全文获取类型
收费全文 | 291篇 |
免费 | 20篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 6篇 |
2019年 | 4篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 10篇 |
2015年 | 16篇 |
2014年 | 18篇 |
2013年 | 17篇 |
2012年 | 32篇 |
2011年 | 21篇 |
2010年 | 16篇 |
2009年 | 15篇 |
2008年 | 12篇 |
2007年 | 13篇 |
2006年 | 8篇 |
2005年 | 8篇 |
2004年 | 9篇 |
2003年 | 11篇 |
2002年 | 16篇 |
2001年 | 6篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1941年 | 1篇 |
排序方式: 共有311条查询结果,搜索用时 234 毫秒
1.
Aim We evaluate how closely diversity patterns of endemic species of vascular plants, beetles, butterflies, molluscs and spiders are correlated with each other, and to what extent similar environmental requirements or survival in common glacial refugia and comparable dispersal limitations account for their existing congruence. Location Austria. Methods We calculated pairwise correlations among species numbers of the five taxonomic groups in 1405 cells of a 3′ × 5′ raster (c. 35 km2) using the raw data as well as the residuals of regression models that accounted for: (1) environmental variables, (2) environmental variables and the occurrence of potential refugia during the Last Glacial Maximum, or (3) environmental variables, refugia and spatial filters. Results Pairwise cross‐taxonomic group Spearman’s rank correlations in the raw data were significantly positive in most cases, but only moderate (0.3 < ρ < 0.5) to weak (ρ < 0.3) throughout. Correlations were closest between plants and beetles, plants and butterflies, and plants and snails, respectively, whereas the distribution of endemic spiders was largely uncorrelated with those of the other groups. Environmental variables explained only a moderate proportion of the variance in endemic richness patterns, and the response of individual groups to environmental gradients was only partly consistent. The inclusion of refugium locations and the spatial filters increased the goodness of model fit for all five taxonomic groups. Moreover, removing the effects of environmental conditions reduced congruence in endemic richness patterns to a lesser extent than did filtering the influence of refugium locations and spatial autocorrelation, except for spiders, which are probably the least dispersal‐limited of the five groups. Main conclusions The moderate to weak congruence of endemic richness patterns clearly limits the usefulness of a surrogacy approach for designating areas for the protection of regional endemics. On the other hand, our results suggest that dispersal limitations still shape the distributions of many endemic plant, snail, beetle and butterfly species, even at the regional scale; that is, survival in shared refugia and subsequent restricted spread retain a detectable signal in existing correlations. Concentrating conservation efforts on well‐known Pleistocene refugia hence appears to be a reasonable first step towards a strategy for protecting regional endemics of at least the less mobile invertebrate groups. 相似文献
2.
Odontophrynus americanus (Amphibia, Anura) genomic DNA from diploid and tetraploid specimens was treated with restriction enzymes sensitive to cytosine and adenine methylation (5 meC and 6 meA). In both diploids and tetraploids a high proportion of the total DNA was not cleaved by 5 meC-sensitive enzymes as observed on agarose gels stained with ethidium bromide. The DNAs were transferred to nitrocellulose filters and hybridized with cloned fragments containing sequences of Xenopus laevis 28S and 18S ribosomal DNA (rDNA). A high level of methylation of the ribosomal repeat units was revealed by 5 meC-sensitive enzymes in blood, liver, kidney and testis tissues. Adenine was methylated to a lesser degree and similarly in the rDNA from both germinative and somatic tissues. Comparison of the results obtained with DNA of diploids and tetraploids showed that methylation of ribosomal genes was increased in tetraploid genomes of adult frogs, but exact quantitative determinations could not be performed by this methodology. Cloning of the 28S region of the rDNA repeat unit was performed in the gtWESC vector. Restriction patterns obtained with methylation-sensitive enzymes using diploid and tetraploid derived clones confirmed the high level of methylation of the corresponding region of the ribosomal repeat unit in genomic DNAs. The implications of these results in the regulation of expression of the ribosomal genes in diploids and tetraploids are discussed. 相似文献
3.
Itamar B. Abrass 《The Western journal of medicine》1990,153(6):641-645
4.
Ariela Baruch Moshe Shani David R. Hurwitz Itamar Barash 《Genesis (New York, N.Y. : 2000)》1995,16(3):241-252
5.
May J. Reed Robert B. Vernon Itamar B. Abrass E. Helene Sage 《Journal of cellular physiology》1994,158(1):169-179
Fibroblasts have a major role in the synthesis and reorganization of extracellular matrix that occur during wound repair. An impaired biosynthetic or functional response of these cells to stimulation by growth factors might contribute to the delayed wound healing noted in aging. We, therefore, compared the responses of dermal fibroblasts from young and elderly individuals (26, 29, 65, 89, 90, and 92 years of age) to transforming growth factor-β1 (TGF-β1) with respect to: (1) the synthesis of type I collagen and SPARC (two extracellular matrix proteins that are highly expressed by dermal fibroblasts during the remodeling phase of wound repair) and (2) the contraction of collagen gels, an in vitro assay of wound contraction. With the exception of one young donor, all cultures exposed for 44 hours to 10 ng/ml TGF-β1 exhibited a 1.6- to 5.5-fold increase in the levels of secreted type 1 collagen and SPARC, relative to untreated cultures, and exhibited a 2.0- to 6.2-fold increase in the amounts of the corresponding mRNAs. Moreover, the dose-response to TGF-β1 (0.1–10 ng/ml), as determined by synthesis of type I collagen and SPARC mRNA, was as vigorous in cells from aged donors as in cells from a young donor. In assays of collagen gel contraction, fibroblasts from all donors were stimulated to a similar degree by 10 ng/ml TGF-β1. In conclusion, cells from both young and aged donors exhibited similar biosynthetic and contractile properties with exposure to TGF-β1. It therefore appears that the impaired wound healing noted in the aged does not result from a failure of their dermal fibroblasts to respond to this cytokine. © 1994 Wiley-Liss, Inc. 相似文献
6.
Itamar Barash Alexander Faerman Tamar Ratovitsky Raisa Puzis Margaret Nathan David R. Hurwitz Moshe Shani 《Transgenic research》1994,3(3):141-151
We produced transgenic mice carrying the native sheep -lactoglobulin (BLG) or fusion genes composed of the BLG promoter and human serum albumin (HSA) minigenes. BLG was expressed exclusively in the mammary glands of the virgin and lactating transgenic mice evaluated. In contrast, transgenic females carrying the BLG/HSA fusion constructs also expressed the HSA RNA ectopically in skeletal muscle, kidney, brain, spleen, salivary gland and skin. Ectopic expression of HSA RNA was detected only in strains that express the transgene in the mammary gland. There was no obvious correlation between the level of the HSA RNA expressed in the mammary gland and that found ectopically. In three transgenic strains analysed, the expression of HSA RNA in kidney and skeletal muscle increased during pregnancy and lactation, whereas in the brain HSA expression decreased during lactation in one of the strains. HSA protein was synthesized in skeletal muscle and skin of strain #23 and its level was higher in lactating mice compared with virgin mice. Expression of HSA was also analysed in males and was found to be more stringently controlled than in females of the same strains.In situ hybridization analyses localized the expressed transgene in the skin, kidney, brain and salivary glands of various transgenic strains. Distinct strain-specific and cell-type specific HSA expression patterns were observed in the skin. This is in contrast to the exclusive expression of the HSA transgene in epithelial cells surrounding the alveoli of the mammary gland. Taken together, these results suggest that the absence of sufficient mammary-specific regulatory elements in the BLG promoter sequences and/or the juxtaposition of the BLG promoter with the HSA coding sequences leads to novel tissue- and cell-specific expression in ectopic tissues of transgenic mice. 相似文献
7.
8.
9.
Ziv Bar-Joseph Georg K Gerber David K Gifford Tommi S Jaakkola Itamar Simon 《Journal of computational biology》2003,10(3-4):341-356
We present algorithms for time-series gene expression analysis that permit the principled estimation of unobserved time points, clustering, and dataset alignment. Each expression profile is modeled as a cubic spline (piecewise polynomial) that is estimated from the observed data and every time point influences the overall smooth expression curve. We constrain the spline coefficients of genes in the same class to have similar expression patterns, while also allowing for gene specific parameters. We show that unobserved time points can be reconstructed using our method with 10-15% less error when compared to previous best methods. Our clustering algorithm operates directly on the continuous representations of gene expression profiles, and we demonstrate that this is particularly effective when applied to nonuniformly sampled data. Our continuous alignment algorithm also avoids difficulties encountered by discrete approaches. In particular, our method allows for control of the number of degrees of freedom of the warp through the specification of parameterized functions, which helps to avoid overfitting. We demonstrate that our algorithm produces stable low-error alignments on real expression data and further show a specific application to yeast knock-out data that produces biologically meaningful results. 相似文献
10.
Barry E. Willner Chien-Ping Lu Willard L. Miranker 《Journal of mathematical biology》1995,33(8):829-866
Hebbian dynamics is used to derive the differential equations for the synaptic strengths in the neural circuitry of the locomotive oscillator. Initially, neural connection are random. Under a specified arborization hypothesis relating to the density of neural connections, the differential equations are shown to model the self-organization and the stability of the oscillator. 相似文献