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   71篇
  免费   8篇
  79篇
  2018年   1篇
  2017年   4篇
  2016年   1篇
  2015年   1篇
  2014年   3篇
  2013年   4篇
  2012年   2篇
  2011年   3篇
  2009年   2篇
  2008年   1篇
  2007年   1篇
  2006年   1篇
  2005年   1篇
  2003年   3篇
  2002年   2篇
  2001年   4篇
  1999年   1篇
  1998年   2篇
  1995年   1篇
  1992年   4篇
  1991年   3篇
  1989年   6篇
  1988年   5篇
  1986年   3篇
  1985年   2篇
  1984年   2篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1980年   1篇
  1977年   1篇
  1976年   2篇
  1974年   2篇
  1973年   2篇
  1971年   1篇
  1970年   2篇
  1967年   1篇
排序方式: 共有79条查询结果,搜索用时 14 毫秒
21.
The genus Plutella was thought to be represented in Australia by a single introduced species, Plutella xylostella (Linnaeus), the diamondback moth. Its status as a major pest of cruciferous crops, and the difficulty in developing control strategies has motivated broad-ranging studies on its biology. Prior genetic work has generally supported the conclusion that populations of this migratory species are connected by substantial gene flow. However, the present study reveals the presence of two genetically divergent lineages of this taxonin Australia. One shows close genetic and morphological similarity with the nearly cosmopolitan Plutella xylostella. The second lineage possesses a similar external morphology, but marked sequence divergence in the barcode region of the cytochrome c oxidase I gene, coupled with clear differences in genitalia. As a consequence, members of this lineage are described as a new species, Plutella australiana Landry & Hebert, which is broadly distributed in the eastern half of Australia.  相似文献   
22.
The prototype JHM strain of murine hepatitis virus (MHV) is an enveloped, RNA-containing coronavirus that has been selected in vivo for extreme neurovirulence. This virus encodes spike (S) glycoproteins that are extraordinarily effective mediators of intercellular membrane fusion, unique in their ability to initiate fusion even without prior interaction with the primary MHV receptor, a murine carcinoembryonic antigen-related cell adhesion molecule (CEACAM). In considering the possible role of this hyperactive membrane fusion activity in neurovirulence, we discovered that the growth of JHM in tissue culture selected for variants that had lost murine CEACAM-independent fusion activity. Among the collection of variants, mutations were identified in regions encoding both the receptor-binding (S1) and fusion-inducing (S2) subunits of the spike protein. Each mutation was separately introduced into cDNA encoding the prototype JHM spike, and the set of cDNAs was expressed using vaccinia virus vectors. The variant spikes were similar to that of JHM in their assembly into oligomers, their proteolysis into S1 and S2 cleavage products, their transport to cell surfaces, and their affinity for a soluble form of murine CEACAM. However, these tissue culture-adapted spikes were significantly stabilized as S1-S2 heteromers, and their entirely CEACAM-dependent fusion activity was delayed or reduced relative to prototype JHM spikes. The mutations that we have identified therefore point to regions of the S protein that specifically regulate the membrane fusion reaction. We suggest that cultured cells, unlike certain in vivo environments, select for S proteins with delayed, CEACAM-dependent fusion activities that may increase the likelihood of virus internalization prior to the irreversible uncoating process.  相似文献   
23.
The metabolic fate of nitrogen from 15N-labeled sodium nitrate has been investigated in four healthy Polish Merino ewes. 15N-labeled sodium nitrate was administered intravenously at the dosage of 400 micromol.kg(-1) body weight. Blood plasma and urine concentrations of nitrate, ammonia, and urea and 15N enrichment of ammonia and urea were estimated over a 50-h period following 15N-nitrate administration. Nitrate (NO3-) was slowly eliminated from the blood plasma, and the presence of NO3(-) in the blood plasma above the nitrate "background" was observed for 50 h. 15N enrichment of blood plasma urea already appeared at 15 min and reached the maximum 6 h after 15N-nitrate administration. The urinary excretion of nitrate occured during 50 h after 15N-nitrate injection; the total urine excretion of NO3(-) was 23.63+/-2.39% of the administered dose. The mean urinary recoveries of nitrogen as 15N-urea and 15N-ammonia were 14.76+/-1.32% and 0.096+/-0.015% of the administered 15N-nitrate dose, respectively. It should be pointed out that in total only 38.49% of the administered nitrate-N was excreted in urine (as nitrate, ammonia and urea nitrogen) during 50 h. The results obtained indicate that sheep are able to store nitrate nitrogen in their body. The fate of the remaining approximately 60% of the 15NO3(-) administered dose is unknown. The results obtained do not allow one to conclude what fraction of the unrecovered approximately 60% of the 15NO3(-) dose was utilized by gastrointestinal microorganisms, and (or) metabolized, or stored in sheep tissues.  相似文献   
24.
Cloning of a cDNA encoding porcine brain natriuretic peptide   总被引:3,自引:0,他引:3  
Complimentary DNA (cDNA) clones encoding porcine brain natriuretic peptide (BNP) were isolated from a porcine atrial cDNA library. The longest of the cDNA clones (1507 nucleotides) apparently originated from an unprocessed messenger RNA, since the nucleotide sequence encoding BNP-26 was interrupted by an intron of 554 nucleotides. A partial cDNA clone representing processed BNP mRNA was prepared by polymerase chain reaction. A comparison of the sequence of these two cDNAs reveals the presence of an additional intron within the sequence encoding the BNP precursor. The identification of these introns suggests that the BNP gene structure differs from the atrial natriuretic peptide gene in the location of intron 2. BNP mRNA encodes a propeptide of 131 amino acids, including a signal peptide domain (25 amino acids) and a prohormone domain (106 amino acids). Like atrial natriuretic peptide, the bioactive BNP sequence is localized at the carboxyl terminus of the prohormone. Although the carboxyl-terminal peptide sequences of porcine atrial natriuretic peptide and BNP are well conserved, there is relatively little homology within their propeptide regions.  相似文献   
25.
Analyses with segmental reassortants of lymphocytic choriomeningitis virus (LCMV) RNA have shown that cytotoxic T lymphocytes (CTL) are induced by and recognize proteins encoded by the viral short segment, which specifies two virus structural proteins, glycoprotein (GP) and nucleoprotein (NP). Expression of cDNA copies of these genes in vaccinia virus vectors demonstrates that C57BL/6 (H2bb) mice mount significant CTL responses to both GP and NP. We have used LCMV-specific H2bb-restricted CTL clones and a family of serial C-terminal truncations of the LCMV GP expressed in vaccinia virus to map the precise specificities of the anti-GP clones. Of the 18 CTL clones studied, 1 recognizes NP and the other 17 recognize GP. The reactivities of 14 of the 17 anti-GP CTL clones against the deleted GP molecules have been fully characterized, and two clear patterns of anti-GP activity have emerged, defining at least two CTL epitopes. The first epitope, recognized by only two of the clones, lies within GP residues 1 to 218. The second is recognized by all 12 of the remaining clones and was mapped, by using the GP deletions, to a 22-amino-acid region comprising GP residues 272 to 293. A synthetic peptide representing this area sensitized uninfected syngeneic target cells to lysis both by bulk CTL obtained from the spleen after a primary immunization and by appropriate CTL clones. Two sets of criteria are available which are said to identify potential T-cell epitopes, one based on primary amino acid sequence and the second based on protein secondary structure. Neither of these predictive schemes would have identified region 272 to 293 as a CTL recognition motif, indicating that such programs are of limited usefulness as presently conceived. Analysis of the CTL clones shows clearly that all three families (anti-NP and anti-GP 1 to 218 and 272 to 293) direct efficient cross-reactive killing against a variety of serologically distinct strains of LCMV.  相似文献   
26.

Purpose

This article is the second part of a series of articles presenting the results of research on the implementation of lifecycle management tools in small- and medium-sized companies in Poland. This work is part of a project financed by the Polish Agency for Enterprise Development (PAED), which began in February 2011. It was carried out by the Wielkopolska Quality Institute, a business environment institution associated with the Polish Centre for life cycle assessment (PCLCA). The main practical objective of the project was to support small and medium enterprises (SMEs) in their business development, e.g. by expanding their horizons beyond the sphere of their operation and identifying new areas for the improvement and promotion of the products and services they offer. The specific objective of the analysis on the environmental impact was an attempt to answer the question of whether environmental LCA is a good management tool for this type of business. Part 2 describes results of the evaluation of the implementation of LCA in SMEs conducted in 46 companies involved in the project.

Methods

In order to assess the effectiveness of the project and the effectiveness of the implementation of LCA and life cycle costing (LCC), a survey was conducted of small and medium businesses where the implementation work had been fully completed. In total, 46 organisations agreed to participate in the LCA survey, which was almost 66 % of all the companies where the LCA and LCC studies had been carried out within the project. The survey was conducted using individual in-depth interviews. Questions to the representatives of the companies referred both to aspects of their functioning in the market (characteristics of a company, its market share, management systems, environmental policy, suppliers and clients) and the operation of their environmental service (assessment of its effectiveness, motivation and difficulties in its implementation), as well as opinions on the potential applications of LCA in their current operations.

Results and discussion

The experience and observations of LCA experts resulting from their cooperation with the organisations analysed are largely supported by the results of the survey. The overall impression gained from the project is that the small- and medium-sized enterprises analysed have a problem with accepting and understanding the life cycle perspective and show limited interest in taking liability for environmental aspects beyond the mandatory legal standards and boundaries of their business operations. The survey shows that the companies rarely analyse environmental aspects appearing on many different stages of the life cycle of their products. Most of them focus on their current operations while trying to meet the mandatory legal requirements relating to environmental protection. It should be noted, however, that SMEs taking part in the studies appreciate the opportunities offered by LCA, their usefulness in business practice, recognise the potential for using life cycle techniques in the future and their impact on the management process, procedure or thinking about the products they manufacture. The result of the study is the identification of four key areas relevant to SMEs which may affect their willingness to adopt the life cycle perspective and undertake environmental measures.

Conclusions

It seems that implementing LCT in small- and medium-sized enterprises requires a special approach. These are often companies with limited human resources (often just a few people) and financial resources (often operating on the verge of survival), with a weak position in a supply chain and, therefore, having various priorities in their daily operation. The researchers also encountered awareness barriers as a result of which the idea of going beyond an organisation and making an entire LCA of a product was often simply misunderstood. The studies conducted among SMEs have shown that managers' own intuition and research on customer preferences were largely conducive to improve existing or introducing new products or services, while changes were mostly introduced due to the requirements of the market, or the desire to reduce costs. It can be assumed that their non-obligatory nature also contributed to the relatively low interest in LCA initiatives and not recognising their usefulness. It seems that it would be useful to carry out relatively simple, but integrated, LCA/LCC analyses in SMEs so that the companies would clearly see the economic effect of the proposed environmental improvements. The analyses conducted lead to the conclusion that the incentive for SMEs to take measures should come from outside, e.g. as requirements for green public procurements, or as part of assessment made by suppliers in a supply chain.  相似文献   
27.
28.
BackgroundLittle is known about the epidemiology and etiology of Waldenstrom macroglobulinemia (WM). Despite several studies of the relation between family history and B-cell disorders and WM, family history of non-hematologic cancers has not been systematically investigated. We thus examined associations of family history of breast, colorectal, lung, ovarian, and prostate cancers with WM.MethodsAll probands aged 20–79 years with bone marrow biopsy-confirmed diagnosis of WM between May 1, 1999 and January 1, 2010 at the Bing Center for Waldenstrom Macroglobulinemia were eligible for inclusion in our analysis. We reviewed medical records for eligible probands to determine family history of cancer (defined as a cancer diagnosis for ≥1 first-degree relative(s) of the proband). Using expected values constructed from the United States National Health Interview Survey, we estimated age- and race-standardized rate ratios (RRs) for family history of breast, colorectal, lung, ovarian, and prostate cancers by WM subtype.ResultsFamily history of prostate cancer had the largest overall rate ratio (RR = 1.4, 95% confidence limits [CL]: 1.1, 1.7), and among sporadic cases, family history of prostate and breast cancer had the largest rate ratios (prostate: RR = 1.3, 95% CL: 1.1, 1.7; breast: RR = 1.3, 95% CL: 1.2, 1.6).ConclusionOur study suggests that it may be worthwhile to pursue these associations in a case–control study with uniform selection and data collection for cases and controls, and at least some record-based information on family history.  相似文献   
29.

Background

Angiogenesis is widely investigated in conjunction with cancer development, in particular because of the possibility of early stage detection and of new therapeutic strategies. However, such studies are negatively affected by the limitations of imaging techniques in the detection of microscopic blood vessels (diameter 3-5 ??m) grown under angiogenic stress. We report that synchrotron-based X-ray imaging techniques with very high spatial resolution can overcome this obstacle, provided that suitable contrast agents are used.

Results

We tested different contrast agents based on gold nanoparticles (AuNPs) for the detection of cancer-related angiogenesis by synchrotron microradiology, microtomography and high resolution X-ray microscopy. Among them only bare-AuNPs in conjunction with heparin injection provided sufficient contrast to allow in vivo detection of small capillary species (the smallest measured lumen diameters were 3-5 ??m). The detected vessel density was 3-7 times higher than with other nanoparticles. We also found that bare-AuNPs with heparin allows detecting symptoms of local extravascular nanoparticle diffusion in tumor areas where capillary leakage appeared.

Conclusions

Although high-Z AuNPs are natural candidates as radiology contrast agents, their success is not guaranteed, in particular when targeting very small blood vessels in tumor-related angiography. We found that AuNPs injected with heparin produced the contrast level needed to reveal--for the first time by X-ray imaging--tumor microvessels with 3-5 ??m diameter as well as extravascular diffusion due to basal membrane defenestration. These results open the interesting possibility of functional imaging of the tumor microvasculature, of its development and organization, as well as of the effects of anti-angiogenic drugs.  相似文献   
30.
Guanylate cyclase was purified from the soluble fraction of rat lung using a modification of procedures published previously. The purified enzyme exhibited specific activities, at pH 7.6, of 219-438 nmoles/mg protein/min and 34-60 nmoles/mg protein/min with Mn2+ and Mg2+ as cation cofactors, respectively. The specific activity changed as a function of the protein concentration due to a change in Vmax with no alteration of the Km for GTP. The enzyme migrated as a single band coincident wih guanylate cyclase activity on nondenaturing polyacrylamide and isoelectric focusing gels (isoelectric point = 5.9). Purified guanylate cyclase had an apparent molecular weight of 150,000 daltons as determined by gel filtration chromatography and polyacrylamide gel electrophoresis. Electrophoresis in the presence of sodium dodecyl sulfate revealed a single subunit of 72,000 daltons, suggesting that the enzyme is a dimer of an identical subunit. The purified enzyme could be activated by nitric oxide, indicating that this compound interacts directly with the enzyme.  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