全文获取类型
收费全文 | 550篇 |
免费 | 2篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 25篇 |
教育与普及 | 1篇 |
理论与方法论 | 3篇 |
现状及发展 | 77篇 |
研究方法 | 86篇 |
综合类 | 331篇 |
自然研究 | 31篇 |
出版年
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 10篇 |
2017年 | 3篇 |
2016年 | 7篇 |
2015年 | 6篇 |
2014年 | 3篇 |
2013年 | 5篇 |
2012年 | 28篇 |
2011年 | 80篇 |
2010年 | 14篇 |
2009年 | 5篇 |
2008年 | 40篇 |
2007年 | 50篇 |
2006年 | 39篇 |
2005年 | 25篇 |
2004年 | 38篇 |
2003年 | 37篇 |
2002年 | 32篇 |
2001年 | 6篇 |
2000年 | 10篇 |
1999年 | 10篇 |
1996年 | 3篇 |
1992年 | 5篇 |
1990年 | 2篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1970年 | 7篇 |
1969年 | 6篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1965年 | 1篇 |
1964年 | 2篇 |
排序方式: 共有554条查询结果,搜索用时 15 毫秒
1.
Beardsley T 《Nature》1985,313(6002):421
A physiologist and the chief veterinarian at the University of Western Ontario are facing possible prosecution for cruelty to animals because of an experiment in which a baboon was kept for several months in a restraining chair. The case is believed to be the first in Canada brought against researchers following complaints by an animal welfare group. The experiment was supported by the Canadian Medical Research Council and the Ontario Heart and Stroke Fund, and the protocol and laboratory were approved by the Canadian Council on Animal Care. 相似文献
2.
Beardsley T 《Nature》1985,313(5998):85
The dispute among federal agencies over regulation of commercial biotechnology has been addressed in the Federal Register by the Office of Science and Technology Policy (OSTP), which says the Recombinant DNA Advisory Committee of the National Institutes of Health should continue to oversee biomedical research in this area, while the Environmental Protection Agency should formulate regulatory policies for industry and resolve such issues as when a microorganism constitutes a new chemical substance. OSTP also proposes four new independent expert committees to advise the EPA, the Food and Drug Administration, the Department of Agriculture, and the National Science Foundation, and a Biotechnology Science Board to coordinate the work of the committees. 相似文献
3.
4.
This paper is a case study of a major System Dynamics (SD) intervention in policy design in the Australian Taxation Office. The project, which lasted two years, tested a number of hypotheses regarding the application of Stafford Beer's Viable Systems Model (VSM) to the structure and process of the project. The paper reports that the success of the VSM within the ATO was qualified by political changes in the external environment. 相似文献
5.
6.
7.
Milne JC Lambert PD Schenk S Carney DP Smith JJ Gagne DJ Jin L Boss O Perni RB Vu CB Bemis JE Xie R Disch JS Ng PY Nunes JJ Lynch AV Yang H Galonek H Israelian K Choy W Iffland A Lavu S Medvedik O Sinclair DA Olefsky JM Jirousek MR Elliott PJ Westphal CH 《Nature》2007,450(7170):712-716
Calorie restriction extends lifespan and produces a metabolic profile desirable for treating diseases of ageing such as type 2 diabetes. SIRT1, an NAD+-dependent deacetylase, is a principal modulator of pathways downstream of calorie restriction that produce beneficial effects on glucose homeostasis and insulin sensitivity. Resveratrol, a polyphenolic SIRT1 activator, mimics the anti-ageing effects of calorie restriction in lower organisms and in mice fed a high-fat diet ameliorates insulin resistance, increases mitochondrial content, and prolongs survival. Here we describe the identification and characterization of small molecule activators of SIRT1 that are structurally unrelated to, and 1,000-fold more potent than, resveratrol. These compounds bind to the SIRT1 enzyme-peptide substrate complex at an allosteric site amino-terminal to the catalytic domain and lower the Michaelis constant for acetylated substrates. In diet-induced obese and genetically obese mice, these compounds improve insulin sensitivity, lower plasma glucose, and increase mitochondrial capacity. In Zucker fa/fa rats, hyperinsulinaemic-euglycaemic clamp studies demonstrate that SIRT1 activators improve whole-body glucose homeostasis and insulin sensitivity in adipose tissue, skeletal muscle and liver. Thus, SIRT1 activation is a promising new therapeutic approach for treating diseases of ageing such as type 2 diabetes. 相似文献
8.
Sexually antagonistic genetic variation for fitness in red deer 总被引:1,自引:0,他引:1
Foerster K Coulson T Sheldon BC Pemberton JM Clutton-Brock TH Kruuk LE 《Nature》2007,447(7148):1107-1110
Evolutionary theory predicts the depletion of genetic variation in natural populations as a result of the effects of selection, but genetic variation is nevertheless abundant for many traits that are under directional or stabilizing selection. Evolutionary geneticists commonly try to explain this paradox with mechanisms that lead to a balance between mutation and selection. However, theoretical predictions of equilibrium genetic variance under mutation-selection balance are usually lower than the observed values, and the reason for this is unknown. The potential role of sexually antagonistic selection in maintaining genetic variation has received little attention in this debate, surprisingly given its potential ubiquity in dioecious organisms. At fitness-related loci, a given genotype may be selected in opposite directions in the two sexes. Such sexually antagonistic selection will reduce the otherwise-expected positive genetic correlation between male and female fitness. Both theory and experimental data suggest that males and females of the same species may have divergent genetic optima, but supporting data from wild populations are still scarce. Here we present evidence for sexually antagonistic fitness variation in a natural population, using data from a long-term study of red deer (Cervus elaphus). We show that male red deer with relatively high fitness fathered, on average, daughters with relatively low fitness. This was due to a negative genetic correlation between estimates of fitness in males and females. In particular, we show that selection favours males that carry low breeding values for female fitness. Our results demonstrate that sexually antagonistic selection can lead to a trade-off between the optimal genotypes for males and females; this mechanism will have profound effects on the operation of selection and the maintenance of genetic variation in natural populations. 相似文献
9.
10.