全文获取类型
收费全文 | 7854篇 |
免费 | 653篇 |
国内免费 | 1篇 |
专业分类
8508篇 |
出版年
2023年 | 33篇 |
2022年 | 87篇 |
2021年 | 176篇 |
2020年 | 77篇 |
2019年 | 116篇 |
2018年 | 132篇 |
2017年 | 114篇 |
2016年 | 246篇 |
2015年 | 374篇 |
2014年 | 445篇 |
2013年 | 514篇 |
2012年 | 667篇 |
2011年 | 649篇 |
2010年 | 450篇 |
2009年 | 434篇 |
2008年 | 531篇 |
2007年 | 524篇 |
2006年 | 466篇 |
2005年 | 458篇 |
2004年 | 433篇 |
2003年 | 412篇 |
2002年 | 366篇 |
2001年 | 67篇 |
2000年 | 46篇 |
1999年 | 64篇 |
1998年 | 80篇 |
1997年 | 59篇 |
1996年 | 51篇 |
1995年 | 40篇 |
1994年 | 49篇 |
1993年 | 52篇 |
1992年 | 31篇 |
1991年 | 34篇 |
1990年 | 22篇 |
1989年 | 23篇 |
1988年 | 25篇 |
1987年 | 16篇 |
1986年 | 17篇 |
1985年 | 13篇 |
1984年 | 8篇 |
1983年 | 17篇 |
1982年 | 8篇 |
1981年 | 15篇 |
1980年 | 6篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1972年 | 3篇 |
排序方式: 共有8508条查询结果,搜索用时 0 毫秒
101.
Dana M. Blumenthal Julie A. Kray William Ortmans Lewis H. Ziska Elise Pendall 《Global Change Biology》2016,22(9):3026-3038
Elevated CO2 and warming may alter terrestrial ecosystems by promoting invasive plants with strong community and ecosystem impacts. Invasive plant responses to elevated CO2 and warming are difficult to predict, however, because of the many mechanisms involved, including modification of phenology, physiology, and cycling of nitrogen and water. Understanding the relative and interactive importance of these processes requires multifactor experiments under realistic field conditions. Here, we test how free‐air CO2 enrichment (to 600 ppmv) and infrared warming (+1.5 °C day/3 °C night) influence a functionally and phenologically distinct invasive plant in semi‐arid mixed‐grass prairie. Bromus tectorum (cheatgrass), a fast‐growing Eurasian winter annual grass, increases fire frequency and reduces biological diversity across millions of hectares in western North America. Across 2 years, we found that warming more than tripled B. tectorum biomass and seed production, due to a combination of increased recruitment and increased growth. These results were observed with and without competition from native species, under wet and dry conditions (corresponding with tenfold differences in B. tectorum biomass), and despite the fact that warming reduced soil water. In contrast, elevated CO2 had little effect on B. tectorum invasion or soil water, while reducing soil and plant nitrogen (N). We conclude that (1) warming may expand B. tectorum's phenological niche, allowing it to more successfully colonize the extensive, invasion‐resistant northern mixed‐grass prairie, and (2) in ecosystems where elevated CO2 decreases N availability, CO2 may have limited effects on B. tectorum and other nitrophilic invasive species. 相似文献
102.
Vanessa Julie Roden Imelda M. Hausmann Alexander Nützel Barbara Seuss Mike Reich Max Urlichs Hans Hagdorn Wolfgang Kiessling 《Palaeontology》2020,63(1):85-102
With 1429 animal species, the Triassic Cassian Formation in the Dolomites, Southern Alps (Italy), yields the highest species richness reported from any spatially constrained pre-Quaternary formation known to science. The high preserved diversity is partly attributable to a high primary diversity governed by the tropical setting, increasing alpha diversity, and the breadth of habitats spurring beta diversity. More important is the excellent preservation of fossils and the ease with which they can be extracted from the poorly lithified sediments. We propose the term ‘liberation Lagerstätten’ to capture this preservational window. In contrast to conservation Lagerstätten, liberation Lagerstätten like the Cassian Formation originate from normal marine conditions but low-grade diagenesis. Molluscs contribute substantially to species richness, comprising 67% of all invertebrate species in the Cassian Formation. The gastropod dominance (39% of all species) is nearly as great as in Recent tropical settings, contradicting the concept of a substantial Cenozoic rise. 相似文献
103.
An obligate intermediate along the slow folding pathway of a group II intron ribozyme 总被引:4,自引:1,他引:4
Most RNA molecules collapse rapidly and reach the native state through a pathway that contains numerous traps and unproductive intermediates. The D135 group II intron ribozyme is unusual in that it can fold slowly and directly to the native state, despite its large size and structural complexity. Here we use hydroxyl radical footprinting and native gel analysis to monitor the timescale of tertiary structure collapse and to detect the presence of obligate intermediates along the folding pathway of D135. We find that structural collapse and native folding of Domain 1 precede assembly of the entire ribozyme, indicating that D1 contains an on-pathway intermediate to folding of the D135 ribozyme. Subsequent docking of Domains 3 and 5, for which D1 provides a preorganized scaffold, appears to be very fast and independent of one another. In contrast to other RNAs, the D135 ribozyme undergoes slow tertiary collapse to a compacted state, with a rate constant that is also limited by the formation D1. These findings provide a new paradigm for RNA folding and they underscore the diversity of RNA biophysical behaviors. 相似文献
104.
105.
Diane C. Saunders Marcela Brissova Neil Phillips Shristi Shrestha John T. Walker Radhika Aramandla Greg Poffenberger David K. Flaherty Kevin P. Weller Julie Pelletier Tracy Cooper Matt T. Goff John Virostko Alena Shostak E. Danielle Dean Dale L. Greiner Leonard D. Shultz Nripesh Prasad Alvin C. Powers 《Cell metabolism》2019,29(3):745-754.e4
106.
Vaulot D Lepère C Toulza E De la Iglesia R Poulain J Gaboyer F Moreau H Vandepoele K Ulloa O Gavory F Piganeau G 《PloS one》2012,7(6):e39648
Among small photosynthetic eukaryotes that play a key role in oceanic food webs, picoplanktonic Mamiellophyceae such as Bathycoccus, Micromonas, and Ostreococcus are particularly important in coastal regions. By using a combination of cell sorting by flow cytometry, whole genome amplification (WGA), and 454 pyrosequencing, we obtained metagenomic data for two natural picophytoplankton populations from the coastal upwelling waters off central Chile. About 60% of the reads of each sample could be mapped to the genome of Bathycoccus strain from the Mediterranean Sea (RCC1105), representing a total of 9 Mbp (sample T142) and 13 Mbp (sample T149) of non-redundant Bathycoccus genome sequences. WGA did not amplify all regions uniformly, resulting in unequal coverage along a given chromosome and between chromosomes. The identity at the DNA level between the metagenomes and the cultured genome was very high (96.3% identical bases for the three larger chromosomes over a 360 kbp alignment). At least two to three different genotypes seemed to be present in each natural sample based on read mapping to Bathycoccus RCC1105 genome. 相似文献
107.
Julie A. Woods John A. Hadfield Alan T. McGown Brian W. Fox 《Bioorganic & medicinal chemistry》1993,1(5)
Bis(2-bromo-4,5-dimethoxyphenyl)sulfide (5) and bis(2-bromo-4,5-dimethoxyphenyl) selenide (7) have been shown to block cells in the G2/M phase of the cell cycle, whereas the debromo (4, 6) equivalents do not. The dibromoselenide (7) is cytotoxic to tumour cells in vitro and has been shown to increase the mitotic index of treated cells. These biological effects are consistent with disruption of the mitotic apparatus. This agent does not inhibit microtubule assembly in vitro, but does bind to tubulin. Molecular modelling of these structures indicates that their spatial and electronic structures may make an important contribution to the biological activity. 相似文献
108.
Jacklyn N. Hellwege Nicholette D. Palmer W. Mark Brown Julie T. Ziegler S. Sandy An Xiuqing Guo Y.-D. Ida Chen Kent Taylor Gregory A. Hawkins Maggie C. Y. Ng Elizabeth K. Speliotes Carlos Lorenzo Jill M. Norris Jerome I. Rotter Lynne E. Wagenknecht Carl D. Langefeld Donald W. Bowden 《Human genetics》2015,134(2):215-215
109.
Joanne Blanchfield Julie Dutton Ron Hogg David Craik David Adams Richard Lewis Paul Alewood Istvan Toth 《Letters in Peptide Science》2001,8(3-5):235-239
The -conotoxin MII is a 16 amino acid long peptide toxinisolated from the marine snail, Conus magus. This toxinhas been found to be a highly selective and potent inhibitorof neuronal nicotinic acetylcholine receptors of the subtype32. To improve the bioavailability of this peptide, we havecoupled to the N-terminus of conotoxin MII, 2-amino-D,L-dodecanoic acid (Laa) creating a lipidic linear peptide whichwas then successfully oxidised to produce the correctly foldedconotoxin MII construct. 相似文献
110.
A primary assembly of a bovine haplotype block map based on a 15,036-single-nucleotide polymorphism panel genotyped in holstein-friesian cattle 下载免费PDF全文
Khatkar MS Zenger KR Hobbs M Hawken RJ Cavanagh JA Barris W McClintock AE McClintock S Thomson PC Tier B Nicholas FW Raadsma HW 《Genetics》2007,176(2):763-772
Analysis of data on 1000 Holstein-Friesian bulls genotyped for 15,036 single-nucleotide polymorphisms (SNPs) has enabled genomewide identification of haplotype blocks and tag SNPs. A final subset of 9195 SNPs in Hardy-Weinberg equilibrium and mapped on autosomes on the bovine sequence assembly (release Btau 3.1) was used in this study. The average intermarker spacing was 251.8 kb. The average minor allele frequency (MAF) was 0.29 (0.05-0.5). Following recent precedents in human HapMap studies, a haplotype block was defined where 95% of combinations of SNPs within a region are in very high linkage disequilibrium. A total of 727 haplotype blocks consisting of > or =3 SNPs were identified. The average block length was 69.7 +/- 7.7 kb, which is approximately 5-10 times larger than in humans. These blocks comprised a total of 2964 SNPs and covered 50,638 kb of the sequence map, which constitutes 2.18% of the length of all autosomes. A set of tag SNPs, which will be useful for further fine-mapping studies, has been identified. Overall, the results suggest that as many as 75,000-100,000 tag SNPs would be needed to track all important haplotype blocks in the bovine genome. This would require approximately 250,000 SNPs in the discovery phase. 相似文献