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   1561篇
  免费   72篇
  1633篇
  2022年   10篇
  2021年   23篇
  2020年   15篇
  2019年   18篇
  2018年   27篇
  2017年   27篇
  2016年   29篇
  2015年   44篇
  2014年   58篇
  2013年   81篇
  2012年   113篇
  2011年   85篇
  2010年   65篇
  2009年   34篇
  2008年   61篇
  2007年   67篇
  2006年   61篇
  2005年   51篇
  2004年   39篇
  2003年   44篇
  2002年   38篇
  2001年   49篇
  2000年   39篇
  1999年   40篇
  1998年   17篇
  1997年   17篇
  1996年   13篇
  1995年   18篇
  1994年   18篇
  1993年   10篇
  1992年   25篇
  1991年   38篇
  1990年   34篇
  1989年   29篇
  1988年   31篇
  1987年   26篇
  1986年   15篇
  1985年   12篇
  1984年   12篇
  1983年   12篇
  1981年   12篇
  1979年   19篇
  1978年   12篇
  1975年   9篇
  1974年   9篇
  1973年   15篇
  1972年   11篇
  1971年   15篇
  1970年   13篇
  1969年   14篇
排序方式: 共有1633条查询结果,搜索用时 15 毫秒
41.
42.
43.
Follicle-stimulating hormone of a high state of physicochemical and biological purity was isolated from acetone-preserved human pituitary glands. The follicle-stimulating hormone was dissociated into alpha and beta subunits by treatment with 8 M urea and the subunits were separated by ion exchange chromatography on DEAE-Sephadex A-25. The subunits were freed of undissociated or reassociated follicle-stimulating hormone by gel filtration on Sephadex G-100. For the establishment of the primary amino acid sequence, the alpha subunit was reduced and either carboxyamidomethylated or S-aminoethylated prior to a thermolytic or a tryptic digestion. Each digest was gel filtered on a column of Sephadex G-50 to separate the glycopeptides from the peptides. The glycopeptides and the peptides were purified further by sequential gel filtration on Sephadex G-25, G-15, and Bio-Gel-P-2 and were isolated by high voltage electrophoresis at pH 6, 3.5, and 2. The purity of the isolated peptides was ascertained further by amino acid analysis. The amino acid sequences of the peptides were determined by Edman degradation followed by subtractive amino acid analysis. COOH-terminal sequences were established by digestion with carboxypeptidases A and B. The primary amino acid sequence of human follicle-stimulating hormone-alpha is identical to that of human chorionic gonadotropin-alpha and differs from that of human luteinizing hormone-alpha in having the tripeptide Ala-Pro-Asx- at the NH2-terminal end.  相似文献   
44.
I M Saxena  K Kudlicka  K Okuda    R M Brown  Jr 《Journal of bacteriology》1994,176(18):5735-5752
The synthesis of an extracellular ribbon of cellulose in the bacterium Acetobacter xylinum takes place from linearly arranged, membrane-localized, cellulose-synthesizing and extrusion complexes that direct the coupled steps of polymerization and crystallization. To identify the different components involved in this process, we isolated an Acetobacter cellulose-synthesizing (acs) operon from this bacterium. Analysis of DNA sequence shows the presence of three genes in the acs operon, in which the first gene (acsAB) codes for a polypeptide with a molecular mass of 168 kDa, which was identified as the cellulose synthase. A single base change in the previously reported DNA sequence of this gene, resulting in a frameshift and synthesis of a larger protein, is described in the present paper, along with the sequences of the other two genes (acsC and acsD). The requirement of the acs operon genes for cellulose production was determined using site-determined TnphoA/Kanr GenBlock insertion mutants. Mutant analysis showed that while the acsAB and acsC genes were essential for cellulose production in vivo, the acsD mutant produced reduced amounts of two cellulose allomorphs (cellulose I and cellulose II), suggesting that the acsD gene is involved in cellulose crystallization. The role of the acs operon genes in determining the linear array of intramembranous particles, which are believed to be sites of cellulose synthesis, was investigated for the different mutants; however, this arrangement was observed only in cells that actively produced cellulose microfibrils, suggesting that it may be influenced by the crystallization of the nascent glucan chains.  相似文献   
45.
Aspergillus japonicus andA. foetidus were found to solubilize, five types of Indian rock phosphates at pH 8 and 9. Solubilization was higher in the presence of pyrite than in controls lacking either pyrite or fungal, inoculum. Both the aspergilli were found to be good pyrite solubilizers and could grow over a wide pH range. Solubilization of rock phosphates was the result of organic acid release and pyrite oxidation.  相似文献   
46.

Cryopreservation is a valuable tool that could potentially create an alternate plant preservation strategy for species at risk such as Hill’s thistle. The present study is focused on a successful paradigm involving conservation, propagation and redistribution (CPR), emaphasizing the usefulness of cryopreservation techniques for plant conservation using Hill’s thistle (Cirsium hillii. (Canby) Fernald). A cryopreservation protocol was established using the droplet-vitrification method for 5-week-old shoot tips of in vitro grown cultures. More than 90% of shoot tips showed regrowth and nearly all regenerated plants were able to survive in the greenhouse. The survival, growth, and development of plants from cryopreserved shoot buds and their performance in field conditions were all comparable or better than the plants from non-cryopreserved buds. Reintroduced plants flowered following overwintering and the magnitude of flowering was site dependent with ca. 80% flowering observed in one site. The site dependent flowering patterns were assessed using phytohormone profiling and compared to herbivory, a common biotic stressor of these plants. Lower tryptophan concentrations led to higher flowering except in alvars, where the limestone resisted root penetration resulting in poor flowering. The presence of tryptamine in the greenhouse acclimatized or alvar field leaves suggested the preparedness of the plants for herbivory/grazing. Serotonin and melatonin concentrations were lower in flowering plants and in sites where the biotic/abiotic stress was minimal. This study provides evidence of the effectiveness of the CPR model in species recovery programs for endangered species. Physiological characterization of plants developed from cryopreserved tissues can be useful for fundamental and applied research in stress adaptation and reproductive biology of plants.

  相似文献   
47.
Baculoviruses have enormous potential for use as biopesticides to control insect pest populations without the adverse environmental effects posed by the widespread use of chemical pesticides. However, continuous baculovirus production is susceptible to DNA mutation and the subsequent production of defective interfering particles (DIPs). The amount of DIPs produced and their genome length distribution are of great interest not only for baculoviruses but for many other DNA and RNA viruses. In this study, we elucidate this aspect of virus replication using baculovirus as an example system and both experimental and modeling studies. The existing mathematical models for the virus replication process consider DIPs as a lumped quantity and do not consider the genome length distribution of the DIPs. In this study, a detailed population balance model for the cell‐virus culture is presented, which predicts the genome length distribution of the DIP population along with their relative proportion. The model is simulated using the kinetic Monte Carlo algorithm, and the results agree well with the experimental results. Using this model, a practical strategy to maintain the DIP fraction to near to its maximum and minimum limits has been demonstrated.  相似文献   
48.
Inflammatory bowel diseases (IBDs) are chronic inflammatory disorders of the gastrointestinal tract that affect more than 3 million people worldwide, but the pathological etiology is still unknown. The overall purpose of our investigations was to elucidate the possibility of pathological causes of IBD, and therefore, we determined the difference of inflammatory cytokine profiles in adipose tissue macrophages (ATMs) and T lymphocytes (ATTs) obtained near active lesions of IBD; investigated whether the alteration in ATM activation induces genes involved in collagen formation; and evaluated the effects of fatty acid oxidation inhibitors on factors involved in inflammation and collagen production by ATMs in IBD. Adipose tissues (ATs) were collected near active lesions and also at the margin of resected segments of the bowel from IBD patients with ulcerative colitis (UC) and CD (n = 14/group). Normal appearing ATs from control subjects (n = 14) who had colon resection for adenocarcinoma were collected as far away from the cancer lesion as possible to rule out possible changes. Compared with inactive disease lesions, ATMs and ATTs from active lesions released more IL-6, IL-4 and IL-13. Treatments of cytokine IL-4 and/or IL-13 to ATMs reduced iNOS expression but increased Arg-I expression which were exacerbated when treated with T cell- and adipocyte-conditioned medium. However, fatty acid oxidation inhibitors prevented the effects of cytokines IL-4 and/or IL-13 on iNOS and Arg-I expressions. This study was the first to show the effect of IL-4 and IL-13 on collagen formation, through iNOS and Arg-I expressions, that was exacerbated in a condition that mimics in vivo condition of active lesions. Moreover, our study was the first to provide potential benefits of fatty acid oxidation inhibitors to ATMs on preventing collagen formation; thus, providing therapeutic implications for individuals with intestinal fibrosis and stricture lesions, although future study should be guaranteed to elucidate the underlying mechanisms.  相似文献   
49.
Heat shock protein induction is often associated with a cellular response to a harmful environment or to adverse life conditions. The main aims of our study were (1) to evaluate the cytotoxic potential of cypermethrin; and (2) to investigate the suitability of stress-induced heat shock protein Hsp70 as a biomarker for environmental pollutants in transgenic Drosophila melanogaster (Hsp70-lacZ)Bg9. Different concentrations of cypermethrin (0.002, 0.2, 0.5 and 50.0 p.p.m.) were mixed with food. Third instar larvae of transgenic Drosophila melanogaster were allowed to feed on these mixtures for different time intervals (2, 4, 6, 12, 24 and 48h). Following feeding, hsp70 induction and tissue damage were evaluated. In the highest concentration treatment group (50 p.p.m.), 100% larval mortality was recorded after 12 h exposure. Hsp70 was found to be induced even at the lowest concentration (0.002 p.p.m.) of the insecticide, while tissue damage was observed in the larvae exposed for 48 h. While an insignificant decline in hsp70 expression was observed in the larvae exposed to cypermethrin at a dietary concentration of 0.002 p.p.m. after 48 h compared with those exposed for 24 h, in the next two higher concentrations of the toxicant, a similar but significant decline in hsp70 expression was evident in the exposed larvae after 48 h. The present study reveals the cytotoxic potential of cypermethrin and further proposes that hsp70 induction in transgenic Drosophila melanogaster could be used as a sensitive biomarker in risk assessment.  相似文献   
50.
Despite recent advances in understanding mechanism of toxicity, the development of biomarkers (biochemicals that vary significantly with exposure to chemicals) for pesticides and environmental contaminants exposure is still a challenging task. Carbofuran is one of the most commonly used pesticides in agriculture and said to be most toxic carbamate pesticide. It is necessary to identify the biochemicals that can vary significantly after carbofuran exposure on earthworms which will help to assess the soil ecotoxicity. Initially, we have optimized the extraction conditions which are suitable for high-throughput gas chromatography mass spectrometry (GC-MS) based metabolomics for the tissue of earthworm, Metaphire posthuma. Upon evaluation of five different extraction solvent systems, 80% methanol was found to have good extraction efficiency based on the yields of metabolites, multivariate analysis, total number of peaks and reproducibility of metabolites. Later the toxicity evaluation was performed to characterize the tissue specific metabolomic perturbation of earthworm, Metaphire posthuma after exposure to carbofuran at three different concentration levels (0.15, 0.3 and 0.6 mg/kg of soil). Seventeen metabolites, contributing to the best classification performance of highest dose dependent carbofuran exposed earthworms from healthy controls were identified. This study suggests that GC-MS based metabolomic approach was precise and sensitive to measure the earthworm responses to carbofuran exposure in soil, and can be used as a promising tool for environmental eco-toxicological studies.  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