全文获取类型
收费全文 | 41459篇 |
免费 | 78篇 |
国内免费 | 104篇 |
专业分类
系统科学 | 208篇 |
丛书文集 | 800篇 |
教育与普及 | 101篇 |
理论与方法论 | 245篇 |
现状及发展 | 17488篇 |
研究方法 | 1654篇 |
综合类 | 20513篇 |
自然研究 | 632篇 |
出版年
2013年 | 322篇 |
2012年 | 556篇 |
2011年 | 1223篇 |
2010年 | 244篇 |
2008年 | 697篇 |
2007年 | 761篇 |
2006年 | 792篇 |
2005年 | 770篇 |
2004年 | 709篇 |
2003年 | 736篇 |
2002年 | 748篇 |
2001年 | 1348篇 |
2000年 | 1253篇 |
1999年 | 792篇 |
1992年 | 744篇 |
1991年 | 614篇 |
1990年 | 626篇 |
1989年 | 660篇 |
1988年 | 638篇 |
1987年 | 647篇 |
1986年 | 660篇 |
1985年 | 766篇 |
1984年 | 648篇 |
1983年 | 543篇 |
1982年 | 474篇 |
1981年 | 456篇 |
1980年 | 580篇 |
1979年 | 1282篇 |
1978年 | 1106篇 |
1977年 | 1063篇 |
1976年 | 779篇 |
1975年 | 859篇 |
1974年 | 1247篇 |
1973年 | 1045篇 |
1972年 | 1074篇 |
1971年 | 1341篇 |
1970年 | 1732篇 |
1969年 | 1275篇 |
1968年 | 1256篇 |
1967年 | 1238篇 |
1966年 | 1108篇 |
1965年 | 787篇 |
1964年 | 180篇 |
1959年 | 450篇 |
1958年 | 693篇 |
1957年 | 554篇 |
1956年 | 449篇 |
1955年 | 386篇 |
1954年 | 445篇 |
1948年 | 260篇 |
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
991.
Karen Schrader Jisen Huai Lars Jöckel Carolin Oberle Christoph Borner 《Cellular and molecular life sciences : CMLS》2010,67(10):1607-1618
Caspases are the most important effectors of apoptosis, the major form of programmed cell death (PCD) in multicellular organisms. This is best reflected by the appearance of serious development defects in mice deficient for caspase-8, -9, and -3. Meanwhile, caspase-independent PCD, mediated by other proteases or signaling components has been described in numerous publications. Although we do not doubt that such cell death exists, we propose that it has evolved later during evolution and is most likely not designed to execute, but to amplify and speed-up caspase-dependent cell death. This review shall provide evidence for such a concept. 相似文献
992.
993.
Christian Dölle Marc Niere Emilia Lohndal Mathias Ziegler 《Cellular and molecular life sciences : CMLS》2010,67(3):433-443
Poly-ADP-ribose polymerases (PARPs) use NAD+ as substrate to generate polymers of ADP-ribose. We targeted the catalytic domain of human PARP1 as molecular NAD+ detector into cellular organelles. Immunochemical detection of polymers demonstrated distinct subcellular NAD+ pools in mitochondria, peroxisomes and, surprisingly, in the endoplasmic reticulum and the Golgi complex. Polymers did not
accumulate within the mitochondrial intermembrane space or the cytosol. We demonstrate the suitability of this compartment-specific
NAD+ and poly-ADP-ribose turnover to establish intra-organellar protein localization. For overexpressed proteins, genetically
endowed with PARP activity, detection of polymers indicates segregation from the cytosol and consequently intra-organellar
residence. In mitochondria, polymer build-up reveals matrix localization of the PARP fusion protein. Compared to presently
used fusion tags for subcellular protein localization, these are substantial improvements in resolution. We thus established
a novel molecular tool applicable for studies of subcellular NAD metabolism and protein localization. 相似文献
994.
There is substantial evidence that the martian volatile inventory and climate have changed markedly throughout the planet's history. Clues come from areas as disparate as the history and properties of the deep interior, the composition of the crust and regolith, the morphology of the surface, composition of the present-day atmosphere, and the nature of the interactions between the upper atmosphere and the solar wind. We piece together the relevant observations into a coherent view of the evolution of the martian climate, focusing in particular on the observations that provide the strongest constraints. 相似文献
995.
The stable propagation of genetic information requires that the entire genome of an organism be faithfully replicated once and only once each cell cycle. In eukaryotes, this replication is initiated at hundreds to thousands of replication origins distributed over the genome, each of which must be prohibited from re-initiating DNA replication within every cell cycle. How cells prevent re-initiation has been a long-standing question in cell biology. In several eukaryotes, cyclin-dependent kinases (CDKs) have been implicated in promoting the block to re-initiation, but exactly how they perform this function is unclear. Here we show that B-type CDKs in Saccharomyces cerevisiae prevent re-initiation through multiple overlapping mechanisms, including phosphorylation of the origin recognition complex (ORC), downregulation of Cdc6 activity, and nuclear exclusion of the Mcm2-7 complex. Only when all three inhibitory pathways are disrupted do origins re-initiate DNA replication in G2/M cells. These studies show that each of these three independent mechanisms of regulation is functionally important. 相似文献
996.
997.
本文报导用活性炭盒吸附方法对香港室内氡浓度的测量结果及其浓度分布规律。对室内氡浓度与建筑物表面氡析出率的关系进行了分析研究。证实室内空气中的氡主要来源于建材中的镭,而氡浓度水平只决定于室内建筑物表面氡的析出率及通风状况。 相似文献
998.
Self peptides bound to self major histocompatibility complex (MHC) molecules have been implicated both in positive and in negative selection of T cells during intrathymic development. We report here that the novel MHC-restricted monoclonal antibody Y-Ae detects the MHC class II bound form of a major self peptide. Y-Ae binds approximately 12% of the relevant MHC class II molecules on self antigen presenting cells. The peptide detected by Y-Ae is one of several major peptides eluted from the MHC molecule. These data suggest that self peptides presented by self MHC class II molecules at densities sufficient to signal a CD4 T cell are of very limited complexity. Furthermore, as Y-Ae stains antigen presenting cells that mediate negative selection but not thymic cortical epithelial cells that drive positive selection, differential expression of self peptide:self MHC class II complexes may be a key feature of intrathymic selection. 相似文献
999.
R E Hill J Favor B L Hogan C C Ton G F Saunders I M Hanson J Prosser T Jordan N D Hastie V van Heyningen 《Nature》1991,354(6354):522-525
1000.
The L-type voltage-dependent calcium channel is an important link in excitation-contraction coupling of muscle cells (reviewed in refs 2 and 3). The channel has two functional characteristics: calcium permeation and receptor sites for calcium antagonists. In skeletal muscle the channel is a complex of five subunits, alpha 1, alpha 2, beta, gamma and delta. Complementary DNAs to these subunits have been cloned and their amino-acid sequences deduced. The skeletal muscle alpha 1 subunit cDNA expressed in L cells manifests as specific calcium-ion permeation, as well as sensitivity to the three classes of organic calcium-channel blockers. We report here that coexpression of the alpha 1 subunit with other subunits results in significant changes in dihydropyridine binding and gating properties. The available number of drug receptor sites increases 10-fold with an alpha 1 beta combination, whereas the affinity of the dihydropyridine binding site remains unchanged. Also, the presence of the beta subunit accelerates activation and inactivation kinetics of the calcium-channel current. 相似文献