全文获取类型
收费全文 | 320篇 |
免费 | 0篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 1篇 |
丛书文集 | 1篇 |
教育与普及 | 25篇 |
现状及发展 | 21篇 |
研究方法 | 42篇 |
综合类 | 232篇 |
出版年
2022年 | 7篇 |
2021年 | 3篇 |
2020年 | 1篇 |
2019年 | 4篇 |
2018年 | 2篇 |
2017年 | 1篇 |
2015年 | 4篇 |
2013年 | 2篇 |
2012年 | 12篇 |
2011年 | 32篇 |
2010年 | 8篇 |
2009年 | 3篇 |
2008年 | 27篇 |
2007年 | 21篇 |
2006年 | 22篇 |
2005年 | 26篇 |
2004年 | 18篇 |
2003年 | 23篇 |
2002年 | 12篇 |
2001年 | 11篇 |
2000年 | 11篇 |
1999年 | 2篇 |
1997年 | 2篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1977年 | 1篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1970年 | 5篇 |
1959年 | 5篇 |
1958年 | 7篇 |
1957年 | 1篇 |
1956年 | 3篇 |
1955年 | 6篇 |
1954年 | 4篇 |
1948年 | 11篇 |
1947年 | 2篇 |
排序方式: 共有322条查询结果,搜索用时 0 毫秒
171.
Many organs are composed of tubular networks that arise by branching morphogenesis in which cells bud from an epithelium and organize into a tube. Fibroblast growth factors (FGFs) and other signalling molecules have been shown to guide branch budding and outgrowth, but it is not known how epithelial cells coordinate their movements and morphogenesis. Here we use genetic mosaic analysis in Drosophila melanogaster to show that there are two functionally distinct classes of cells in budding tracheal branches: cells at the tip that respond directly to Branchless FGF and lead branch outgrowth, and trailing cells that receive a secondary signal to follow the lead cells and form a tube. These roles are not pre-specified; rather, there is competition between cells such that those with the highest FGF receptor activity take the lead positions, whereas those with less FGF receptor activity assume subsidiary positions and form the branch stalk. Competition appears to involve Notch-mediated lateral inhibition that prevents extra cells from assuming the lead. There may also be cooperation between budding cells, because in a mosaic epithelium, cells that cannot respond to the chemoattractant, or respond only poorly, allow other cells in the epithelium to move ahead of them. 相似文献
172.
173.
174.
175.
176.
Recent advances in the genetics of schizophrenia 总被引:13,自引:0,他引:13
Waterwort DM Bassett AS Brzustowicz LM 《Cellular and molecular life sciences : CMLS》2002,59(2):331-348
The genetic etiology of schizophrenia, a common and debilitating psychiatric disorder, is supported by a wealth of data.
Review of the current findings suggests that considerable progress has been made in recent years, with a number of chromosomal
regions consistently implicated by linkage analysis. Three groups have shown linkage to 1q21-22 using similar models, with
HLOD scores of 6.5, 3.2, and 2.4. Other replicated loci include 13q32 that has been implicated by two independent groups with
significant HLOD scores (4.42) or NPL values (4.18), and 5p14.1-13.1, 5q21-33, 8p21-22, and 10p11-15, each of which have been
reported as suggestive by at least three separate groups. Different studies have also replicated evidence for a modest number
of candidate genes that were not ascertained through linkage. Of these, the greatest support exists for the DRD3 (3q13.3),
HTR2A (13q14.2), and CHRNA7 (15q13-q14) genes. The refinement of phenotypes, the use of endophenotypes, reduction of heterogeneity,
and extensive genetic mapping have all contributed to this progress. The rapid expansion of information from the human genome
project will likely further accelerate this progress and assist in the discovery of susceptibility genes for schizophrenia.
A greater understanding of disease mechanisms and the application of pharmacogenetics should also lead to improvements in
therapeutic interventions.
Received 11 May 2001; received after revision: 20 July 2001; accepted 18 September 2001 相似文献
177.
Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models 总被引:10,自引:0,他引:10
Gurney KR Law RM Denning AS Rayner PJ Baker D Bousquet P Bruhwiler L Chen YH Ciais P Fan S Fung IY Gloor M Heimann M Higuchi K John J Maki T Maksyutov S Masarie K Peylin P Prather M Pak BC Randerson J Sarmiento J Taguchi S Takahashi T Yuen CW 《Nature》2002,415(6872):626-630
Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network within the tropics. 相似文献
178.
Testing Hamilton's rule with competition between relatives 总被引:18,自引:0,他引:18
Hamilton's theory of kin selection suggests that individuals should show less aggression, and more altruism, towards closer kin. Recent theoretical work has, however, suggested that competition between relatives can counteract kin selection for altruism. Unfortunately, factors that tend to increase the average relatedness of interacting individuals--such as limited dispersal--also tend to increase the amount of competition between relatives. Therefore, in most natural systems, the conflicting influences of increased competition and increased relatedness are confounded, limiting attempts to test theory. Fig wasp taxa exhibit varying levels of aggression among non-dispersing males that show a range of average relatedness levels. Thus, across species, the effects of relatedness and competition between relatives can be separated. Here we report that--contrary to Hamilton's original prediction but in agreement with recent theory--the level of fighting between males shows no correlation with the estimated relatedness of interacting males, but is negatively correlated with future mating opportunities. 相似文献
179.
Peptide bond formation is the principal reaction of protein synthesis. It takes place in the peptidyl transferase centre of the large (50S) ribosomal subunit. In the course of the reaction, the polypeptide is transferred from peptidyl transfer RNA to the alpha-amino group of amino acyl-tRNA. The crystallographic structure of the 50S subunit showed no proteins within 18 A from the active site, revealing peptidyl transferase as an RNA enzyme. Reported unique structural and biochemical features of the universally conserved adenine residue A2451 in 23S ribosomal RNA (Escherichia coli numbering) led to the proposal of a mechanism of rRNA catalysis that implicates this nucleotide as the principal catalytic residue. In vitro genetics allowed us to test the importance of A2451 for the overall rate of peptide bond formation. Here we report that large ribosomal subunits with mutated A2451 showed significant peptidyl transferase activity in several independent assays. Mutations at another nucleotide, G2447, which is essential to render catalytic properties to A2451 (refs 2, 3), also did not dramatically change the transpeptidation activity. As alterations of the putative catalytic residues do not severely affect the rate of peptidyl transfer the ribosome apparently promotes transpeptidation not through chemical catalysis, but by properly positioning the substrates of protein synthesis. 相似文献
180.