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   268篇
  免费   22篇
  国内免费   10篇
  300篇
  2021年   2篇
  2020年   2篇
  2019年   4篇
  2018年   5篇
  2017年   3篇
  2016年   4篇
  2015年   18篇
  2014年   17篇
  2013年   12篇
  2012年   12篇
  2011年   17篇
  2010年   13篇
  2009年   10篇
  2008年   8篇
  2007年   8篇
  2006年   17篇
  2005年   2篇
  2004年   13篇
  2003年   8篇
  2002年   8篇
  2001年   6篇
  2000年   7篇
  1999年   7篇
  1998年   10篇
  1997年   6篇
  1996年   4篇
  1995年   4篇
  1994年   12篇
  1992年   4篇
  1989年   2篇
  1988年   2篇
  1987年   5篇
  1986年   2篇
  1985年   2篇
  1982年   2篇
  1980年   2篇
  1979年   5篇
  1978年   3篇
  1977年   4篇
  1976年   2篇
  1975年   2篇
  1974年   5篇
  1973年   1篇
  1972年   1篇
  1971年   3篇
  1970年   3篇
  1968年   1篇
  1967年   1篇
  1955年   1篇
  1954年   2篇
排序方式: 共有300条查询结果,搜索用时 15 毫秒
41.
42.
A Taenia solium 2-Cys peroxiredoxin (Ts2-CysPrx) clone was isolated from a T. solium adult cDNA library. The clone encodes a polypeptide comprising 197 amino acids with a predictive Mr = 21,836. It has the 2 classical cysteine domains from the typical 2-Cys peroxiredoxins, and its primary amino acid sequence shows higher identity with 2 Echinococcus 2-Cys peroxiredoxins. Northern and Southern blot hybridizations exhibit an mRNA with a size of -1.0 kb, encoded by 1 gene. Ts2-CysPrx was expressed in Escherichia coli and purified by anion-exchange chromatography. Biochemical analysis showed Ts2-CysPrx is a dimer composed by monomers of -22 kDa that presented activity with hydrogen peroxide (H2O2) and cumene hydroperoxide. It presented the catalytic mechanism for a typical 2-CysPrx because the homodimeric oxidized form is reduced to a monomeric form by thioredoxin (Trx) and by dithiothreitol (DTT) and was converted to a homodimeric oxidized form by H2O2. Western blot studies using antibodies against Ts2-CysPrx revealed that the protein is expressed during the entire T. solium life cycle, as in other Taenia species. Immunohistochemical studies indicated that Ts2-CysPrx is localized on the tegument and in tegumentary and muscle cells of cysticerci. We also show that T. crassiceps cysticerci can tolerate H2O2 levels of 2.5 mM for 2.5 hr.  相似文献   
43.
Peru is the top exporter of fishmeal and fish oil (FMFO) worldwide and is responsible for half and a third of global production, respectively. Landings of “anchoveta” (Engraulis ringens) are used nearly exclusively for FMFO production, despite a proactive national food policy aimed at favoring the direct human consumption of this inexpensive species. It may be surprising that in a country where malnutrition and caloric deficit constitute major issues, a low-priced and highly nutritious fish such as anchovy does not have stronger domestic demand as a food fish. Here, we review and assess eight potential politico-socio-economic processes that can explain this situation. The main explanation are dietary habits, the preference for broiler and the higher profit from anchovy sold as feed fish compared to its use as a food fish due to historically high FMFO prices, boosted by an increasing demand for aquaculture in a context of finite forage and trash fish resources. In addition, the recent introduction of an individual quota system has shifted bargaining power from processors to fishers, thereby increasing competition for the raw material. This competition results in an increase in anchovy prices offered by the feed fish industry due to its onshore processing overcapacity, which is detrimental to the food fish industry. In the end, although the dominant use of anchovy for fish feed is largely explained by integrating these market mechanisms and other minor ones, this use raises other issues, such as rent redistribution through public policies, employment, equitability and utility (low social costs), and resource management (threats to ecosystems or global change). Different policy scenarios are proposed in relation to these issues.  相似文献   
44.

Background

The tephritid fruit flies include a number of economically important pests of horticulture, with a large accumulated body of research on their biology and control. Amongst the Tephritidae, the genus Bactrocera, containing over 400 species, presents various species groups of potential utility for genetic studies of speciation, behaviour or pest control. In Australia, there exists a triad of closely-related, sympatric Bactrocera species which do not mate in the wild but which, despite distinct morphologies and behaviours, can be force-mated in the laboratory to produce fertile hybrid offspring. To exploit the opportunities offered by genomics, such as the efficient identification of genetic loci central to pest behaviour and to the earliest stages of speciation, investigators require genomic resources for future investigations.

Results

We produced a draft de novo genome assembly of Australia’s major tephritid pest species, Bactrocera tryoni. The male genome (650 -700 Mbp) includes approximately 150Mb of interspersed repetitive DNA sequences and 60Mb of satellite DNA. Assessment using conserved core eukaryotic sequences indicated 98% completeness. Over 16,000 MAKER-derived gene models showed a large degree of overlap with other Dipteran reference genomes. The sequence of the ribosomal RNA transcribed unit was also determined. Unscaffolded assemblies of B. neohumeralis and B. jarvisi were then produced; comparison with B. tryoni showed that the species are more closely related than any Drosophila species pair. The similarity of the genomes was exploited to identify 4924 potentially diagnostic indels between the species, all of which occur in non-coding regions.

Conclusions

This first draft B. tryoni genome resembles other dipteran genomes in terms of size and putative coding sequences. For all three species included in this study, we have identified a comprehensive set of non-redundant repetitive sequences, including the ribosomal RNA unit, and have quantified the major satellite DNA families. These genetic resources will facilitate the further investigations of genetic mechanisms responsible for the behavioural and morphological differences between these three species and other tephritids. We have also shown how whole genome sequence data can be used to generate simple diagnostic tests between very closely-related species where only one of the species is scaffolded.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1153) contains supplementary material, which is available to authorized users.  相似文献   
45.
Sulfate reduction and S-oxidation in a moorland pool sediment   总被引:1,自引:2,他引:1  
In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO4 2– reduction rates in the sediment, (2) depletion of SO4 2– in the overlying water column and (3) release of35S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO4 2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core35SO4 2– injection. Rates of SO4 2– consumption in the overlying water were estimated by changes in SO4 2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m–2 d–1. Rates of SO4 2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m–2 d–1 (November) to 0.43–1.81 mmol m–2 d–1 (July, August and April). Maximum rates of oxidation to SO4 2– in July 1990 estimated by combination of SO4 2– reduction rates and rates of in situ SO4 2– uptake in the enclosed water column were 10.3 and 10.5 mmol m–2 d–1 at an organic rich and at a sandy site respectively.Experiments with35S2– and35SO4 2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO4 2– and (2) the presence of mainly non SO4 2–-S derived from reduced S transported from the sediment into the overlying water. A35S2– tracer experiment showed that about 7% of35S2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author  相似文献   
46.
47.
We recently showed that diencephalic TRH may mediate the central leptin-induced pressor effect. Here, to study the role of TRH in obesity-induced hypertension (OIH), we used a model of OIH produced by a high-fat diet (HFD, 45 days) in male Wistar rats. After 4 wk, body weight and systolic arterial blood pressure (SABP) increased in HFD animals. Plasma leptin was correlated with peritoneal adipose tissue. Then, we treated OIH animals with an antisense oligodeoxynucleotide and small interfering (si)RNA against the prepro-TRH. Antisense significantly decreased diencephalic TRH content and SABP at 24 and 48 h posttreatment. Similar effects were observed with siRNA against prepro-TRH but for up to 4 wk. Conversely, vehicle, an inverted antisense sequence and siRNA against green fluorescence protein, produced no changes. SABP decrease seems to be owing to an inhibition of the obesity-enhanced sympathetic outflow but not to an alteration in thyroid status. Using a simple OIH model we demonstrated, for the first time, that central TRH participates in the hypertension induced by body weight gain probably through its well-known action on sympathetic activity. Thus the TRH-leptin interaction may contribute to the strong association between hypertension and obesity.  相似文献   
48.
Characterization of glutathione S-transferase of Taenia solium.   总被引:5,自引:0,他引:5  
A Taenia solium glutathione-S-transferase fraction (SGSTF) was isolated from a metacestode crude extract by affinity chromatography on reduced glutathione (GSH)-sepharose. The purified fraction displayed a specific glutathione S-transferase (GST) activity of 2.8 micromol/min/mg and glutathione peroxidase selenium-independent activity of 0.22 micromol/min/mg. Enzymatic characterization of the fraction suggested that the activity was closer to the mammalian mu-class GSTs. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel filtration, and enzyme activity analysis showed that the fraction was composed of a major band of Mr = 26 kd and that the active enzyme was dimeric. Immunohistochemical studies using specific antibodies against the major 26-kd band of the SGSTF indicated that GST protein was present in the tegument, parenchyma, protonephridial, and tegumentary cytons of the T. solium metacestode. Antibodies generated against the SGSTF tested in western blot showed cross-reactivity against GSTs purified from Taenia saginata, T. taeniaeformis, and T. crassiceps, but did not react with GSTs from Schistosoma mansoni, or mice, rabbit, and pig liver tissue. Furthermore, immunization of mice with SGSTF reduced the metacestode burden up to 74.2%. Our findings argue in favor of GST having an important role in the survival of T. solium in its hosts.  相似文献   
49.
Non-invasive genetic analyses are important for studies of species that are rare, sensitive or at risk of extinction. This study investigates the possibility of using faeces and urine to obtain microsatellite genotypes for individual identification of wolverines (Gulo gulo). The reliability of the employed method was assessed by analysing independent amplifications of non-invasive samples (a multiple-tube approach) and by comparing genotypes obtained from faeces to genotypes obtained from blood or tissue of the same individual. Ten microsatellite markers were successfully amplified in 65% of the faecal samples (n = 32) and 40% of the urine samples (n = 22). Allelic dropout was found in 12 and 14% of the amplifications from extracts of faeces and urine, respectively. Nevertheless, all multi-locus genotypes were correct, as judged from comparison to data from tissue or blood samples, after three replicates. These results suggest that a non-invasive approach based on DNA-analysis of faeces can be a powerful tool in population monitoring of wolverines, potentially providing reliable estimates of population size and immigration rate. A second objective of the study was to develop markers for DNA-based sex identification in wolverines using non-invasive samples. We developed two Y-linked markers, one that was specific to wolverine and one that also successfully identified sex in another mustelid. Importantly, none of the markers amplified potential prey species such as reindeer or rodents.  相似文献   
50.
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