全文获取类型
收费全文 | 3614篇 |
免费 | 254篇 |
国内免费 | 4篇 |
出版年
2023年 | 9篇 |
2022年 | 14篇 |
2021年 | 53篇 |
2020年 | 30篇 |
2019年 | 44篇 |
2018年 | 48篇 |
2017年 | 55篇 |
2016年 | 93篇 |
2015年 | 141篇 |
2014年 | 153篇 |
2013年 | 219篇 |
2012年 | 270篇 |
2011年 | 209篇 |
2010年 | 176篇 |
2009年 | 129篇 |
2008年 | 230篇 |
2007年 | 231篇 |
2006年 | 177篇 |
2005年 | 197篇 |
2004年 | 189篇 |
2003年 | 181篇 |
2002年 | 170篇 |
2001年 | 47篇 |
2000年 | 43篇 |
1999年 | 42篇 |
1998年 | 50篇 |
1997年 | 31篇 |
1996年 | 32篇 |
1995年 | 40篇 |
1994年 | 45篇 |
1993年 | 37篇 |
1992年 | 30篇 |
1991年 | 26篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 24篇 |
1987年 | 32篇 |
1986年 | 20篇 |
1985年 | 32篇 |
1984年 | 24篇 |
1983年 | 22篇 |
1982年 | 26篇 |
1981年 | 27篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1975年 | 17篇 |
1974年 | 13篇 |
1973年 | 8篇 |
排序方式: 共有3872条查询结果,搜索用时 296 毫秒
31.
Cysteamine (2-aminoethanethiol) inhibited cell division in synchronously dividing cultures of Euglena gracilis at relatively low concentrations (0.005 M), Cystamine (2,2′-dithiobis(ethylamine). however, was only partially inhibitory at high concentrations (0.1 M). This differential inhibition may reflect certain unique features of nuclear division in euglenoid flagellates. 相似文献
32.
33.
34.
Linker insertion analysis of the FimH adhesin of type 1 fimbriae in an Escherichia coli fimH-null background 总被引:2,自引:0,他引:2
Mark A. Schembri Lars Pallesen Hugh Connell David L. Hasty Per Klemm 《FEMS microbiology letters》1996,137(2-3):257-263
Abstract The gene encoding the Escherichia coli FimH adhesin of type 1 fimbriae has been subjected to linker insertion mutagenesis. Amino acid changes were introduced at a number of positions spanning the entire sequence in order to probe the structure-function relationship of the FimH protein. The effect of these mutations on the ability of bacteria to express a D-mannose binding phenotype was assessed in a fimH null mutant (MS4) constructed by allelic exchange in the E. coli K-12 strain PC31. Mutations mapping at amino acid residues 36, 58 and 279 of the mature FimH protein were shown to completely abolish binding to D-mannose receptors. Differences in the level of fimbriation were also observed as a result of some of the mutations in the fimH gene. These mutants may prove useful in dissecting receptor-ligand interactions by defining regions of the FimH protein that are important in erythrocyte binding. 相似文献
35.
The region encompassing theMa, Mb1, Mb2, andLmp2 genes of the mouse class II major histocompatibility complex (MHC) was sequenced. Since this region contains clusters of genes required for efficient class I and class II antigen presentation, it was interesting to search for putative additional genes in the 21 kilobase gap between theMb1 andLmp2 genes. Computer predictions of coding regions and CpG islands, exon trapping experiments, and cross-species comparison with the corresponding human sequence indicate that no additional functional gene is present in that stretch. However, computer analysis revealed the possible existence of an alternative 3 exon forMb1. Except for the fact that the mouse MHC contains twoMb genes, the genomic organization of theH2-M loci was found to be almost identical to the organization of the humanHLA-DM genes. The promoter regions of theMa andMb genes also resemble classical class II promoters, containing typical S, X, and Y boxes. Like the human genes, the threeH2-M genes displayed very limited polymorphism when we compared the cDNA sequences from six haplotypes. Finally, comparison ofDMB withMb1 andMb2, both at the genomic level and in their coding regions, suggests that theMb gene was recently duplicated, probably only in certain rodents. 相似文献
36.
Latitudinal gradients in species diversity are well established for a number of plant and animal taxa. Both historical and present-day environmental factors have been suggested to be responsible for observed patterns. We tested the hypothesis that current environmental features of the environment (primary productivity and regional landscape structure) may explain the longitudinal variation in species richness of mammalian herbivores in the Holarctic boreal zone. Mammalian herbivore species diversity was strongly correlated with a number of environmental variables measured. We reduced the data set by a principal components analysis (PCA), and found that in the Palearctic, species richness is positively related to warm climate (high temperature sum), the number of hardwood species, and the area of boreal forest. In the Nearctic, species richness increases as the length of the growing season and the number of coniferous tree species increase. Thus indirect measures of primary productivity as well as tree species number may accurately predict species richness in mammalian herbivores. In addition, there seems to be a strong species-area effect at the regional level. The larger and more homogeneous in terms of forest coverage a boreal section is, the more species coexist there. 相似文献
37.
Per Wretenberg Ulf P. Arborelius 《European journal of applied physiology and occupational physiology》1994,68(5):413-417
The aim of this study was to determine the power output and work done by different muscle groups at the hip and knee joints during a rising movement, to be able to tell the degree of activation of the muscle groups and the relationship between concentric and eccentric work. Nine healthy male subjects rose from a chair with the seat at knee level. The moments of force about the hip and knee joints were calculated semidynamically. The power output (P) and work in the different muscle groups surrounding the joints was calculated as moment of force times joint angular velocity. Work was calculated as: work = f Pdt. The mean peak concentric power output was for the hip extensors 49.9 W, hip flexors 7.9 W and knee extensor 89.5 W. This power output corresponded to a net concentric work of 20.7 J, 1.0 J and 55.6 J, respectively. There was no concentric power output from the knee flexor muscles. Energy absorption through eccentric muscle action was produced by the hip extensors and hip flexors with a mean peak power output of 4.8 W and 7.4 W, respectively. It was concluded that during rising, the hip and knee muscles mainly worked concentrically and that the greatest power output and work were produced during concentric contraction of the knee and hip extensor muscles. There was however also a demand for eccentric work by the hip extensors as well as both concentric and eccentric work by the hip flexors. The knee flexor muscles were unloaded. 相似文献
38.
Tova Rahn Martin Ridderstrle Hans Tornqvist Vincent Manganiello Gudrun Fredrikson Per Belfrage Eva Degerman 《FEBS letters》1994,350(2-3):314-318
Incubation of rat adipocytes with wortmannin, a potent and selective phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor, completely blocked the antilipolytic action of insulin (IC50≈ 100 nM), the insulin-induced activation and phosphorylation of cGMP-inhibited cAMP phosphodiesterase (cGI-PDE) as well as the activation of the insulin-stimulated cGI-PDE kinase (IC50≈ 10–30 nM). No direct effects of the inhibitor on the insulin-stimulated cGI-PDE kinase, the cGI-PDE and the hormone-sensitive lipase were observed. These data suggest that activation of PI 3-kinase upstream of the insulin-stimulated cGI-PDE kinase in the antilipolytic insulin signalchain has an essential role for insulin-induced cGI-PDE activation/ phosphorylation and anti-lipolysis. 相似文献
39.
New formulas for deriving the sensitivities of stable stage structures and reproductive values to changes in vital rates are
presented. They enable comparison of the sensities to changes of different elements in the projection matrix; in other words,
comparison of partial derivatives of the eigenvectors. These kinds of sensitivities can be used in applied problems such as
an analysis of the effect of harvesting on the population structure. However, in this paper, we examine the application of
the sensitivities in a more general ecological context. We investigate why the stable stage structure of the mustard aphid,Lipaphis erysimi, changes very little in the temperature interval 10–30°C. The sensitivities of the stable stage structure at 15°C and 25°C
were derived. The character of the sensitivites were the same in both temperatures although the stage structure was more sensitive
to changes at 15°C than at 25°C. The sensitivity analysis also revealed that the temperature variation results in changes
in fecundity and developmental rate that have a counteractive effect on the population structure. 相似文献
40.
Molecular characterization of Arabidopsis thaliana cDNAs encoding three purine biosynthetic enzymes 总被引:2,自引:0,他引:2
Kirk M. Schnorr Per Nygaard Michel Laloue 《The Plant journal : for cell and molecular biology》1994,6(1):113-121
Glycinamide ribonucleotide (GAR) synthetase, GAR transformylase and aminoimidazole ribonucleotide (AIR) synthetase are the second, third and fifth enzymes in the 10-step de novo purine biosynthetic pathway. From a cDNA library of Arabidopsis thaliana, cDNAs encoding the above three enzymes were cloned by functional complementation of corresponding Escherichia coli mutants. Each of the cDNAs encode peptides comprising the complete enzymatic domain of either GAR synthetase, GAR transformylase or AIR synthetase. Comparisons of the three Arabidopsis purine biosynthetic enzymes with corresponding enzymes/polypeptide-fragments from procaryotic and eucaryotic sources indicate a high degree of conserved homology at the amino acid level, in particular with procaryotic enzymes. Assays from extracts of E. coli expressing the complementing clones verified the specific enzymatic activity of Arabidopsis GAR synthetase and GAR transformylase. Sequence analysis, as well as Northern blot analysis indicate that Arabidopsis has single and monofunctional enzymes. In this respect the organization of these three plant purine biosynthesis genes is fundamentally different from the multifunctional purine biosynthesis enzymes characteristic of other eucaryotes and instead resembles the one gene, one enzyme relationship found in procaryotes. 相似文献