全文获取类型
收费全文 | 281篇 |
免费 | 33篇 |
国内免费 | 32篇 |
专业分类
346篇 |
出版年
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 6篇 |
2019年 | 4篇 |
2018年 | 9篇 |
2017年 | 6篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 9篇 |
2013年 | 12篇 |
2012年 | 17篇 |
2011年 | 21篇 |
2010年 | 17篇 |
2009年 | 17篇 |
2008年 | 12篇 |
2007年 | 8篇 |
2006年 | 21篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 7篇 |
2002年 | 7篇 |
2001年 | 8篇 |
2000年 | 3篇 |
1999年 | 6篇 |
1998年 | 13篇 |
1997年 | 9篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1992年 | 7篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1971年 | 2篇 |
1963年 | 2篇 |
1959年 | 2篇 |
1958年 | 3篇 |
1957年 | 2篇 |
1955年 | 3篇 |
1953年 | 1篇 |
1952年 | 1篇 |
1949年 | 5篇 |
1948年 | 2篇 |
排序方式: 共有346条查询结果,搜索用时 8 毫秒
101.
102.
Effect of Ethidium Bromide on the Oxidative Metabolism and Enzyme Profiles of Crithidia fasciculata*
SYNOPSIS. Changes in the metabolism of Crithidia fasciculata ATCC 11745 when grown in the presence of ethidium bromide were studied. Ethidium bromide-grown cells had decreased respiratory activity as measured by oxygen consumption. More than 50% of the organisms cultivated in a defined medium containing 1.0 mg/liter of ethidium bromide became dyskine-toplastic and had decreased activities of particulate succinate and NADH-linked dehydrogenases as well as of soluble isocitrate dehydrogenase. These cells also had increased activities of particulate α-glycerophosphate dehydrogenase, soluble α-glycerophosphate dehydrogenase, malic enzyme, hexokinase, and malate dehydrogenase. Ethidium bromide-grown cells had a lower level of ATP and contained less DNA than cells grown in its absence. 相似文献
103.
Hierarchical genetic structure and gene flow in three sympatric species of Amazonian rodents 总被引:2,自引:0,他引:2
The population genetic structure of three species of Amazonian rodents ( Oligoryzomys microtis, Oryzomys capito , and Mesomys hispidus ) is examined for mtDNA sequence haplotypes of the cytochrome b gene by hierarchical analysis of variance and gene flow estimates based on fixation indices ( N ST ) and coalescence methods. Species samples are from the same localities along 1000 km of the Rio Juruá in western Amazonian Brazil, but each species differs in important life history traits such as population size and reproductive rate. Average haplotype differentiation, hierarchical haplotype apportionment, and gene flow estimates are contrasted in discussing the current and past population structure. Two species exhibit isolation by distance patterns wherein gene flow is largely limited to geographically adjacent localities. Mesomys exhibits this pattern throughout its range along the river. More than 75% of haplotype variation is apportioned among localities and regions, and estimates of Nm for pair-wise comparisons are nearly always less than 1. Oligoryzomys shows weak isolation by distance, but only over the largest geographical distances. Nm values for this species are nearly always above 1 and most (about 80%) of haplotype variation is contained within local populations. In contrast, Oryzomys exhibits no genetic structure throughout its entire distribution; Nm values average 17 and nearly 90% of the total haplotype variance is contained within local populations. Although gene flow estimates are high, the pattern of Nm as a function of geographical distance suggests that this species experienced a more recent invasion of the region and is still in genetic disequilibrium under its current demographic conditions. 相似文献
104.
Gram-Chromotrope: a New Technique that Enhances Detection of Microsporidial Spores in Clinical Samples 总被引:3,自引:0,他引:3
HERCULES MOURA JORGE L. NUNES DA SILVA FERNANDO C. SODRÉ PATRICIA BRASIL KRISTY WALLMO SUSANNE WAHLQUIST SARA WALLACE GIAN P. CROPPO GOVINDA S. VISVESVARA 《The Journal of eukaryotic microbiology》1996,43(5):94S-95S
We have developed a new staining procedure that combines the traditional Gram staining for bacteria and the Weber's chromotrope staining method, the standard technique for the detection of microsporidia spores in clinical Specimens. This “Gram-chromotrope” staining technique enhances the staining characteristics of microsporidia spores and facilitates the easy detection and differentiation of spores from other microorganisms that are found in clinical specimens, especially stool samples. This new technique is fast, reliable, and simple to perform, and can be easily adapted for use in clinical laboratories. 相似文献
105.
POLYANNA DA CONCEIÇÃO BISPO MÁRCIO DE MORISSON VALERIANO JOÃO ROBERTO DOS SANTOS 《Austral ecology》2012,37(4):491-499
The objective of this study was to identify the effects of local geomorphometry on the abundance, richness and floristic composition of tree species in the central Brazilian Amazon. Forty‐six 0.25‐ha plots in different phyto‐ecologic sites were sampled, and their trees were inventoried. Geomorphometric data (elevation, slope, aspect, plan and profile curvatures) were derived from Shuttle Radar Topography Mission data. A detrended correspondence analysis (DCA) was used to examine the floristic distribution patterns among plots. In addition, geomorphometric variables were submitted to multiple regression analysis to identify the variables influencing floristic composition (represented by the first DCA component), abundance and species richness. Correlation analyses between the number of individuals from each species and the first DCA component were performed to evaluate the contribution of each species. Analysis of the results could not confirm an effect of geomorphometry alone on species richness and abundance, although floristic composition was significantly influenced by profile curvature and elevation. Despite the relatively low variation in altitude at the study site, species were found to be sensitive to terrain peculiarities such as elevation and profile curvature, which can constrain particular ecologic niches and contribute to the spatial distribution patterns of species. 相似文献
106.
用RACE技术扩增并克隆牛BMP4基因3'端序列 总被引:4,自引:1,他引:4
RACE技术是一项扩增基因末端序列的新技术。该研究从牛BMP4基因出发,以牛软骨的RNA为模板,按照不同物种BMP4基因的相似性设计特异引物,运用PCR和RACE技术扩增并获得了特异片段,该片段经PCR、酶切和测序验证,证实所克隆序列为牛BMP4的3′端序列,包含有1170bp组成的开放读码框(ORF),编码389个氨基酸,3′非编码区121bp个核苷酸和poly(A)15。同源性分析结果表明,牛BMP4 cDNA最大开放读码框所推测的氨基酸序列与已知人、小鼠、大鼠、狗、羊和鸡等真核生物BMP4氨基酸序列进行比较,分别有94.5%、93.1%、91.9%、87.4%、94.2%、79%的同源性。这为克隆其他物种的BMP4基因提供了依据,同时牛骨形态发生蛋白的测序为我们更好的理解牛的生骨机理提供帮助。 相似文献
107.
The originality of a species is how much that species contributes to the rarity of traits in a community. Here we tested the relation between abundance and both phylogenetic and phenotypic originality. We measured nine traits associated with defence against herbivory, as well as phylogenetic information and abundance for woody plant species in a woodland cerrado in southeastern Brazil. About 90% of the species accounted for about 50% of the phylogenetic and phenotypic originality: most woody species had low originality. Abundance was related to tougher leaves, lower specific leaf area and lower originality based on nutritional quality. Our results suggest that herbivory may reduce the abundance of species with low resistance to herbivory and with different nutritional quality. Nevertheless, abundance was not related to either phylogenetic or phenotypic originality, so extinction of rare species may not endanger overall community function as long as more abundant species are retained. We argue that this is a consequence of the low complementarity of a large number of woody species. 相似文献
108.
细胞外三磷酸腺苷(extracellular adenosine-5'-triphosphate)是植物细胞的重要信号分子。以烟草悬浮细胞BY-2(Nicotiana tabacum L.cv.Bright Yellow-2)为材料,探讨了胞外三磷酸腺苷对铅胁迫下细胞损伤、H2O2(过氧化氢)含量及H2O2清除酶活性的影响。结果显示,随着Pb(NO3)2浓度的不断提高(30~400 μmol·L-1),细胞外三磷酸腺苷含量呈现出逐渐下降的趋势,但胞内三磷酸腺苷含量及细胞的受损伤程度逐渐增大;同时,H2O2含量和过氧化氢酶的活性均有所上升,并在200 μmol·L-1 Pb(NO3)2处理下达到最大值,而过氧化物酶的活性则不断降低。较之Pb(NO3)2胁迫下的细胞,对Pb(NO3)2胁迫的细胞加入外源三磷酸腺苷使得细胞受损伤程度显著降低,H2O2含量减少,过氧化氢酶活性减弱,而过氧化物酶活性增强。实验结果表明,Pb(NO3)2胁迫诱导的植物细胞损伤和H2O2及其清除酶水平的变化能受到细胞外三磷酸腺苷水平的调节。 相似文献
109.
110.
Evolution of the primary and secondary structures of the E1a mRNAs of the adenovirus 总被引:1,自引:0,他引:1
In this paper we investigate and compare (evolutionary) patterns in the
primary and secondary structure of four homologous E1a mRNAs of the
adenovirus. Our main results are as follows: (1) The similarity of the
coding regions of the mRNA sequences reflects both similarity in function
(i.e., oncogenicity) and evolutionary divergence. (2) The similarity of the
leader and the trailer regions reflects host specificity (i.e., human or
simian) and must therefore arise from convergence. (3) Minimal energy
foldings of the mRNAs show similar secondary structures (in particular
around the splice sites). The conservation of pre-mRNA secondary structure
shows that mRNAs are subject to selection constraints in addition to those
associated with proteins. (4) The conserved secondary (helical) structures
consist of nonhomologous subsequences, i.e., shifts have occurred. The
observed shifts near the splice sites seem to be the simplest way of
dealing with the dual constraints.
相似文献