | 14篇 |
综合类 | 194篇 |
出版年
2014年 | 4篇 |
2013年 | 2篇 |
2012年 | 6篇 |
2011年 | 2篇 |
2010年 | 2篇 |
2008年 | 9篇 |
2007年 | 3篇 |
2006年 | 7篇 |
2005年 | 8篇 |
2004年 | 5篇 |
2003年 | 7篇 |
2002年 | 7篇 |
2001年 | 12篇 |
2000年 | 10篇 |
1999年 | 7篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 17篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 10篇 |
1973年 | 15篇 |
1972年 | 10篇 |
1971年 | 8篇 |
1970年 | 12篇 |
1969年 | 8篇 |
1968年 | 10篇 |
1967年 | 10篇 |
1966年 | 3篇 |
1965年 | 1篇 |
1964年 | 2篇 |
1958年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有303条查询结果,搜索用时 15 毫秒
121.
122.
Properties of protective malarial antibody 总被引:4,自引:0,他引:4
123.
Platelet contractile regulation in an isometric system 总被引:4,自引:0,他引:4
124.
Jaillon O Bouhouche K Gout JF Aury JM Noel B Saudemont B Nowacki M Serrano V Porcel BM Ségurens B Le Mouël A Lepère G Schächter V Bétermier M Cohen J Wincker P Sperling L Duret L Meyer E 《Nature》2008,451(7176):359-362
Most eukaryotic genes are interrupted by non-coding introns that must be accurately removed from pre-messenger RNAs to produce translatable mRNAs. Splicing is guided locally by short conserved sequences, but genes typically contain many potential splice sites, and the mechanisms specifying the correct sites remain poorly understood. In most organisms, short introns recognized by the intron definition mechanism cannot be efficiently predicted solely on the basis of sequence motifs. In multicellular eukaryotes, long introns are recognized through exon definition and most genes produce multiple mRNA variants through alternative splicing. The nonsense-mediated mRNA decay (NMD) pathway may further shape the observed sets of variants by selectively degrading those containing premature termination codons, which are frequently produced in mammals. Here we show that the tiny introns of the ciliate Paramecium tetraurelia are under strong selective pressure to cause premature termination of mRNA translation in the event of intron retention, and that the same bias is observed among the short introns of plants, fungi and animals. By knocking down the two P. tetraurelia genes encoding UPF1, a protein that is crucial in NMD, we show that the intrinsic efficiency of splicing varies widely among introns and that NMD activity can significantly reduce the fraction of unspliced mRNAs. The results suggest that, independently of alternative splicing, species with large intron numbers universally rely on NMD to compensate for suboptimal splicing efficiency and accuracy. 相似文献
125.
The subduction of oceanic plateaux, which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood-basalt provinces, is an important factor in many current models of plate motion and provides a potential mechanism for triggering plate reorganization. To evaluate such models, it is essential to decipher the history of the collision between the largest and thickest of the world's oceanic plateaux, the Ontong Java plateau, and the Australian plate, but this has been hindered by poor constraints for the arrival of the plateau at the Melanesian trench. Here we present (40)Ar-(39)Ar geochronological data on hotspot volcanoes in eastern Australian that reveal a strong link between collision of the Greenland-sized Ontong Java plateau with the Melanesian arc and motion of the Australian plate. The new ages define a short-lived period of reduced northward plate motion between 26 and 23 Myr ago, coincident with an eastward offset in the contemporaneous tracks of seamount chains in the Tasman Sea east of Australia. These features record a brief westward deflection of the Australian plate as the plateau entered and choked the Melanesian trench 26 Myr ago. From 23 Myr ago, Australia returned to a rapid northerly trajectory at roughly the same time that southwest-directed subduction began along the Trobriand trough. The timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the Pacific plate, including the archetypal Hawaiian chain, and thus provide strong evidence that immense oceanic plateaux, like the Ontong Java, can contribute to initiating rapid change in plate boundaries and motions on a global scale. 相似文献
126.
Cohen RI 《Cellular and molecular life sciences : CMLS》2005,62(5):505-510
Oligodendrocytes, the myelinating cells of the central nervous system (CNS), originate early in the formation of the brain in specific foci, and migrate throughout the parenchyma. The instructional cues guiding the migration of these progenitor cells must be encoded into their developing environment. Soluble factors as well as membrane-bound cues most likely synergize to create a complex thoroughfare needed to sculpt and organize the brain into a functional organ with white and gray matter. Classically, the focus of many guidance related studies in the CNS has been limited to neuron physiology. However, It is becoming increasingly clear that their lifelong partners, oligodendrocytes, express both ligands and receptors able to both present and respond to these classical cues. In this short review, some recent findings in the Semaphorin and Eph fields will be presented with respect to oligodendrocyte expression and function.Received 4 November 2004; received after revision 1 December 2004; accepted 7 December 2004 相似文献
127.
A pronounced negative carbon-isotope (delta13C) excursion of approximately 5-7 per thousand (refs 1-7) indicates the occurrence of a significant perturbation to the global carbon cycle during the Early Jurassic period (early Toarcian age, approximately 183 million years ago). The rapid release of 12C-enriched biogenic methane as a result of continental-shelf methane hydrate dissociation has been put forward as a possible explanation for this observation. Here we report high-resolution organic carbon-isotope data from well-preserved mudrocks in Yorkshire, UK, which demonstrate that the carbon-isotope excursion occurred in three abrupt stages, each showing a shift of -2 per thousand to -3 per thousand. Spectral analysis of these carbon-isotope measurements and of high-resolution carbonate abundance data reveals a regular cyclicity. We interpret these results as providing strong evidence that methane release proceeded in three rapid pulses and that these pulses were controlled by astronomically forced changes in climate, superimposed upon longer-term global warming. We also find that the first two pulses of methane release each coincided with the extinction of a large proportion of marine species. 相似文献
128.
129.
The surround response mechanism in on-center X-cells in cat retina was found to be bimodally distributed and weak or nonexistent in the receptive field middle. An on-inhibition measure was used to assess surround mechanism gain. 相似文献
130.
The adaptation field of the surround mechanism of X and Y retinal ganglion cells in the cat was assessed with variable size, unmodulated adapting spots. Both an on-inhibition measure and an off-discharge measure of surround gain was used. Results suggest that the surround mechanism in Y-cells is strongest in the receptive field middle but weak or nonexistent in the middle of X-cell receptive fields. 相似文献