全文获取类型
收费全文 | 1541篇 |
免费 | 108篇 |
出版年
2022年 | 11篇 |
2021年 | 14篇 |
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 17篇 |
2017年 | 18篇 |
2016年 | 20篇 |
2015年 | 53篇 |
2014年 | 62篇 |
2013年 | 66篇 |
2012年 | 89篇 |
2011年 | 87篇 |
2010年 | 69篇 |
2009年 | 46篇 |
2008年 | 58篇 |
2007年 | 82篇 |
2006年 | 62篇 |
2005年 | 67篇 |
2004年 | 49篇 |
2003年 | 71篇 |
2002年 | 72篇 |
2001年 | 12篇 |
2000年 | 9篇 |
1999年 | 12篇 |
1998年 | 19篇 |
1997年 | 13篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1988年 | 9篇 |
1986年 | 14篇 |
1985年 | 9篇 |
1984年 | 19篇 |
1983年 | 19篇 |
1982年 | 19篇 |
1980年 | 15篇 |
1979年 | 12篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1976年 | 11篇 |
1974年 | 11篇 |
1973年 | 10篇 |
1972年 | 10篇 |
1971年 | 10篇 |
1970年 | 18篇 |
1968年 | 17篇 |
1967年 | 10篇 |
1941年 | 8篇 |
1937年 | 13篇 |
1934年 | 9篇 |
1930年 | 9篇 |
排序方式: 共有1649条查询结果,搜索用时 15 毫秒
91.
92.
Hirth F Kammermeier L Frei E Walldorf U Noll M Reichert H 《Development (Cambridge, England)》2003,130(11):2365-2373
Studies on expression and function of key developmental control genes suggest that the embryonic vertebrate brain has a tripartite ground plan that consists of a forebrain/midbrain, a hindbrain and an intervening midbrain/hindbrain boundary region, which are characterized by the specific expression of the Otx, Hox and Pax2/5/8 genes, respectively. We show that the embryonic brain of the fruitfly Drosophila melanogaster expresses all three sets of homologous genes in a similar tripartite pattern. Thus, a Pax2/5/8 expression domain is located at the interface of brain-specific otd/Otx2 and unpg/Gbx2 expression domains anterior to Hox expression regions. We identify this territory as the deutocerebral/tritocerebral boundary region in the embryonic Drosophila brain. Mutational inactivation of otd/Otx2 and unpg/Gbx2 result in the loss or misplacement of the brain-specific expression domains of Pax2/5/8 and Hox genes. In addition, otd/Otx2 and unpg/Gbx2 appear to negatively regulate each other at the interface of their brain-specific expression domains. Our studies demonstrate that the deutocerebral/tritocerebral boundary region in the embryonic Drosophila brain displays developmental genetic features similar to those observed for the midbrain/hindbrain boundary region in vertebrate brain development. This suggests that a tripartite organization of the embryonic brain was already established in the last common urbilaterian ancestor of protostomes and deuterostomes. 相似文献
93.
We have developed a new strategy designated SIMF (Systematic Insertional Mutagenesis of Families), to identify DNA insertions
in many members of a gene family simultaneously. This method requires only a short amino acid sequence conserved in all members
of the family to make a degenerate oligonucleotide, and a sequence from the end of the DNA insertion. The SIMF strategy was
successfully applied to the large maize R2R3 Myb family of regulatory genes, and Mutator insertions in several novel Myb genes were identified. Application of this technique to identify insertions in other large gene families could significantly
decrease the effort involved in screening at the same time for insertions in all members of groups of genes that share a limited
sequence identity.
Received: 1 June 2000 / Accepted: 26 June 2000 相似文献
94.
When far red light preincubated cells of Euglena gracilis are transferred to dark or light, chlorophyll fluorescence (F0 and Fm) decreases. Non-photochemical quenching in the dark is suggested to be induced partly by chlororespiration and partly by
changes in the distribution of excitation energy between the photosystems. Depending on the light intensities it was possible
to resolve the non-photochemical quenching into at least three different components. The slowest relaxation phase of non-photochemical
quenching occurred only after exposure to high light and was assigned to photoinhibition. The other two components were an
energy-dependent quenching (qE), and the one which we attribute to a spill over mechanism. We suggest that both photosystems
use a common antenna system consisting of LHC I and LHC II proteins. In contrast to higher plants, qE in Euglena gracilis is independent of the xanthophyll cycle and an aggregation of LHC II.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
95.
Determination of nifedipine in human plasma by flow-injection tandem mass spectrometry 总被引:1,自引:0,他引:1
Jan Dankers Jos van den Elshout Gertrude Ahr Erich Brendel Cees van der Heiden 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1998,710(1-2):115-120
For use in clinical studies, a fast and sensitive assay method was developed for the determination of nifedipine in human plasma samples. The assay method is based on tandem mass spectrometry detection (HPLC–MS–MS). The effect of flow injection as well as HPLC separation on the results of the nifedipine determination were evaluated. The limit of quantification is 0.5 ng/ml and the accuracy (as determined by spiking recovery) was found to be good. 相似文献
96.
Martin Roos Ernst Pittenauer Erich Schmid Manfred Beyer Bernhard Reinike Günter Allmaier Harald Labischinski 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1998,705(2):M499
Reversed-phase high-performance liquid chromatography (RP-HPLC) of muropeptides, obtained by muramidase digestion of peptidoglycan in combination with amino acid analysis and plasma desorption time-of-flight mass spectrometry is today by far the best tool to analyze the fine structure of the peptidoglycans. Here we report further improvements of the RP-HPLC separation of muropeptides for analyzing the peptidoglycans of various methicillin-resistant strains of Staphylococcus aureus, with emphasis on a more detailed characterization of the interpeptide bridge of the peptidoglycans of this species. 相似文献
97.
Relationship between T-wave amplitude and oxygen pulse in guinea pigs in hyperbaric helium and hydrogen 总被引:1,自引:0,他引:1
Kayar Susan R.; Parker Erich C.; Aukhert Eugenia O. 《Journal of applied physiology》1998,85(3):798-806
Diving isknown to induce a change in the amplitude of the T wave(ATw) ofelectrocardiograms, but it is unknown whether this is linked to achange in cardiovascular performance. We analyzed ATw in guinea pigs at 10-60atm and 25-36°C, breathing 2%O2 in either helium (heliox;n = 10) or hydrogen (hydrox;n = 9) for 1 h at each pressure. Coretemperature and electrocardiograms were detected by using implantedradiotelemeters. O2 consumption rate was measured by using gas chromatography. In a previous study (S. R. Kayar and E. C. Parker. J. Appl.Physiol. 82: 988-997, 1997), we analyzed theO2 pulse, i.e., theO2 consumption rate per heartbeat, in the same animals. By multivariate regression analysis, weidentified variables that were significant toO2 pulse: body surface area,chamber temperature, core temperature, and pressure. In this study,inclusion of ATw made asignificantly better model with fewer variables. After normalizing forchamber temperature and pressure, theO2 pulse increased with increasing ATw in heliox(P = 0.001) but with decreasingATw in hydrox(P < 0.001). ThusATw is associated with thedifferences in O2 pulse foranimals breathing heliox vs. hydrox. 相似文献
98.
99.
Philine G. D. Feulner Frédéric J. J. Chain Mahesh Panchal Yun Huang Christophe Eizaguirre Martin Kalbe Tobias L. Lenz Irene E. Samonte Monika Stoll Erich Bornberg-Bauer Thorsten B. H. Reusch Manfred Milinski 《PLoS genetics》2015,11(2)
The patterns of genomic divergence during ecological speciation are shaped by a combination of evolutionary forces. Processes such as genetic drift, local reduction of gene flow around genes causing reproductive isolation, hitchhiking around selected variants, variation in recombination and mutation rates are all factors that can contribute to the heterogeneity of genomic divergence. On the basis of 60 fully sequenced three-spined stickleback genomes, we explore these different mechanisms explaining the heterogeneity of genomic divergence across five parapatric lake and river population pairs varying in their degree of genetic differentiation. We find that divergent regions of the genome are mostly specific for each population pair, while their size and abundance are not correlated with the extent of genome-wide population differentiation. In each pair-wise comparison, an analysis of allele frequency spectra reveals that 25–55% of the divergent regions are consistent with a local restriction of gene flow. Another large proportion of divergent regions (38–75%) appears to be mainly shaped by hitchhiking effects around positively selected variants. We provide empirical evidence that alternative mechanisms determining the evolution of genomic patterns of divergence are not mutually exclusive, but rather act in concert to shape the genome during population differentiation, a first necessary step towards ecological speciation. 相似文献
100.