全文获取类型
收费全文 | 20009篇 |
免费 | 1650篇 |
国内免费 | 5篇 |
专业分类
21664篇 |
出版年
2023年 | 88篇 |
2022年 | 232篇 |
2021年 | 432篇 |
2020年 | 233篇 |
2019年 | 306篇 |
2018年 | 423篇 |
2017年 | 353篇 |
2016年 | 636篇 |
2015年 | 1030篇 |
2014年 | 1169篇 |
2013年 | 1409篇 |
2012年 | 1748篇 |
2011年 | 1649篇 |
2010年 | 1062篇 |
2009年 | 936篇 |
2008年 | 1264篇 |
2007年 | 1291篇 |
2006年 | 1105篇 |
2005年 | 1060篇 |
2004年 | 1034篇 |
2003年 | 937篇 |
2002年 | 891篇 |
2001年 | 203篇 |
2000年 | 144篇 |
1999年 | 153篇 |
1998年 | 233篇 |
1997年 | 149篇 |
1996年 | 136篇 |
1995年 | 125篇 |
1994年 | 112篇 |
1993年 | 101篇 |
1992年 | 80篇 |
1991年 | 64篇 |
1990年 | 67篇 |
1989年 | 60篇 |
1988年 | 50篇 |
1987年 | 55篇 |
1986年 | 42篇 |
1985年 | 46篇 |
1984年 | 43篇 |
1983年 | 62篇 |
1982年 | 37篇 |
1981年 | 32篇 |
1980年 | 36篇 |
1979年 | 28篇 |
1978年 | 42篇 |
1977年 | 35篇 |
1976年 | 32篇 |
1975年 | 25篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Thomas Wachtler Christian Wehrhahn Barry B. Lee 《Journal of computational neuroscience》1996,3(1):73-82
We developed a physiologically plausible model of the first steps of spatial visual information processing in the fovea of the human retina. With the predictions of this model we could support the hypothesis that, for moderate contrasts ( 40%), hyperacuity is mediated by the magnocellular (MC-) pathway. Despite the lower sampling density in the MC pathway, as compared to the parvocellular (PC-) pathway, the information that is transferred by the MC ganglion cells is sufficient to achieve thresholds comparable to those of human subjects in psychophysical tasks. This is a result of the much higher signal-to-noise ratio of the MC pathway cell signals. The PC pathway cells do not transfer enough information for hyperacuity thresholds. 相似文献
62.
63.
64.
65.
66.
Christian Chervaux Nathalie Sauvonnet Annick Le Clainche Brendan Kenny A. Lesley Hunt Jenny K. Broome-Smith I. Barry Holland 《Molecular & general genetics : MGG》1995,249(2):237-245
An in frame gene fusion containing the coding region for mature -lactamase and the 3-end of hylA encoding the haemolysin secretion signal, was constructed under the control of a lac promoter. The resulting 53 kDa hybrid protein was specifically secreted to the external medium in the presence of the haemolysin translocator proteins, HlyB and HlyD. The specific activity of the -lactamase portion of the secreted protein (measured by the hydrolysis of penicillin G), approximately 1 U/g protein, was close to that of authentic, purified TEM--lactamase. This is an important example of a hybrid protein that is enzymatically active, and secreted via the haemolysin pathway. Previous studies have indicated that haemolysin is secreted directly into the medium, bypassing the periplasm, to which -lactamase is normally targeted. This study indicated, therefore, that normal folding of an active -lactamase, can occur, at least when fused to the HlyA C-terminus, without the necessity of entering the periplasm. Despite the secretion of approximately 5 g/ml levels of the active -lactamase fusion into the medium, there was maximally only a 50% detectable increase in the LD50 for resistance to ampicillin at the individual cell level. This result suggests that, normally, resistance to ampicillin requires a high concentration of the enzyme close to killing targets, i.e. in the periplasm, in order to achieve significant levels of protection.These authors made an equal contribution to this work 相似文献
67.
Statistical analysis of vertebrate sequences reveals that long genes are scarce in GC-rich isochores 总被引:9,自引:0,他引:9
Laurent Duret Dominique Mouchiroud Christian Gautier 《Journal of molecular evolution》1995,40(3):308-317
We compared the exon/intron organization of vertebrate genes belonging to different isochore classes, as predicted by their GC content at third codon position. Two main features have emerged from the analysis of sequences published in GenBank: (1) genes coding for long proteins (i.e., 500 aa) are almost two times more frequent in GC-poor than in GC-rich isochores; (2) intervening sequences (=sum of introns) are on average three times longer in GC-poor than in GC-rich isochores. These patterns are observed among human, mouse, rat, cow, and even chicken genes and are therefore likely to be common to all warm-blooded vertebrates. Analysis of Xenopus sequences suggests that the same patterns exist in cold-blooded vertebrates. It could be argued that such results do not reflect the reality because sequence databases are not representative of entire genomes. However, analysis of biases in GenBank revealed that the observed discrepancies between GC-rich and GC-poor isochores are not artifactual, and are probably largely underestimated. We investigated the distribution of microsatellites and interspersed repeats in introns of human and mouse genes from different isochores. This analysis confirmed previous studies showing that Ll repeats are almost absent from GC-rich isochores. Microsatellites and SINES (Alu, B1, B2) are found at roughly equal frequencies in introns from all isochore classes. Globally, the presence of repeated sequences does not account for the increased intron length in GC-poor isochores. The relationships between gene structure and global genome organization and evolution are discussed. 相似文献
68.
We present ab-initio periodic Hartree–Fock calculations (crystal program) of small molecules on TiO2 and MgO. The adsorption of the molecules may be molecular or dissociative. This depends on their acid and basic properties in the gas phase. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages: thus, the dissociation of MeOH leads to a preferential basic cleavage (the fragment HO– is adsorbed on a Ti+4 ion and the fragment Me+ is adsorbed on a O2– ion of the oxide). The opposite result is obtained with MeSH. Another important factor is the adsorbate–adsorbate interaction: favorable cases are a sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO2 molecules to bend over the surface so that their mutual orientation resembles the geometry of the CO2 dimer. With respect to water adsorption, MgO appears to be a basic oxide. As experimentally observed, NH3 adsorbs preferentially on TiO2 and CO2 on MgO. However, this difference of reactivity should not be expressed in terms of acid vs. basic behaviour but in terms of hard and soft acidity. The MgO surface is a 'soft' acidic surface that reacts preferentially with the soft base, CO2. 相似文献
69.
70.
Post-translational modifications are fundamental to processes controlling behaviour, including cellular signaling, growth and transformation. As the molecular basis of protein modifications in normal and disease processes are becoming better defined, so new strategies for designing therapeutic entities to control complex disease processes are emerging. 相似文献