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   332篇
  免费   99篇
  2019年   3篇
  2018年   3篇
  2017年   10篇
  2016年   9篇
  2015年   16篇
  2014年   16篇
  2013年   17篇
  2012年   16篇
  2011年   19篇
  2010年   12篇
  2009年   4篇
  2008年   5篇
  2007年   11篇
  2006年   10篇
  2005年   13篇
  2004年   8篇
  2003年   10篇
  2002年   4篇
  2001年   13篇
  2000年   10篇
  1999年   9篇
  1998年   9篇
  1997年   6篇
  1996年   8篇
  1995年   6篇
  1994年   4篇
  1992年   4篇
  1989年   4篇
  1988年   4篇
  1987年   8篇
  1986年   9篇
  1985年   9篇
  1984年   5篇
  1983年   3篇
  1982年   7篇
  1981年   8篇
  1980年   12篇
  1979年   8篇
  1978年   16篇
  1977年   6篇
  1976年   4篇
  1975年   7篇
  1974年   9篇
  1973年   10篇
  1972年   7篇
  1969年   4篇
  1968年   6篇
  1967年   3篇
  1961年   4篇
  1957年   2篇
排序方式: 共有431条查询结果,搜索用时 15 毫秒
61.
Distinguishing morphologically cryptic taxa, by definition, requires genetic data such as DNA sequences. However, DNA sequences may not be obtained easily for taxa from remote sites. Here we provide the details of a high-resolution melt-curve-based method using taxon-specific primers that can distinguish two taxa of Adélie penguins, and that will be usable in Antarctica when combined with some of the newly developed field-deployable thermal cyclers. We suggest that the wider adoption of field-deployable polymerase-chain-reaction-based techniques will enable faster assignation of haplotype to individuals in situ, and so allow the targeting of observations and sample collection to specimens relevant to the research question. Targeting individuals will also reduce the need to repeatedly handle animals and reduce the time and travel required to complete field work.  相似文献   
62.
63.
64.
Tuna species support some of the world’s largest commercial and recreational fisheries. Their extensive migratory patterns expose them to multiple national and international fisheries and fishery management regimes. Several prized species have become the focus of global conservation efforts and there is a growing worldwide interest in establishing optimal strategies for sustainable tuna fisheries. Although this task has proven to be very challenging, it has taken on a new sense of urgency in the face of the potential effects of global climate change. A better understanding of the interactions between environmental conditions and tuna physiology and how they affect tuna behavior will offer population and stock assessment modelers and fisheries biologists a more mechanistic understanding of tuna distribution patterns and may help predict changes in both geographic and depth-related movement patterns. Indeed, physiological data comprise a growing component of multi-trait analysis approaches to species conservation. Our review aims to summarize what is known about differences among tuna species in distribution patterns, tolerances to environmental conditions, and physiological characteristics that correlate with the capacity to inhabit cooler (deeper, higher latitude) and even hypoxic waters. To achieve this goal, we discuss how these physiological traits are associated with habitat partitioning within the three-dimensional oceanic environment and with niche expansion into cooler and hypoxic waters. We also point out areas where additional research is needed to predict more accurately how future changes in oceanographic conditions will affect the distributions and movement patterns of tunas and their availability to fisheries.  相似文献   
65.
Elastin‐Like Polypeptides (ELPs) reversibly phase separate in response to changes in temperature, pressure, concentration, pH, and ionic species. While powerful triggers, biological microenvironments present a multitude of more specific biological cues, such as antibodies, cytokines, and cell‐surface receptors. To develop better biosensors and bioresponsive drug carriers, rational strategies are required to sense and respond to these target proteins. We recently reported that noncovalent association of two ELP fusion proteins to a “chemical inducer of dimerization” small molecule (1.5 kDa) induces phase separation at physiological temperatures. Having detected a small molecule, here we present the first evidence that ELP multimerization can also detect a much larger (60 kDa) protein target. To demonstrate this strategy, ELPs were biotinylated at their amino terminus and mixed with tetrameric streptavidin. At a stoichiometric ratio of [4:1], two to three biotin‐ELPs associate with streptavidin into multimeric complexes with an apparent Kd of 5 nM. The increased ELP density around a streptavidin core strongly promotes isothermal phase separation, which was tuned to occur at physiological temperature. This phase separation reverses upon saturation with excess streptavidin, which only favors [1:1] complexes. Together, these findings suggest that ELP association with multimeric biomolecules is a viable strategy to deliberately engineer ELPs that respond to multimeric protein substrates.  相似文献   
66.
The invasive freshwater snail Tarebia granifera (Lamarck, 1822) was first reported in South Africa in 1999 and it has become widespread across the country, with some evidence to suggest that it reduces benthic macroinvertebrate biodiversity. The current study aimed to identify the primary abiotic drivers behind abundance patterns of T. granifera, by comparing the current abundance of the snail in three different regions, and at three depths, of the highly modified Nseleni River in KwaZulu-Natal, South Africa. Tarebia granifera was well established throughout the Nseleni River system, with an overall preference for shallow waters and seasonal temporal patterns of abundance. Although it is uncertain what the ecological impacts of the snail in this system are, its high abundances suggest that it should be controlled where possible and prevented from invading other systems in the region.  相似文献   
67.
Despite aggressive pre- or postoperative treatment, feline fibrosarcomas have high recurrence rates. Immunostimulatory gene therapy is a promising approach in veterinary oncology. This phase I dose-escalation study was performed to determine toxicity and feasibility of gene therapy with feline granulocyte-macrophage colony-stimulating factor (feGM-CSF) in cats with fibrosarcomas. Twenty cats were treated with plasmid coding for feGM-CSF attached to magnetic nanoparticles in doses of 50, 250, 750 and 1250 microg. Two preoperative intratumoral injections followed by magnetofection were given. Four control cats received only surgical treatment. Adverse events were recorded and correlated according to the veterinary co-operative oncology group toxicity scale. An enzyme-linked immunosorbent assay was performed to detect plasma feGM-CSF concentrations. No significant treatment related toxicity was observed. Preliminary recurrence results were encouraging as, on day 360, ten of 20 treated cats were recurrence-free. In conclusion, 1250 microg of feGM-CSF plasmid DNA applied by magnetofection is safe and feasible for phase II testing.  相似文献   
68.

Background

In the past decade there has been increasing visibility of malaria control efforts at the national and international levels. The factors that have enhanced this scenario are the availability of proven interventions such as artemisinin-based combination therapy, the wide scale use of insecticide-treated nets (ITNs) and a renewed emphasis in indoor residual house-spraying. Concurrently, there has been a window of opportunity of financial commitments from organizations such as the Global Fund for HIV/AIDS, Tuberculosis and Malaria (GFATM), the President's Malaria Initiative and the World Bank Booster programme.

Methods

The case study uses the health policy analysis framework to analyse the implementation of a public-private partnership approach embarked upon by the government of Tanzania in malaria control – 'The Tanzania National Voucher Scheme'- and in this synthesis, emphasis is on the challenges faced by the scheme during the pre-implementation (2001 – 2004) and implementation phases (2004 – 2005). Qualitative research tools used include: document review, interview with key informants, stakeholder's analysis, force-field analysis, time line of events, policy characteristic analysis and focus group discussions. The study is also complemented by a cross-sectional survey, which was conducted at the Rufiji Health Demographic Surveillance Site, where a cohort of women of child-bearing age were followed up regarding access and use of ITNs.

Results

The major challenges observed include: the re-introduction of taxes on mosquito nets and related products, procurement and tendering procedures in the implementation of the GFATM, and organizational arrangements and free delivery of mosquito nets through a Presidential initiative.

Conclusion

The lessons gleaned from this synthesis include: (a) the consistency of the stakeholders with a common vision, was an important strength in overcoming obstacles, (b) senior politicians often steered the policy agenda when the policy in question was a 'crisis event', the stakes and the visibility were high, (c) national stakeholders in policy making have an advantage in strengthening alliances with international organizations, where the latter can become extremely influential in solving bottlenecks as the need arises, and (d) conflict can be turned into an opportunity, for example the Presidential initiative has inadvertently provided Tanzania with important lessons in the organization of 'catch-up' campaigns.  相似文献   
69.
70.
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