首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2221篇
  免费   228篇
  国内免费   1篇
  2023年   11篇
  2022年   7篇
  2021年   45篇
  2020年   30篇
  2019年   39篇
  2018年   41篇
  2017年   38篇
  2016年   79篇
  2015年   125篇
  2014年   128篇
  2013年   157篇
  2012年   157篇
  2011年   193篇
  2010年   131篇
  2009年   105篇
  2008年   113篇
  2007年   155篇
  2006年   98篇
  2005年   117篇
  2004年   124篇
  2003年   126篇
  2002年   103篇
  2001年   31篇
  2000年   11篇
  1999年   28篇
  1998年   20篇
  1997年   16篇
  1996年   13篇
  1995年   17篇
  1994年   18篇
  1993年   19篇
  1992年   14篇
  1991年   14篇
  1990年   12篇
  1989年   16篇
  1988年   11篇
  1987年   6篇
  1986年   5篇
  1985年   5篇
  1984年   6篇
  1983年   7篇
  1982年   5篇
  1981年   11篇
  1980年   5篇
  1979年   4篇
  1977年   4篇
  1976年   4篇
  1974年   9篇
  1973年   2篇
  1970年   3篇
排序方式: 共有2450条查询结果,搜索用时 15 毫秒
51.
52.
53.
This report summarizes major changes in previously published protocols for DNA extraction to improve the quality of DNA extracted from plants. Here, we highlight the critical modifications in the original protocols. The efficiency of these changes results in high-quality DNA ready to use in a variety of phytogenetically distant plant families, in particular species with mucopolysaccharides. The DNA obtained can be used without further purification in various molecular biology assays, including direct sequencing and AFLP and RAPD (random-amplified polymorphic DNA) analyses. The effectiveness of this method is proven by the amplification and sequencing of PCR products of up to 1 kb with DNA extracted from herbarium tissue ≥60 years old. This versatility is not usually found in DNA extraction protocols. In addition, this method is quick, adaptable to standard laboratories, and most important, safer and more cost-effective.  相似文献   
54.
Cyanogenic glycosides are ancient biomolecules found in more than 2,650 higher plant species as well as in a few arthropod species. Cyanogenic glycosides are amino acid-derived β-glycosides of α-hydroxynitriles. In analogy to cyanogenic plants, cyanogenic arthropods may use cyanogenic glycosides as defence compounds. Many of these arthropod species have been shown to de novo synthesize cyanogenic glycosides by biochemical pathways that involve identical intermediates to those known from plants, while the ability to sequester cyanogenic glycosides appears to be restricted to Lepidopteran species. In plants, two atypical multifunctional cytochromes P450 and a soluble family 1 glycosyltransferase form a metabolon to facilitate channelling of the otherwise toxic and reactive intermediates to the end product in the pathway, the cyanogenic glycoside. The glucosinolate pathway present in Brassicales and the pathway for cyanoalk(en)yl glucoside synthesis such as rhodiocyanosides A and D in Lotus japonicus exemplify how cytochromes P450 in the course of evolution may be recruited for novel pathways. The use of metabolic engineering using cytochromes P450 involved in biosynthesis of cyanogenic glycosides allows for the generation of acyanogenic cassava plants or cyanogenic Arabidopsis thaliana plants as well as L. japonicus and A. thaliana plants with altered cyanogenic, cyanoalkenyl or glucosinolate profiles.  相似文献   
55.
56.
57.
On September 22, 2017, “Engaging the Forgotten Parent: Conference of Experts on Fathers’ Role in Children’s Weight‐Related Behaviours and Outcomes” brought researchers, students, health professionals, community stakeholders, and knowledge users from Canada, Australia, and the United States together to Toronto, Ontario, for a 1‐day gathering focused on furthering scientific understanding of effective strategies to recruit, engage, and retain fathers in family‐based obesity research. Furthermore this conference focused on identifying key knowledge gaps and opportunities for collaborative research to further our understanding of fathers’ roles in the development of their children’s behaviors and weight outcomes. The following four themes emerged from the conference: (1) underrepresentation of fathers in family‐based obesity research, (2) fathers’ engagement in children’s weight‐related behaviors and desire to participate in research, (3) best practices in recruiting, engaging, and retaining fathers in research, and (4) key lessons learned from fatherhood research in developmental science.  相似文献   
58.
The sensitivity of amphibian species to shifts in environmental conditions has been exhibited through long-term population studies and the projection of ecological niche models under expected conditions. Species in biodiversity hotspots have been the focus of ample predictive modeling studies, while, despite their significant ecological value, wide-ranging and common taxa have received less attention. We focused on predicting range restriction of the spotted salamander (Ambystoma maculatum), blue-spotted salamander (A. laterale), four-toed salamander (Hemidactylium scutatum), and red-backed salamander (Plethodon cinereus) under future climate scenarios. Using bias-corrected future climate data and biodiversity database records, we developed maximum entropy (MaxEnt) models under current conditions and for climate change projections in 2050 and 2070. We calculated positivity rates of species localities to represent proportions of habitat expected to remain climatically suitable with continued climate change. Models projected under future conditions predicted average positivity rates of 91% (89–93%) for the blue-spotted salamander, 23% (2–41%) for the spotted salamander, 4% (0.7–9%) for the four-toed salamander, and 61% (42–76%) for the red-backed salamander. Range restriction increased with time and greenhouse gas concentration for the spotted salamander, four-toed salamander, and red-backed salamander. Common, widespread taxa that often receive less conservation resources than other species are at risk of experiencing significant losses to their climatic ranges as climate change continues. Efforts to maintain populations of species should be focused on regions expected to experience fewer climatic shifts such as the interior and northern zones of species' distributions.  相似文献   
59.
60.
Molluscs, comprising one of the most successful phyla, lack clear evidence of adaptive immunity and yet thrive in the oceans, which are rich in viruses. There are thought to be nearly 120,000 species of Mollusca, most living in marine habitats. Despite the extraordinary abundance of viruses in oceans, molluscs often have very long life spans (10 to 100 years). Thus, their innate immunity must be highly effective at countering viral infections. Antiviral compounds are a crucial component of molluscan defenses against viruses and have diverse mechanisms of action against a wide variety of viruses, including many that are human pathogens. Antiviral compounds found in abalone, oyster, mussels, and other cultured molluscs are available in large supply, providing good opportunities for future research and development. However, most members of the phylum Mollusca have not been examined for the presence of antiviral compounds. The enormous diversity and adaptations of molluscs imply a potential source of novel antiviral compounds for future drug discovery.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号