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   981篇
  免费   54篇
  1035篇
  2023年   12篇
  2022年   36篇
  2021年   55篇
  2020年   21篇
  2019年   30篇
  2018年   43篇
  2017年   33篇
  2016年   39篇
  2015年   62篇
  2014年   54篇
  2013年   62篇
  2012年   82篇
  2011年   70篇
  2010年   41篇
  2009年   31篇
  2008年   35篇
  2007年   45篇
  2006年   49篇
  2005年   40篇
  2004年   31篇
  2003年   26篇
  2002年   21篇
  2001年   10篇
  2000年   9篇
  1999年   6篇
  1998年   10篇
  1997年   8篇
  1996年   2篇
  1995年   3篇
  1994年   2篇
  1993年   3篇
  1992年   7篇
  1991年   3篇
  1990年   6篇
  1988年   7篇
  1987年   3篇
  1986年   5篇
  1985年   3篇
  1984年   3篇
  1983年   5篇
  1982年   3篇
  1981年   2篇
  1980年   2篇
  1979年   4篇
  1977年   2篇
  1976年   1篇
  1975年   3篇
  1974年   1篇
  1973年   2篇
  1972年   1篇
排序方式: 共有1035条查询结果,搜索用时 10 毫秒
111.
Farah JA  Cromie G  Davis L  Steiner WW  Smith GR 《Genetics》2005,171(4):1499-1511
Spo11 or a homologous protein appears to be essential for meiotic DNA double-strand break (DSB) formation and recombination in all organisms tested. We report here the first example of an alternative, mutationally activated pathway for meiotic recombination in the absence of Rec12, the Spo11 homolog of Schizosaccharomyces pombe. Rad2, a FEN-1 flap endonuclease homolog, is involved in processing Okazaki fragments. In its absence, meiotic recombination and proper segregation of chromosomes were restored in rec12Delta mutants to nearly wild-type levels. Although readily detectable in wild-type strains, meiosis-specific DSBs were undetectable in recombination-proficient rad2Delta rec12Delta strains. On the basis of the biochemical properties of Rad2, we propose that meiotic recombination by this alternative (Rec*) pathway can be initiated by non-DSB lesions, such as nicks and gaps, which accumulate during premeiotic DNA replication in the absence of Okazaki fragment processing. We compare the Rec* pathway to alternative pathways of homologous recombination in other organisms.  相似文献   
112.
This paper presents phylogenies of the small but ecologically and horticulturally important Australian genus Correa (Rutaceae). Consensus phylogenies generated using parsimony were congruent with their counterparts generated by Bayesian analysis, although usually less well resolved. The phylogeny generated from the second internal transcribed spacer region of the nuclear ribosomal DNA supported the monophyly of Correa and identified two well supported clades (one comprising C. lawrenceana and C. baeuerlenii and the other containing all other species of the genus). Phylogenetic reconstructions based on the combined trnL-trnF spacer and the trnK intron (including the matK gene) regions of chloroplast DNA also supported the monophyly of Correa and of the C. lawrenceana/C. baeuerlenii clade, but the topology among the other species differed markedly from that in the ITS-based phylogeny. The major clades identified in the chloroplast phylogenies seemed to follow geographic patterns rather than species boundaries, with different samples of C. glabra bearing chloroplast genotypes from different clades. These patterns are likely to be because of independent evolution of the chloroplast and nuclear genomes, and are typical of cases of introgressive hybridisation among species or incomplete lineage sorting of chloroplast genomes leading to incongruence between chloroplast and nuclear phylogenies. Thus, the phylogenies based on nuclear DNA should reflect species relations better than the chloroplast phylogeny in Correa, and we propose a new subgeneric classification of the genus on the basis of the ITS-based phylogeny and morphology. Correa subgenus Persistens Othman, Duretto and G.J. Jord., containing C. lawrenceana and C. baeuerlenii, is formally described.  相似文献   
113.
Macrophage migration inhibitory factor (MIF), a proinflammatory cytokine, is considered an attractive therapeutic target in multiple inflammatory and autoimmune disorders. In addition to its known biologic activities, MIF can also function as a tautomerase. Several small molecules have been reported to be effective inhibitors of MIF tautomerase activity in vitro. Herein we employed a robust activity-based assay to identify different classes of novel inhibitors of the catalytic and biological activities of MIF. Several novel chemical classes of inhibitors of the catalytic activity of MIF with IC50 values in the range of 0.2–15.5 μm were identified and validated. The interaction site and mechanism of action of these inhibitors were defined using structure-activity studies and a battery of biochemical and biophysical methods. MIF inhibitors emerging from these studies could be divided into three categories based on their mechanism of action: 1) molecules that covalently modify the catalytic site at the N-terminal proline residue, Pro1; 2) a novel class of catalytic site inhibitors; and finally 3) molecules that disrupt the trimeric structure of MIF. Importantly, all inhibitors demonstrated total inhibition of MIF-mediated glucocorticoid overriding and AKT phosphorylation, whereas ebselen, a trimer-disrupting inhibitor, additionally acted as a potent hyperagonist in MIF-mediated chemotactic migration. The identification of biologically active compounds with known toxicity, pharmacokinetic properties, and biological activities in vivo should accelerate the development of clinically relevant MIF inhibitors. Furthermore, the diversity of chemical structures and mechanisms of action of our inhibitors makes them ideal mechanistic probes for elucidating the structure-function relationships of MIF and to further determine the role of the oligomerization state and catalytic activity of MIF in regulating the function(s) of MIF in health and disease.  相似文献   
114.
115.
IL-10 has been suggested as a possible parameter for human African trypanosomiasis stage determination. However, conclusive experimental studies have not been carried out to evaluate this, which is a prerequisite before a potential test can be validated in humans for diagnostic purposes. We used the vervet monkey model of trypanosomiasis to scrutinize IL-10 in blood and cerebrospinal fluid (CSF). Five adult males were experimentally infected with T. b. rhodesiense. The infected animals became anemic and exhibited weight loss. Parasitemia was patent after 3 days and fluctuated around 3.7 × 107 trypanosomes/ml throughout the experimental period. The total CSF white cell counts increased from pre-infection means around 3 cells/μl to a peak of 30 cells/μl, 42 days post-infection (DPI). IL-10 was not detectable (< 2 pg/ml) in serum prior to infection. IL-10 serum concentrations increased to 273 pg/ml 10 DPI coinciding with the first peak of parasitemia. Thereafter the levels declined to a mean value of 77 pg/ml 34 DPI followed by a significant rise to a second peak of 304 pg/ml (p < 0.008) 42 DPI. There was no detectable IL-10 in CSF. IL-10 synthesis is thus stimulated both in the early and transitional stages of experimental trypanosomiasis. That IL-10 is produced in early stage disease is an interesting finding unlikely to be detected in humans where it is difficult to determine the exact time of infection. The IL-10 peak observed on day 42 of infection might indicate onset of parasite neuroinvasion coinciding with a peak in white blood cell counts in the blood and CSF.  相似文献   
116.
A cucumber green mosaic mottle virus (CGMMV) full-length clone was developed for the expression of Hepatitis B surface antigen (HBsAg). The expression of the surface displayed HBsAg by the chimeric virus was confirmed through a double antibody sandwich ELISA. Assessment of the coat protein composition of the chimeric virus particles by SDS-PAGE analysis showed that 50% of the coat proteins were fused to the HBsAg. Biological activity of the expressed HBsAg was assessed through the stimulation of in vitro antibody production by cultured peripheral blood mononuclear cells (PBMC). PBMC that were cultured in the presence of the chimeric virus showed up to an approximately three-fold increase in the level of anti HBsAg immunoglobulin thus suggesting the possible use of this new chimeric virus as an effective Hepatitis B vaccine.  相似文献   
117.
CbgA plays a role in cortex formation and the acquisition of a subset of stress resistance properties in Myxococcus xanthus spores. The cbgA mutant produces spores with thin or no cortex layers, and these spores are more sensitive to heat and sodium dodecyl sulfate than their wild-type counterparts.  相似文献   
118.
Twigs (young and old) from Nothapodytes foetida growing in the Jammu and Mahabaleshwar regions in India were used for the isolation of 52 strains of endophytic fungi and were tested for their ability to produce the anticancer alkaloid camptothecin. One of the isolates from the inner bark tissue of the N. foetida plant growing in the Jammu region of J&K state, India, was found to produce detectable quantities of camptothecin and its derivatives when grown in a semi-synthetic liquid medium. Camptothecin was identified by physicochemical analysis and further confirmed by spectroscopic studies. No camptothecin was detected in zero time cultures or in uninoculated culture broth. The maximum yield of camptothecin was 0.575 +/- 0.031 mg/100 g of dry cell mass in 96 h in shake flasks, whereas 4.96 +/- 0.73 mg/100 g of dry mass was recorded in 48 h in a bioreactor.  相似文献   
119.
The Cre-recombination system has become an important tool for genetic manipulation of higher organisms and a model for site-specific DNA-recombination mechanisms employed by the λ-Int superfamily of recombinases. We report a novel quantitative approach for characterizing the probability of DNA-loop formation in solution using time-dependent ensemble Förster resonance energy transfer measurements of intra- and inter-molecular Cre-recombination kinetics. Our method uses an innovative technique for incorporating multiple covalent modifications at specific sites in covalently closed DNA. Because the mechanism of Cre recombinase does not conform to a simple kinetic scheme, we employ numerical methods to extract rate constants for fundamental steps that pertain to Cre-mediated loop closure. Cre recombination does not require accessory proteins, DNA supercoiling or particular metal-ion cofactors and is thus a highly flexible system for quantitatively analyzing DNA-loop formation in vitro and in vivo.  相似文献   
120.
Gemcitabine (2',2'-difluoro-2'-deoxycytidine; dFdCyd) is one of the anti-metabolites drugs that target DNA replication. We evaluated dFdCyd cytotoxicity and its radiosensitizing ability in human breast cancer cell lines, MCF-7 (wild-type p53) and MDA-MB-231 (mutant-type p53) along with normal mammary epithelial cell line (MCF-12) for comparison. Radiosensitivity and cytotoxicity were measured by the clonogenic survival assays. DNA DSBs was studied by Pulse Field Gel Electrophoresis (PFGE) and cell cycle distribution was analyzed by flow cytometry. MDA-MB-231 cells were the most sensitive to the cytotoxicity of dFdCyd (IC(50) 5 nM) then MCF-7 (IC(50) 10nM), whereas MCF-12 cells were the most resistant to the cytotoxicity of dFdCyd (IC(50) 70 nM). MCF-12 and MCF-7 cell lines did not show any radiosensitization to dFdCyd, whereas the MDA-MB-231 cells showed significantly increased radioresistant to dFdCyd at equimolar concentration (p=0.002) and at IC(50) concentration (p<0.001). The DNA double strand breaks (DSBs) repair showed that dFdCyd neither increases DNA DSBs nor decreases the rate of their repair in MCF-12 and MCF-7 cell lines, while the same treatment in MDA-MB-231 cell line led to decrease the rate of DSBs or increase the rate of DNA repair (p=0.034). Therefore, dFdCyd is a cytotoxic agent, especially in the cancer cells irrespective of having wild-type or mutated p53 protein, but it is not effective as radiosensitizer in the cell lines used in this study. dFdCyd combined with radiation reduces the efficacy of chemo-radiotherapy in p53 mutated cells. Therefore, p53-mutated cancer could be a counter-indication for radiation-gemcitabine combined treatment.  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