全文获取类型
收费全文 | 152篇 |
免费 | 1篇 |
国内免费 | 2篇 |
专业分类
理论与方法论 | 6篇 |
现状及发展 | 36篇 |
研究方法 | 26篇 |
综合类 | 77篇 |
自然研究 | 10篇 |
出版年
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 2篇 |
2017年 | 4篇 |
2016年 | 5篇 |
2015年 | 1篇 |
2012年 | 14篇 |
2011年 | 17篇 |
2010年 | 6篇 |
2008年 | 10篇 |
2007年 | 15篇 |
2006年 | 15篇 |
2005年 | 8篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 9篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1994年 | 1篇 |
1991年 | 4篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有155条查询结果,搜索用时 15 毫秒
91.
92.
Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency 总被引:3,自引:0,他引:3
Dimensionless numbers are important in biomechanics because their constancy can imply dynamic similarity between systems, despite possible differences in medium or scale. A dimensionless parameter that describes the tail or wing kinematics of swimming and flying animals is the Strouhal number, St = fA/U, which divides stroke frequency (f) and amplitude (A) by forward speed (U). St is known to govern a well-defined series of vortex growth and shedding regimes for airfoils undergoing pitching and heaving motions. Propulsive efficiency is high over a narrow range of St and usually peaks within the interval 0.2 < St < 0.4 (refs 3-8). Because natural selection is likely to tune animals for high propulsive efficiency, we expect it to constrain the range of St that animals use. This seems to be true for dolphins, sharks and bony fish, which swim at 0.2 < St < 0.4. Here we show that birds, bats and insects also converge on the same narrow range of St, but only when cruising. Tuning cruise kinematics to optimize St therefore seems to be a general principle of oscillatory lift-based propulsion. 相似文献
93.
McGregor L Makela V Darling SM Vrontou S Chalepakis G Roberts C Smart N Rutland P Prescott N Hopkins J Bentley E Shaw A Roberts E Mueller R Jadeja S Philip N Nelson J Francannet C Perez-Aytes A Megarbane A Kerr B Wainwright B Woolf AS Winter RM Scambler PJ 《Nature genetics》2003,34(2):203-208
Fraser syndrome (OMIM 219000) is a multisystem malformation usually comprising cryptophthalmos, syndactyly and renal defects. Here we report autozygosity mapping and show that the locus FS1 at chromosome 4q21 is associated with Fraser syndrome, although the condition is genetically heterogeneous. Mutation analysis identified five frameshift mutations in FRAS1, which encodes one member of a family of novel proteins related to an extracellular matrix (ECM) blastocoelar protein found in sea urchin. The FRAS1 protein contains a series of N-terminal cysteine-rich repeat motifs previously implicated in BMP metabolism, suggesting that it has a role in both structure and signal propagation in the ECM. It has been speculated that Fraser syndrome is a human equivalent of the blebbed phenotype in the mouse, which has been associated with mutations in at least five loci including bl. As mapping data were consistent with homology of FRAS1 and bl, we screened DNA from bl/bl mice and identified a premature termination of mouse Fras1. Thus, the bl mouse is a model for Fraser syndrome in humans, a disorder caused by disrupted epithelial integrity in utero. 相似文献
94.
Deficiency of Mbd2 suppresses intestinal tumorigenesis 总被引:16,自引:0,他引:16
Gene silencing through de novo methylation of CpG island promoters contributes to cancer. We find that Mbd2, which recruits co-repressor complexes to methylated DNA, is essential for efficient tumorigenesis in the mouse intestine. As Mbd2-deficient mice are viable and fertile, their resistance to intestinal cancer may be of therapeutic relevance. 相似文献
95.
Ron Wehrens Lutgarde M.C. Buydens Chris Fraley Adrian E. Raftery 《Journal of Classification》2004,21(2):231-253
The rapid increase in the size of data sets makes clustering all the more important
to capture and summarize the information, at the same time making clustering more
difficult to accomplish. If model-based clustering is applied directly to a large data set, it
can be too slow for practical application. A simple and common approach is to first cluster
a random sample of moderate size, and then use the clustering model found in this way
to classify the remainder of the objects. We show that, in its simplest form, this method
may lead to unstable results. Our experiments suggest that a stable method with better performance can be obtained with two straightforward modifications to the simple sampling
method: several tentative models are identified from the sample instead of just one, and
several EM steps are used rather than just one E step to classify the full data set. We find
that there are significant gains from increasing the size of the sample up to about 2,000,
but not from further increases. These conclusions are based on the application of several
alternative strategies to the segmentation of three different multispectral images, and to
several simulated data sets. 相似文献
96.
97.
Loss-of-function mutations in the EGF-CFC gene CFC1 are associated with human left-right laterality defects 总被引:4,自引:0,他引:4
Bamford RN Roessler E Burdine RD Saplakoğlu U dela Cruz J Splitt M Goodship JA Towbin J Bowers P Ferrero GB Marino B Schier AF Shen MM Muenke M Casey B 《Nature genetics》2000,26(3):365-369
All vertebrates display a characteristic asymmetry of internal organs with the cardiac apex, stomach and spleen towards the left, and the liver and gall bladder on the right. Left-right (L-R) axis abnormalities or laterality defects are common in humans (1 in 8,500 live births). Several genes (such as Nodal, Ebaf and Pitx2) have been implicated in L-R organ positioning in model organisms. In humans, relatively few genes have been associated with a small percentage of human situs defects. These include ZIC3 (ref. 5), LEFTB (formerly LEFTY2; ref. 6) and ACVR2B (encoding activin receptor IIB; ref. 7). The EGF-CFC genes, mouse Cfc1 (encoding the Cryptic protein; ref. 9) and zebrafish one-eyed pinhead (oep; refs 10, 11) are essential for the establishment of the L-R axis. EGF-CFC proteins act as co-factors for Nodal-related signals, which have also been implicated in L-R axis development. Here we identify loss-of-function mutations in human CFC1 (encoding the CRYPTIC protein) in patients with heterotaxic phenotypes (randomized organ positioning). The mutant proteins have aberrant cellular localization in transfected cells and are functionally defective in a zebrafish oep-mutant rescue assay. Our findings indicate that the essential role of EGF-CFC genes and Nodal signalling in left-right axis formation is conserved from fish to humans. Moreover, our results support a role for environmental and/or genetic modifiers in determining the ultimate phenotype in humans. 相似文献
98.
Normal mixture models are widely used for statistical modeling of data, including cluster analysis.
However maximum likelihood estimation (MLE) for normal mixtures using the EM algorithm may fail as the result of singularities
or degeneracies. To avoid this, we propose replacing the MLE by a maximum a posteriori (MAP) estimator, also found by the
EM algorithm. For choosing the number of components and the model parameterization, we propose a modified version of BIC,
where the likelihood is evaluated at the MAP instead of the MLE. We use a highly dispersed proper conjugate prior, containing
a small fraction of one observation's worth of information. The resulting method avoids degeneracies and singularities, but
when these are not present it gives similar results to the standard method using MLE, EM and BIC. 相似文献
99.
Chamero P Marton TF Logan DW Flanagan K Cruz JR Saghatelian A Cravatt BF Stowers L 《Nature》2007,450(7171):899-902
Mice use pheromones, compounds emitted and detected by members of the same species, as cues to regulate social behaviours such as pup suckling, aggression and mating. Neurons that detect pheromones are thought to reside in at least two separate organs within the nasal cavity: the vomeronasal organ (VNO) and the main olfactory epithelium (MOE). Each pheromone ligand is thought to activate a dedicated subset of these sensory neurons. However, the nature of the pheromone cues and the identity of the responding neurons that regulate specific social behaviours are largely unknown. Here we show, by direct activation of sensory neurons and analysis of behaviour, that at least two chemically distinct ligands are sufficient to promote male-male aggression and stimulate VNO neurons. We have purified and analysed one of these classes of ligand and found its specific aggression-promoting activity to be dependent on the presence of the protein component of the major urinary protein (MUP) complex, which is known to comprise specialized lipocalin proteins bound to small organic molecules. Using calcium imaging of dissociated vomeronasal neurons (VNs), we have determined that the MUP protein activates a sensory neuron subfamily characterized by the expression of the G-protein Galpha(o) subunit (also known as Gnao) and Vmn2r putative pheromone receptors (V2Rs). Genomic analysis indicates species-specific co-expansions of MUPs and V2Rs, as would be expected among pheromone-signalling components. Finally, we show that the aggressive behaviour induced by the MUPs occurs exclusively through VNO neuronal circuits. Our results substantiate the idea of MUP proteins as pheromone ligands that mediate male-male aggression through the accessory olfactory neural pathway. 相似文献
100.