全文获取类型
收费全文 | 776篇 |
免费 | 0篇 |
国内免费 | 8篇 |
专业分类
系统科学 | 8篇 |
教育与普及 | 2篇 |
理论与方法论 | 4篇 |
现状及发展 | 78篇 |
研究方法 | 146篇 |
综合类 | 512篇 |
自然研究 | 34篇 |
出版年
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 3篇 |
2018年 | 4篇 |
2017年 | 10篇 |
2016年 | 6篇 |
2015年 | 7篇 |
2014年 | 8篇 |
2013年 | 9篇 |
2012年 | 65篇 |
2011年 | 110篇 |
2010年 | 23篇 |
2009年 | 7篇 |
2008年 | 56篇 |
2007年 | 72篇 |
2006年 | 52篇 |
2005年 | 64篇 |
2004年 | 58篇 |
2003年 | 43篇 |
2002年 | 47篇 |
2001年 | 7篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1997年 | 2篇 |
1995年 | 2篇 |
1992年 | 9篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1972年 | 3篇 |
1971年 | 3篇 |
1970年 | 7篇 |
1969年 | 3篇 |
1968年 | 7篇 |
1967年 | 6篇 |
1965年 | 3篇 |
1963年 | 1篇 |
排序方式: 共有784条查询结果,搜索用时 15 毫秒
1.
Y Y He T McNally I Manfield O Navratil I G Old S E Phillips I Saint-Girons P G Stockley 《Nature》1992,359(6394):431-433
The three-dimensional crystal structure of the Escherichia coli methionine repressor, MetJ, complexed with a DNA operator fragment is described in an accompanying article. The complex exhibits several novel features of DNA-protein interaction. DNA sequence recognition is achieved largely by hydrogen-bond contacts between the bases and amino-acid side chains located on a beta-ribbon, a mode of recognition previously hypothesized on the basis of modelling of idealized beta-strands and DNA, and mutagenesis of the Salmonella phage P22 repressors Arc and Mnt. The complex comprises a pair of MetJ repressor dimers which bind to adjacent met-box sites on the DNA, and contact each other by means of a pair of antiparallel alpha-helices. Here we assess the importance of these contacts, and also of contacts that would be made between the C-helices of the protein and DNA in a previous model of the complex, by studying mutations aimed at disrupting them. The role of the carboxy-terminal helix face in operator binding was unclear, but we demonstrate that recognition of operator sequences occurs through side chains in the beta-strand motif and that dimer-dimer interactions are required for effective repression. 相似文献
2.
Christopher Monterola May Lim Jerrold Garcia Caesar Saloma 《Journal of forecasting》2002,21(6):435-449
The problem of pollsters is addressed which is to forecast accurately the final answers of the undecided respondents to the primary question in a public opinion poll. The task is viewed as a pattern‐recognition problem of correlating the answers of the respondents to the peripheral questions in the survey with their primary answers. The underlying pattern is determined with a supervised artificial neural network that is trained using the peripheral answers of the decided respondents whose primary answers are also known. With peripheral answers as inputs, the trained network outputs the most probable primary response of an undecided respondent. For a poll conducted to determine the approval rating of the (former) Philippine president, J. E. Estrada in December 1999 and March 2000, the trained network predicted with a 95% success rate the direct responses of a test population that consists of 24.57% of the decided population who were excluded in the network training set. For the undecided population (22.67% of December respondents; 23.67% of March respondents), the network predicted a final response distribution that is consistent with the approval/disapproval ratio of the decided population. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
3.
4.
The Environmental Protection Agency is preparing to regulate the biotechnology industry, claiming authority under the provisions of the Federal Toxic Substances Control Act (TOSCA) and the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). In the agency's view, the products of recombinant DNA techniques can be compared to new chemicals or pesticides, both of which fall under EPA jurisdiction. Although a legal challenge to EPA's authority in this area is expected from industry, the agency hopes to issue its first set of draft regulations in early 1984. 相似文献
5.
Introduction Nucleate boiling has always been an important heat transfer mechanism in many fields, such as energy utilization, manufacturing processes, and chemical processing and nucleate boiling continues to find new applications in many new areas, such… 相似文献
6.
7.
8.
Immunological properties of oxygen-transport proteins: hemoglobin,hemocyanin and hemerythrin 总被引:2,自引:0,他引:2
It is now well documented that peptides with enhanced or alternative functionality (termed cryptides) can be liberated from larger, and sometimes inactive, proteins. A primary example of this phenomenon is the oxygen-transport protein hemoglobin. Aside from respiration, hemoglobin and hemoglobin-derived peptides have been associated with immune modulation, hematopoiesis, signal transduction and microbicidal activities in metazoans. Likewise, the functional equivalents to hemoglobin in invertebrates, namely hemocyanin and hemerythrin, act as potent immune effectors under certain physiological conditions. The purpose of this review is to evaluate the true extent of oxygen-transport protein dynamics in innate immunity, and to impress upon the reader the multi-functionality of these ancient proteins on the basis of their structures. In this context, erythrocyte–pathogen antibiosis and the immune competences of various erythroid cells are compared across diverse taxa. 相似文献
9.
PTC124 targets genetic disorders caused by nonsense mutations 总被引:1,自引:0,他引:1
Welch EM Barton ER Zhuo J Tomizawa Y Friesen WJ Trifillis P Paushkin S Patel M Trotta CR Hwang S Wilde RG Karp G Takasugi J Chen G Jones S Ren H Moon YC Corson D Turpoff AA Campbell JA Conn MM Khan A Almstead NG Hedrick J Mollin A Risher N Weetall M Yeh S Branstrom AA Colacino JM Babiak J Ju WD Hirawat S Northcutt VJ Miller LL Spatrick P He F Kawana M Feng H Jacobson A Peltz SW Sweeney HL 《Nature》2007,447(7140):87-91
Nonsense mutations promote premature translational termination and cause anywhere from 5-70% of the individual cases of most inherited diseases. Studies on nonsense-mediated cystic fibrosis have indicated that boosting specific protein synthesis from <1% to as little as 5% of normal levels may greatly reduce the severity or eliminate the principal manifestations of disease. To address the need for a drug capable of suppressing premature termination, we identified PTC124-a new chemical entity that selectively induces ribosomal readthrough of premature but not normal termination codons. PTC124 activity, optimized using nonsense-containing reporters, promoted dystrophin production in primary muscle cells from humans and mdx mice expressing dystrophin nonsense alleles, and rescued striated muscle function in mdx mice within 2-8 weeks of drug exposure. PTC124 was well tolerated in animals at plasma exposures substantially in excess of those required for nonsense suppression. The selectivity of PTC124 for premature termination codons, its well characterized activity profile, oral bioavailability and pharmacological properties indicate that this drug may have broad clinical potential for the treatment of a large group of genetic disorders with limited or no therapeutic options. 相似文献
10.