全文获取类型
收费全文 | 14895篇 |
免费 | 1563篇 |
国内免费 | 4篇 |
出版年
2022年 | 101篇 |
2021年 | 211篇 |
2020年 | 199篇 |
2019年 | 275篇 |
2018年 | 330篇 |
2017年 | 329篇 |
2016年 | 420篇 |
2015年 | 698篇 |
2014年 | 703篇 |
2013年 | 936篇 |
2012年 | 989篇 |
2011年 | 916篇 |
2010年 | 685篇 |
2009年 | 625篇 |
2008年 | 783篇 |
2007年 | 800篇 |
2006年 | 732篇 |
2005年 | 759篇 |
2004年 | 718篇 |
2003年 | 756篇 |
2002年 | 748篇 |
2001年 | 148篇 |
2000年 | 114篇 |
1999年 | 179篇 |
1998年 | 225篇 |
1997年 | 164篇 |
1996年 | 155篇 |
1995年 | 156篇 |
1994年 | 138篇 |
1993年 | 100篇 |
1992年 | 136篇 |
1991年 | 108篇 |
1990年 | 95篇 |
1989年 | 99篇 |
1988年 | 74篇 |
1987年 | 87篇 |
1986年 | 82篇 |
1985年 | 101篇 |
1984年 | 108篇 |
1983年 | 87篇 |
1982年 | 110篇 |
1981年 | 126篇 |
1980年 | 98篇 |
1979年 | 99篇 |
1978年 | 96篇 |
1977年 | 83篇 |
1976年 | 86篇 |
1975年 | 53篇 |
1974年 | 78篇 |
1973年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
991.
Stevens JM Rao Saroja N Jaouen M Belghazi M Schmitter JM Mansuy D Artaud I Sari MA 《Protein expression and purification》2003,29(1):70-76
Nitrile hydratases (NHases) are industrially important iron- and cobalt-containing enzymes that are used in the large-scale synthesis of acrylamide. Heterologous expression of NHases has been complicated by the fact that other proteins (activators or metallochaperones) appear to be required to produce NHases in their catalytically active form. We report a novel heterologous system for the expression of catalytically active iron-containing NI1 NHase in Escherichia coli, involving coexpression with the E. coli GroES and GroEL chaperones. The purified recombinant enzyme was found to be highly similar to the enzyme purified from Comamonas testosteroni according to its spectroscopic features, catalytic properties with various substrates, and post-translational modifications. In addition, we report a rapid and convenient spectrophotometric method to monitor the activity of NI1 NHase during purification. 相似文献
992.
The 30 living species of armadillos, anteaters, and sloths (Mammalia: Xenarthra) represent one of the three major clades of placentals. Armadillos (Cingulata: Dasypodidae) are the earliest and most speciose xenarthran lineage with 21 described species. The question of their tricky phylogeny was here studied by adding two mitochondrial genes (NADH dehydrogenase subunit 1 [ND1] and 12S ribosomal RNA [12S rRNA]) to the three protein-coding nuclear genes (alpha2B adrenergic receptor [ADRA2B], breast cancer susceptibility exon 11 [BRCA1], and von Willebrand factor exon 28 [VWF]) yielding a total of 6869 aligned nucleotide sites for thirteen xenarthran species. The two mitochondrial genes were characterized by marked excesses of transitions over transversions-with a strong bias toward CT transitions for the 12S rRNA-and exhibited two- to fivefold faster evolutionary rates than the fastest nuclear gene (ADRA2B). Maximum likelihood and Bayesian phylogenetic analyses supported the monophyly of Dasypodinae, Tolypeutinae, and Euphractinae, with the latter two armadillo subfamilies strongly clustering together. Conflicting branching points between individual genes involved relationships within the subfamilies Tolypeutinae and Euphractinae. Owing to a greater number of informative sites, the overall concatenation favored the mitochondrial topology with the classical grouping of Cabassous and Priodontes within Tolypeutinae, and a close relationship between Euphractus and Chaetophractus within Euphractinae. However, low statistical support values associated with almost equal distributions of apomorphies among alternatives suggested that two parallel events of rapid speciation occurred within these two armadillo subfamilies. 相似文献
993.
Geuze HJ Murk JL Stroobants AK Griffith JM Kleijmeer MJ Koster AJ Verkleij AJ Distel B Tabak HF 《Molecular biology of the cell》2003,14(7):2900-2907
The traditional view holds that peroxisomes are autonomous organelles multiplying by growth and division. More recently, new observations have challenged this concept. Herein, we present evidence supporting the involvement of the endoplasmic reticulum (ER) in peroxisome formation by electron microscopy, immunocytochemistry and three-dimensional image reconstruction of peroxisomes and associated compartments in mouse dendritic cells. We found the peroxisomal membrane protein Pex13p and the ATP-binding cassette transporter protein PMP70 present in specialized subdomains of the ER that were continuous with a peroxisomal reticulum from which mature peroxisomes arose. The matrix proteins catalase and thiolase were only detectable in the reticula and peroxisomes. Our results suggest the existence of a maturation pathway from the ER to peroxisomes and implicate the ER as a major source from which the peroxisomal membrane is derived. 相似文献
994.
Manuel M Borchiellini C Alivon E Le Parco Y Vacelet J Boury-Esnault N 《Systematic biology》2003,52(3):311-333
Because calcareous sponges are triggering renewed interest with respect to basal metazoan evolution, a phylogenetic framework of their internal relationships is needed to clarify the evolutionary history of key morphological characters. Morphological variation was scored at the suprageneric level within Calcispongia, but little phylogenetic information could be retrieved from morphological characters. For the main subdivision of Calcispongia, the analysis of morphological data weakly supports a classification based upon cytological and embryological characters (Calcinea/Calcaronea) rather than the older classification scheme based upon the aquiferous system (Homocoela/Heterocoela). The 18S ribosomal RNA data were then analyzed, both alone and in combination with morphological characters. The monophyly of Calcispongia is highly supported, but the position of this group with respect to other sponge lineages and to eumetazoan taxa is not resolved. The monophyly of both Calcinea and Calcaronea is retrieved, and the data strongly rejected the competing Homocoela/Heterocoela hypothesis. The phylogeny implies that characters of the skeleton architecture are highly homoplastic, as are characters of the aquiferous system. However, axial symmetry seems to be primitive for all Calcispongia, a conclusion that has potentially far-reaching implications for hypotheses of early body plan evolution in Metazoa. 相似文献
995.
996.
997.
Notch signalling in the regulation of peripheral T-cell function 总被引:3,自引:0,他引:3
Mckenzie GJ Young LL Briend E Lamb JR Dallman MJ Champion BR 《Seminars in cell & developmental biology》2003,14(2):127-134
The Notch signalling pathway plays a highly-conserved role in regulating the cellular differentiation and proliferation events that characterise pattern formation in the embryo. As cells in the embryo respond to environmental signals, similarly T-cells in the peripheral immune system must monitor their environment for antigens and respond accordingly by entering one of several potential differentiation pathways. Recent studies have identified a role for the Notch pathway in regulating the responses of T-cells in the periphery. In this review, we discuss these findings in the context of the Notch signalling pathway's role as an orchestrator of cellular differentiation, and propose a central role for Notch as a regulator of immune system function. 相似文献
998.
999.
McKinley J Buchanan-Smith HM Bassett L Morris K 《Journal of applied animal welfare science : JAAWS》2003,6(3):209-220
The first author trained 12 laboratory-housed common marmosets (Callithrix jacchus) in pairs to assess the practicality of positive reinforcement training as a technique in the management of these nonhuman animals. Behaviors taught were (a) target training to allow in homecage weighing and (b) providing urine samples. Between 2 to 13, 10-minute training sessions established desired behaviors. Training aggressive animals only after they had been fed eliminated aggression during training. Trained animals proved extremely reliable, and data collection using trained animals was considerably faster than collection using current laboratory techniques. The results suggest that positive reinforcement training is a practical option in the management of laboratory-housed marmosets. 相似文献
1000.
Charton I Suzenet F Boutin JA Audinot V Delagrange P Bennejean C Renard P Guillaumet G 《Journal of enzyme inhibition and medicinal chemistry》2003,18(2):187-193
The synthesis of benzoxathiins bearing a retroamide function is described from 8-hydroxythiochroman, the key step involving the synthesis of the benzoxathiin ring through a sulfonium salt. These new melatonin analogues were evaluated on human receptors MT1 and MT2 and have a similar affinity to that of melatonin itself. 相似文献