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   233篇
  免费   21篇
  2021年   1篇
  2020年   1篇
  2018年   2篇
  2017年   5篇
  2016年   8篇
  2015年   14篇
  2014年   12篇
  2013年   11篇
  2012年   14篇
  2011年   18篇
  2010年   5篇
  2009年   8篇
  2008年   26篇
  2007年   16篇
  2006年   16篇
  2005年   14篇
  2004年   11篇
  2003年   9篇
  2002年   9篇
  2001年   3篇
  2000年   7篇
  1999年   2篇
  1998年   2篇
  1997年   1篇
  1996年   5篇
  1995年   4篇
  1994年   4篇
  1993年   3篇
  1992年   1篇
  1991年   1篇
  1990年   3篇
  1989年   4篇
  1988年   4篇
  1986年   1篇
  1985年   1篇
  1984年   1篇
  1983年   2篇
  1982年   3篇
  1981年   1篇
  1980年   1篇
排序方式: 共有254条查询结果,搜索用时 437 毫秒
51.
The subcellular localization of glucose oxidase (EC 1.1.3.4) in Aspergillus niger N400 (CBS 120.49) was investigated by (immuno)cytochemical methods. By these methods, the bulk of the enzyme was found to be localized in the cell wall. In addition, four different catalases (EC 1.11.1.6) were demonstrated by nondenaturing polyacrylamide gel electrophoresis of crude extracts of induced and noninduced cells. Comparison of both protoplast and mycelial extracts indicated that, of two constitutive catalases, one is located outside the cell membrane whereas the other is intracellular. Parallel with the induction of glucose oxidase, two other catalases are also induced, one located intracellularly and one located extracellularly. Furthermore, lactonase (EC 3.1.1.17) activity, catalyzing the hydrolysis of glucono-δ-lactone to gluconic acid, was found to be exclusively located outside the cell membrane, indicating that gluconate formation in A. niger occurs extracellularly.  相似文献   
52.
Summary A linkage analysis has been performed in a large Dutch kindred with progressive mixed deafness with perilymphatic gusher during stapes surgery (DFN3) using a panel of X-chromosomal RFLPs. Tight linkage (zmax=3.07 at =0.00) was demonstrated with the locus for phosphoglycerate kinase (PGK), which is located at Xq13. Tight linkage was excluded for DXS9 (probe RC8) and DXS41 (probe 99.6) on Xp and for blood clotting factor 9 (FIX) on distal Xq. Deafness is one of the predominant clinical features in males with deletions of the Xq21 band. Our results suggest that this association may be due to involvement of the DFN3 gene.  相似文献   
53.
Partial nucleotide sequencing of C11p11, a probe mapping close to the gene determining familial adenomatous polyposis (FAP) on human chromosome 5, in 4 unrelated persons has revealed a 4-base-pair deletion variant designated DELI at an arbitrary DNA locus D5S71. For screening the deletion variants that may frequently occur in the non-coding DNA sequences, we set up a non-invasive procedure which involves DNA amplification by PCR, simple polyacrylamide gel electrophoresis and direct visualisation of alleles under long wave ultraviolet light by ethidium bromide staining.  相似文献   
54.
55.
Two approaches to an integration of evolution and development are often distinguished, one “neo-Darwinian” and the other “structuralist”. Should these approaches in turn be integrated? Kelly Smith recently stated that we need a “more complete” theory of biological order, suggesting integration as the ideal. In response to him, I argue that a recognition of different types of scientific questions and causal explanation is more urgent. Do we understand development when we know the crucial factors in the process of differentiation, or rather when we know the laws that govern the transformations of fields? Without a recognition of these different explanatory ideals, “integration” is likely to have the character of annexation.  相似文献   
56.
Citric acid (CA) was evaluated as a functionality-enhancing monomer in biobased polyesters suitable for coating applications. Model reactions of CA with several primary and secondary alcohols and diols, including the 1,4:3,6-dianhydrohexitols, revealed that titanium(IV) n-butoxide catalyzed esterification reactions involving these compounds proceed at relatively low temperatures, often via anhydride intermediates. Interestingly, the facile anhydride formation from CA at temperatures around CA's melting temperature ( T m = 153 degrees C) proved to be crucial in modifying sterically hindered secondary hydroxyl end groups. OH-functional polyesters were reacted with CA in the melt between 150 and 165 degrees C, yielding slightly branched carboxylic acid functional materials with strongly enhanced functionality. The acid/epoxy curing reaction of the acid-functional polymers was simulated with a monofunctional glycidyl ether. Finally, the CA-modified polyesters were applied as coatings, using conventional cross-linking agents. The formulations showed rapid curing, resulting in chemically and mechanically stable coatings. These results demonstrate that citric acid can be applied in a new way, making use of its anhydride formation to functionalize OH-functional polyesters, which is an important new step toward fully biobased coating systems.  相似文献   
57.
The genome sequence of Archaeoglobus fulgidus VC16 encodes three CO dehydrogenase genes. Here we explore the capacity of A. fulgidus to use CO as growth substrate. Archaeoglobus fulgidus VC16 was successfully adapted to growth medium that contained sulfate and CO. In the presence of CO and sulfate the culture OD(660) increased to 0.41 and sulfide, carbon dioxide, acetate and formate were formed. Accumulation of formate was transient. Similar results, except that no sulfide was formed, were obtained when sulfate was omitted. Hydrogen was never detected. Under the conditions tested, the observed concentrations of acetate (18 mM) and formate (8.2 mM) were highest in cultures without sulfate. Proton NMR spectroscopy indicated that CO2, and not CO, is the precursor of formate and the methyl group of acetate. Methylviologen-dependent formate dehydrogenase activity (1.4 micromol formate oxidized min(-1) mg(-1)) was detected in cell-free extracts and expected to have a role in formate reuptake. It is speculated that formate formation proceeds through hydrolysis of formyl-methanofuran or formyl-tetrahydromethanopterin. This study demonstrates that A. fulgidus can grow chemolithoautotrophically with CO as acetogen, and is not strictly dependent on the presence of sulfate, thiosulfate or other sulfur compounds as electron acceptor.  相似文献   
58.
From 2000 to 2004, ticks were collected by dragging a blanket in four habitat areas in The Netherlands: dunes, heather, forest, and a city park. Tick densities were calculated, and infection with Borrelia burgdorferi and Anaplasma and Ehrlichia species was investigated by reverse line blot analysis. The lowest tick density was observed in the heather area (1 to 8/100 m2). In the oak forest and city park, the tick densities ranged from 26 to 45/100 m2. The highest tick density was found in the dune area (139 to 551/100 m2). The infection rates varied significantly for the four study areas and years, ranging from 0.8 to 11. 5% for Borrelia spp. and 1 to 16% for Ehrlichia or Anaplasma (Ehrlichia/Anaplasma) spp. Borrelia infection rates were highest in the dunes, followed by the forest, the city park, and heather area. In contrast, Ehrlichia/Anaplasma was found most often in the forest and less often in the city park. The following Borrelia species were found: Borrelia sensu lato strains not identified to the species level (2.5%), B. afzelii (2.5%), B. valaisiana (0.9%), B. burgdorferi sensu stricto (0.13%), and B. garinii (0.13%). For Ehrlichia/Anaplasma species, Ehrlichia and Anaplasma spp. not identified to the species level (2.5%), Anaplasma schotti variant (3.5%), Anaplasma phagocytophilum variant (0.3%), and Ehrlichia canis (0.19%) were found. E. canis is reported for the first time in ticks in The Netherlands in this study. Borrelia lusitaniae, Ehrlichia chaffeensis, and the human granylocytic anaplasmosis agent were not detected. About 1.6% of the ticks were infected with both Borrelia and Ehrlichia/Anaplasma, which was higher than the frequency predicted from the individual infection rates, suggesting hosts with multiple infections or a possible selective advantage of coinfection.  相似文献   
59.
Yolk androgens affect offspring hatching, begging, growth and survival in many bird species. If these effects are sex-specific, yolk androgen deposition may constitute a mechanism for differential investment in male and female offspring. We tested this hypothesis in zebra finches. In this species, females increase yolk-testosterone levels and produce male-biased sex ratios when paired to more attractive males. We therefore predicted that especially sons benefit from elevated yolk androgens. Eggs were injected with testosterone or sesame oil (controls) after 2 days of incubation. Testosterone had no clear effect on sex-specific embryonic mortality and changed the pattern of early nestling mortality independent of offspring sex. Testosterone-treated eggs took longer to hatch than control eggs. Control males begged significantly longer than females during the first days after hatching and grew significantly faster. These sex differences were reduced in offspring from testosterone-treated eggs due to prolonged begging durations of daughters, enhanced growth of daughters and reduced growth of sons. The results show that variation in maternal testosterone can play an important role in avian sex allocation due to its sex-specific effects on offspring begging and growth.  相似文献   
60.
A new sensitive and accurate analytical method has been developed for quantification of intracellular nucleotides in complex biological samples from cultured cells of different microorganisms such as Saccharomyces cerevisiae, Escherichia coli, and Penicillium chrysogenum. This method is based on ion pair reversed phase liquid chromatography electrospray ionization isotope dilution tandem mass spectrometry (IP-LC-ESI-ID-MS/MS. A good separation and low detection limits were observed for these compounds using dibutylamine as volatile ion pair reagent in the mobile phase of the LC. Uniformly 13C-labeled isotopes of nucleotides were used as internal standards for both extraction and quantification of intracellular nucleotides. The method was validated by determining the linearity, sensitivity, and repeatability.  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