全文获取类型
收费全文 | 481篇 |
免费 | 3篇 |
国内免费 | 5篇 |
专业分类
系统科学 | 6篇 |
丛书文集 | 1篇 |
教育与普及 | 1篇 |
理论与方法论 | 11篇 |
现状及发展 | 122篇 |
研究方法 | 43篇 |
综合类 | 269篇 |
自然研究 | 36篇 |
出版年
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 7篇 |
2015年 | 6篇 |
2014年 | 10篇 |
2013年 | 9篇 |
2012年 | 47篇 |
2011年 | 102篇 |
2010年 | 22篇 |
2009年 | 2篇 |
2008年 | 29篇 |
2007年 | 28篇 |
2006年 | 26篇 |
2005年 | 26篇 |
2004年 | 15篇 |
2003年 | 13篇 |
2002年 | 25篇 |
2000年 | 3篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1994年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1978年 | 2篇 |
1977年 | 7篇 |
1974年 | 6篇 |
1973年 | 7篇 |
1972年 | 1篇 |
1971年 | 5篇 |
1970年 | 4篇 |
1969年 | 2篇 |
1968年 | 6篇 |
1967年 | 4篇 |
1966年 | 3篇 |
1965年 | 3篇 |
1957年 | 1篇 |
1954年 | 1篇 |
1946年 | 1篇 |
排序方式: 共有489条查询结果,搜索用时 9 毫秒
401.
RNA-guided genetic silencing systems in bacteria and archaea 总被引:12,自引:0,他引:12
Clustered regularly interspaced short palindromic repeat (CRISPR) are essential components of nucleic-acid-based adaptive immune systems that are widespread in bacteria and archaea. Similar to RNA interference (RNAi) pathways in eukaryotes, CRISPR-mediated immune systems rely on small RNAs for sequence-specific detection and silencing of foreign nucleic acids, including viruses and plasmids. However, the mechanism of RNA-based bacterial immunity is distinct from RNAi. Understanding how small RNAs are used to find and destroy foreign nucleic acids will provide new insights into the diverse mechanisms of RNA-controlled genetic silencing systems. 相似文献
402.
Buchhave LA Latham DW Johansen A Bizzarro M Torres G Rowe JF Batalha NM Borucki WJ Brugamyer E Caldwell C Bryson ST Ciardi DR Cochran WD Endl M Esquerdo GA Ford EB Geary JC Gilliland RL Hansen T Isaacson H Laird JB Lucas PW Marcy GW Morse JA Robertson P Shporer A Stefanik RP Still M Quinn SN 《Nature》2012,486(7403):375-377
The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a 'fossil' record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation. 相似文献
403.
D Reich N Patterson D Campbell A Tandon S Mazieres N Ray MV Parra W Rojas C Duque N Mesa LF García O Triana S Blair A Maestre JC Dib CM Bravi G Bailliet D Corach T Hünemeier MC Bortolini FM Salzano ML Petzl-Erler V Acuña-Alonzo C Aguilar-Salinas S Canizales-Quinteros T Tusié-Luna L Riba M Rodríguez-Cruz M Lopez-Alarcón R Coral-Vazquez T Canto-Cetina I Silva-Zolezzi JC Fernandez-Lopez AV Contreras G Jimenez-Sanchez MJ Gómez-Vázquez J Molina A Carracedo A Salas C Gallo G Poletti DB Witonsky 《Nature》2012,488(7411):370-374
The peopling of the Americas has been the subject of extensive genetic, archaeological and linguistic research; however, central questions remain unresolved. One contentious issue is whether the settlement occurred by means of a single migration or multiple streams of migration from Siberia. The pattern of dispersals within the Americas is also poorly understood. To address these questions at a higher resolution than was previously possible, we assembled data from 52 Native American and 17 Siberian groups genotyped at 364,470 single nucleotide polymorphisms. Here we show that Native Americans descend from at least three streams of Asian gene flow. Most descend entirely from a single ancestral population that we call 'First American'. However, speakers of Eskimo-Aleut languages from the Arctic inherit almost half their ancestry from a second stream of Asian gene flow, and the Na-Dene-speaking Chipewyan from Canada inherit roughly one-tenth of their ancestry from a third stream. We show that the initial peopling followed a southward expansion facilitated by the coast, with sequential population splits and little gene flow after divergence, especially in South America. A major exception is in Chibchan speakers on both sides of the Panama isthmus, who have ancestry from both North and South America. 相似文献
404.
This study is the foundation of a new interpretation of the introduction and the three first books of Diophantus’s Arithmetica, one that opens the way to a historically correct contextualization of the work. Its purpose, as indicated in the title,
is to renew the traditional discussion on the methods of problem-solving used by Diophantus, through the detailed exposition
of a new analytical framework that aims to give an account of the coherence and progressive nature of the material included
in the three first books of the Arithmetica. One outcome of this new ‘toolbox’ is a new conspectus of the problems and solutions contained in the latter, which is presented
in appendix. The first part of the article clarifies, as a necessary preliminary, the key notions and terminology underlying
our analysis. Among these new concepts is the notion of “method of invention,” which accounts in general for any way, by which
“positions” (hypostaseis) are used in the Arithmetica. The next part proposes a complete inventory of the various methods of invention found in the three first books. Finally
the last part presents the above mentioned conspectus and proposes a series of preliminary conclusions that can be drawn from
it. 相似文献
405.
Samuel H. Wilson William A. Beard David D. Shock Vinod K. Batra Nisha A. Cavanaugh Rajendra Prasad Esther W. Hou Yuan Liu Kenjiro Asagoshi Julie K. Horton Donna F. Stefanick Padmini S. Kedar Michael J. Carrozza Aya Masaoka Michelle L. Heacock 《Cellular and molecular life sciences : CMLS》2010,67(21):3633-3647
Base excision repair (BER) can protect a cell after endogenous or exogenous genotoxic stress, and a deficiency in BER can render a cell hypersensitive to stress-induced apoptotic and necrotic cell death, mutagenesis, and chromosomal rearrangements. However, understanding of the mammalian BER system is not yet complete as it is extraordinarily complex and has many back-up processes that complement a deficiency in any one step. Due of this lack of information, we are unable to make accurate predictions on therapeutic approaches targeting BER. A deeper understanding of BER will eventually allow us to conduct more meaningful clinical interventions. In this review, we will cover historical and recent information on mammalian BER and DNA polymerase β and discuss approaches toward development and use of small molecule inhibitors to manipulate BER. With apologies to others, we will emphasize results obtained in our laboratory and those of our collaborators. 相似文献
406.
Invasion of plant species onto recently exposed beaches of Mono Lake, California, was documented. Three vegetation zones were evident on these beaches. The first was formed entirely of annual species. The second was composed of annuals mixed with salt grass, and the third was formed entirely of salt grass. Stabilization of these beaches was accomplished by the aggressive growth of salt grass rhizomes. 相似文献
407.
The algal flora of selected soils in the Uintah Basin, Uintah County, Utah, was studied. A total of 45 species was identified from the top soils of three different vascular plant habitats. The soil algal flora was dominated in biomass by filamentous Cyanophyta, including Microcoleus vaginatus, Phormidium minnesotense, and three species of Nostoc . These algae formed the algal matrix of the soil within which other Cyanophyta as well as Chlorophyta, Bacillariophyta, and Euglenophyta occurred. 相似文献
408.
Three diatom species recently collected from Great Basin localities represent new records of these taxa from this region of western North America. Cocconeis scutellum Ehr. and Melosira dubia Kuetz. were collected from a thermal spring in Tooele County, Utah. Nitzschia hustedtiana Salah was collected from newly flooded marshes at the south end of the Great Salt Lake, Tooele Countv, Utah. 相似文献
409.
Blue Lake Warm Spring in Tooele County, Utah, contains a rich diatom flora with a significant number of taxa that are unusual in this region. We have identified a total of 136 taxa in our samples from this locality, several of which are new records for Utah and North America. 相似文献
410.
Hanging gardens and wet wall habitats have been studied for the past several years in many arid regions of the Intermountain West. One unusual large wet wall habitat in Zion National Park was found to be covered with a mucilaginous red-colored growth of the filamentous gliding bacterium Thioploca ingrica Visloukh. This is the only habitat we have examined where the predominant matrix-forming organism was a bacterium rather than an alga. 相似文献