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   664篇
  免费   86篇
  国内免费   1篇
  751篇
  2024年   2篇
  2023年   3篇
  2022年   36篇
  2021年   39篇
  2020年   29篇
  2019年   85篇
  2018年   67篇
  2017年   47篇
  2016年   41篇
  2015年   39篇
  2014年   44篇
  2013年   64篇
  2012年   53篇
  2011年   47篇
  2010年   30篇
  2009年   13篇
  2008年   15篇
  2007年   20篇
  2006年   8篇
  2005年   9篇
  2004年   8篇
  2003年   9篇
  2002年   8篇
  2001年   2篇
  2000年   4篇
  1999年   2篇
  1998年   1篇
  1997年   1篇
  1996年   2篇
  1995年   2篇
  1994年   2篇
  1993年   1篇
  1992年   3篇
  1991年   1篇
  1990年   3篇
  1989年   3篇
  1978年   2篇
  1975年   1篇
  1974年   2篇
  1972年   1篇
  1966年   1篇
  1933年   1篇
排序方式: 共有751条查询结果,搜索用时 0 毫秒
731.
The enzyme 1-aminocyclopropane-1-carboxylate deaminase (ACCD) converts ACC, the precursor of the plant hormone ethylene, to alpha-ketobutyrate and ammonium. This enzyme has been identified in soil bacteria and has been proposed to play a key role in microbe-plant association. A soluble recombinant ACCD from Pseudomonas putida UW4 of molecular weight 41 kDa has been cloned, expressed, and purified. It showed selectivity and high activity towards the substrate ACC: K(M)=3.4+/-0.2 mM and k(cat)=146+/-5 min(-1) at pH 8.0 and 22 degrees C. The enzyme displayed optimal activity at pH 8.0 with a sharp decline to essentially no activity below pH 6.5 and a slightly less severe tapering in activity at higher pH resulting in loss of activity at pH>10. The major component of the enzyme's secondary structure was determined to be alpha-helical by circular dichroism (CD). P. putida UW4 ACCD unfolded at 60 degrees C as determined by its CD temperature profile as well as by differential scanning microcalorimetry (DSC). Enzyme activity was knocked out in the point mutant Gly44Asp. Modeling this mutation into the known yeast ACCD structure shed light on the role this highly conserved residue plays in allowing substrate accessibility to the active site. This enzyme's biochemical and biophysical properties will serve as an important reference point to which newly isolated ACC deaminases from other organisms can be compared.  相似文献   
732.
733.
The biological synthesis methods have been emerging as a promising new approach for production of nanoparticles due to their simplicity and non-toxicity. In the present study, spores of Bacillus athrophaeus were used to achieve the objective of developing a green synthesis method of silver nanoparticles. Enzyme assay revealed that the spores and their heat inactivated forms (microcapsules) were highly active and their enzymatic contents differed from the vegetative cells. Laccase, glucose oxidase, and alkaline phosphatase activities were detected in the dormant forms, but not in the vegetative cells. Although no nanoparticle was produced by active cells of B. athrophaeus, both spores and microcapsules were efficiently capable of reducing the silver ions (Ag+) to elemental silver (Ag0) leading to the formation of nanoparticles from silver nitrate (AgNO3). The presence of biologically synthesized silver nanoparticles was determined by obtaining broad spectra with maximum absorbance at 400 nm in UV–visible spectroscopy. The X-ray diffraction analysis pattern revealed that the nanoscale particles have crystalline nature with various topologies, as confirmed by transmission electron microscopy (TEM). The TEM micrograph showed the nanocrystal structures with dimensions ranging from 5 to 30 nm. Accordingly, the spore mixture could be employed as a factory for detoxification of heavy metals and subsequent production of nanoparticles. This research introduces an environmental friendly and cost effective biotechnological process for the extracellular synthesis of silver nanoparticles using the bacterial spores.  相似文献   
734.
Sclerotinia wilt of sunflower caused by Sclerotinia sclerotiorum (Lib.) De Bary is the major disease of sunflower (Helianthus annuus L.) in Iran. North-western areas of Iran demonstrate the most yield losses through the disease. Development of cultivars with adequate genetic resistance is necessary to avoid these losses. Evaluation of sunflower genotypes varies based on researchers and environmental conditions. Meanwhile, greenhouse tests of the sunflower genotypes are more reliable because of controlled conditions during the assessment activities. To study the reaction of the host plants under controlled conditions, three stem inoculation techniques, including mycelium plug (MP), oxalic acid solution (OAS) and wheat seeds infested (IWS) with Sclerotinia mycelium, were employed. Wounded and non-wounded treatments were used in the experiment to find their effect on the disease progress. Four genotypes, including Ghalami (local variety in market), Confeta, Alstar and Master, were inoculated in this study. The factor was the lesion length to evaluate the effectiveness of different inoculation procedures. The lesion length was measured after 3, 7, 10 and 14 days post inoculation. The analysis of variance demonstrated significant differences between IWS and two other methods (MP and OAS), where the IWS produced the longest lesion lengths. In contrast, the non-significant differences between MP and OAS methods might help researchers to employ the pathogenicity factor (oxalic acid) as an alternative inoculum for their studies. Master variety was the most tolerant genotype among the treatments and its viability was 100% even at 14?days after inoculation and incubation. Interestingly, there was no mortality in all cultivars before 7-day incubations, regardless of resistant or susceptible reactions.  相似文献   
735.
Affected sib pair and linkage disequilibrium analysis, intrafamilial and case-control association studies were performed on 81 Danish multiplex insulin-dependent diabetes mellitus (IDDM) families (382 individuals) and 82 healthy Danish controls. The results confirm the linkage of D15S107 to IDDM in these Danish IDDM families (P = 0.010). When these data are combined with those of previous studies, an even stronger case for linkage can be made (P = 0.0005). Our analyses show that the D15S107*130 allele provides increased susceptibility (P = 0.02, relative risk = 3.55) and that the D15S107 locus contributes up to 16% of the familial clustering of IDDM. The analysis of affected sib pairs suggests that HLA and D15S107 may possibly act independently of each other. Taken together with our previous findings, our results suggest that three causes of susceptibilities can be discerned in the IDDM patient population: (1) a major susceptibility caused by the HLA-DRB1 alleles; (2) a minor susceptibility caused by the joint action of HLA and other non-HLA gene(s); and (3) a minor susceptibility caused by non-HLA gene(s). Received: 18 March 1996 / Revised: 17 May 1996  相似文献   
736.
Conventional treatments for breast cancer are costly and have serious side effects. Non-conventional natural treatments have gained wide acceptance due to their promise of a cure with minimal or no side effects, but little scientific evidence exists. One such common remedy is the seed of the Lepidium sativum plant. Presented here is the first reported use of the aqueous extract of Lepidium sativum seeds on breast cancer cells. The ability of the extract to induce apoptosis and necrosis in the human breast cancer cell line MCF-7, compared to normal human skin fibroblasts (HFS), was determined by morphological changes in the cells using light microscopy, DNA fragmentation assay, and florescent stains (Annexin V and propidium iodide) using flow cytometry and fluorescent microscopy. Apoptosis was induced in both cells, and more in MCF-7, when they were treated with 25% and 50% extract, while necrosis was observed mainly after exposure to elevated extract concentrations (75%). DNA fragmentation resulted for both cells, in a time and dose-dependent manner. Both cells, at all extract concentrations, showed no significant differences in the number of living, dead, apoptotic, and necrotic cells. Finally, the results may indicate that apoptotic changes in MCF-7 may be independent of caspase-3, which is involved in apoptosis and is lacking in MCF-7 cells.  相似文献   
737.
Bioassay-guided isolation and subsequent structure elucidation of a Bael tree Aegle marmelos lipid extract yielded two unstable acylated geranyloxycoumarin mixtures (12), six geranyloxycoumarins (38), (+)-9′-isovaleroxylariciresinol (9), and dehydromarmeline (10). In a T47D cell-based reporter assay, 1 and 2 potently inhibited hypoxia-induced HIF-1 activation (IC50 values 0.18 and 1.10 μg mL?1, respectively). Insufficient material and chemical instability prevented full delineation of the fatty acyl side chain olefin substitution patterns in 1 and 2. Therefore, five fatty acyl geranyloxycoumarin ester derivatives (1115) were prepared from marmin (3) and commercial fatty acyl chlorides by semisynthesis. The unsaturated C-6′ linoleic acid ester derivative 14 that was structurally most similar to 1 and 2, inhibited HIF-1 activation with comparable potency (IC50 0.92 μM). The octanoyl (11) and undecanoyl (12) ester derivatives also suppressed HIF-1 activation (IC50 values 3.1 and 0.87 μM, respectively). Mechanistic studies revealed that these geranyloxycoumarin derivatives disrupt mitochondrial respiration, primarily at complex I. Thus, these compounds may inhibit HIF-1 activation by suppressing mitochondria-mediated hypoxic signaling. One surprising observation was that, while less potent, the purported cancer chemopreventive agent auraptene (8) was found to act as a mitochondrial poison that disrupts HIF-1 signaling in tumors.  相似文献   
738.
Infectious pathogens can disrupt the microbiome in addition to directly affecting the host. Impacts of disease may be dependent on the ability of the microbiome to recover from such disturbance, yet remarkably little is known about microbiome recovery after disease, particularly in nonhuman animals. We assessed the resilience of the amphibian skin microbial community after disturbance by the pathogen, Batrachochytrium dendrobatidis (Bd). Skin microbial communities of laboratory-reared mountain yellow-legged frogs were tracked through three experimental phases: prior to Bd infection, after Bd infection (disturbance), and after clearing Bd infection (recovery period). Bd infection disturbed microbiome composition and altered the relative abundances of several dominant bacterial taxa. After Bd infection, frogs were treated with an antifungal drug that cleared Bd infection, but this did not lead to recovery of microbiome composition (measured as Unifrac distance) or relative abundances of dominant bacterial groups. These results indicate that Bd infection can lead to an alternate stable state in the microbiome of sensitive amphibians, or that microbiome recovery is extremely slow—in either case resilience is low. Furthermore, antifungal treatment and clearance of Bd infection had the additional effect of reducing microbial community variability, which we hypothesize results from similarity across frogs in the taxa that colonize community vacancies resulting from the removal of Bd. Our results indicate that the skin microbiota of mountain yellow-legged frogs has low resilience following Bd-induced disturbance and is further altered by the process of clearing Bd infection, which may have implications for the conservation of this endangered amphibian.Subject terms: Microbial ecology, Community ecology  相似文献   
739.
We examined sexual size dimorphism of the rock-dwelling lizard Darevskia raddei (Boettger, 1892) with the help of 30 specimens that were provided from various sources. Eleven metric and seven meristic features were examined. Seven characters (gulars, length of basal tail, femoral pores, length of head, width of head, length of fore limb and length of hind limb) were identified as dimorphic between the two sexes. Some of these characters have important roles in copulation for males, especially the hind limb and the tail base. The number of femoral pores is important in the release of signal components because females release these components to attract males during the mating season. The length of the hind limb as locomotor performance plays an important role during mating, so that the male can grasp the female and adopt the correct position during copulation.  相似文献   
740.
The G4C2 repeat expansion in C9orf72 is the most common known cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). We tested the hypothesis that the repeat expansion causes aberrant CpG methylation near the G4C2 repeat, which could be responsible for the downregulation of gene expression. We investigated the CpG methylation profile by two methods using genomic DNA from the blood of individuals with ALS (37 expansion carriers and 64 noncarriers), normal controls (n = 76), and family members of 7 ALS probands with the expansion. We report that hypermethylation of the CpG island 5′ of the G4C2 repeat is associated with the presence of the expansion (p < 0.0001). A higher degree of methylation was significantly correlated with a shorter disease duration (p < 0.01), associated with familial ALS (p = 0.009) and segregated with the expansion in 7 investigated families. Notably, we did not detect methylation for either normal or intermediate alleles (up to 43 repeats), bringing to question the current cutoff of 30 repeats for pathological alleles. Our study raises several important questions for the future investigation of large data sets, such as whether the degree of methylation corresponds to clinical presentation (ALS versus FTLD).  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