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   1213篇
  免费   84篇
  2023年   4篇
  2022年   8篇
  2021年   16篇
  2020年   6篇
  2019年   23篇
  2018年   27篇
  2017年   17篇
  2016年   37篇
  2015年   78篇
  2014年   53篇
  2013年   97篇
  2012年   106篇
  2011年   79篇
  2010年   80篇
  2009年   59篇
  2008年   73篇
  2007年   60篇
  2006年   86篇
  2005年   55篇
  2004年   70篇
  2003年   50篇
  2002年   54篇
  2001年   17篇
  2000年   19篇
  1999年   7篇
  1998年   17篇
  1997年   12篇
  1996年   7篇
  1995年   13篇
  1994年   13篇
  1993年   11篇
  1992年   8篇
  1991年   10篇
  1990年   5篇
  1989年   2篇
  1988年   2篇
  1987年   5篇
  1986年   3篇
  1985年   3篇
  1983年   1篇
  1981年   1篇
  1978年   1篇
  1977年   1篇
  1952年   1篇
排序方式: 共有1297条查询结果,搜索用时 375 毫秒
991.
Lipoxygenases (EC. 1.13.11.12) are a non-heme iron enzymes consisting of one polypeptide chain folded into two domains, the N-terminal domain and the catalytic moiety β-barrel domain. They catalyze the dioxygenation of 1Z,4Z-pentadiene moieties of polyunsaturated fatty acids obtaining hydroperoxy fatty acids. For years, the presence of lipoxygenases was considered a eukaryotic feature, present in mammals, plants, small marine invertebrates, and fungi, but now, some lipoxygenase sequences have been detected on prokaryotic organisms, changing the idea that lipoxygenases are exclusively a eukaryotic affair. Lipoxygenases are involved in different types of reactions on eukaryote organisms where the biological role and the structural characteristics of these enzymes are well studied. However, these aspects of the bacterial lipoxygenases have not yet been elucidated and are unknown. This revision discusses biochemical aspects, biological applications, and some characteristics of these enzymes and tries to determine the existence of a subfamily of bacterial lipoxygenases in the context of the phylogeny of prokaryotic lipoxygenases, supporting the results of phylogenetic analyzes with the comparison and discussion of structural information of the first prokaryotic lipoxygenase crystallized and other eukaryotic lipoxygenases structures.  相似文献   
992.
The complex resistance to cucumber mosaic virus (CMV) present in the exotic melon accession Sonwang Charmi PI161375 (SC) has been studied using two populations, a near-isogenic line (NIL) collection and a doubled haploid line (DHL) collection, both generated from a cross between SC and the cultivar Piel de Sapo as resistant and susceptible parents, respectively. The NIL collection had previously allowed us to describe a single recessive gene, cmv1, which conferred full resistance to CMV strains P9 and P104.82. Screening of the whole DHL population followed by quantitative trait locus (QTL) analysis revealed that resistance to the strains M6 and TL, both non-responsive to cmv1, was quantitative and governed by at least three QTLs. One of them, cmvqw12.1, co-located with cmv1 in linkage group (LG) XII. The QTL analysis mapped another two QTLs in LGIII (cmvqw3.1) and LGX (cmvqw10.1) and showed interaction between cmvqw12.1 and cmvqw3.1. Progeny of crosses between resistant DHLs carrying the three main QTLs showed complete resistance to the strain M6, validating the accuracy of the QTL analysis. However, in our screening, there were resistant DHLs carrying only two QTLs, suggesting that there are other regions involved in resistance to M6 and required when one of the main QTLs is missing. Therefore, resistance to CMV in melon SC is qualitative for some strains and quantitative for the rest. For this late resistance, cmv1 is necessary and explains most of the phenotypic variance, but it is not sufficient, and needs the interaction with other loci.  相似文献   
993.
Aphanomyces astaci (Oomycetes) is responsible for the crayfish plague disease. This species is endemic of North America and five genotypes have been described using RAPD-PCR. The red swamp crayfish, Procambarus clarkii, is one of the most widely spread North American species and invasive in the world. However, no outbreaks on its specific genotype, i.e., genotype D, have ever been described in nature. We investigated three major series of crayfish plague outbreaks in indigenous crayfish populations of Austropotamobius pallipes, located in the areas of influence of P. clarkii. All samples collected tested positive for A. astaci using a rnDNA ITS-PCR test. We also performed an AFLP-PCR analysis on 19 isolates, and found that all isolates belong to genotype D. These isolates exhibited similar properties, i.e., adaptation to warm temperatures. We demonstrate, for the first time, the transmission of A. astaci genotype D to indigenous European populations of crayfish, and confirm that the properties of adaptation to warm water temperatures seem to be a specific character of genotype D. The results of this work emphasize once more the need of controlling invasive species and its trade, since they can carry harmful pathogens with specific adaptations or increased virulence in new environments.  相似文献   
994.
Summary We used the following polymorphic markers: APOC2 (BanI, BglI, TaqI), CKMM (NcoI, TaqI), and D19S63 (PstI) to haplotype 33 Spanish myotonic dystrophy (DM) families. We analysed the allele and haplotype frequencies of our sample, and the possible association of alleles or haplotypes with the disease. We found a slight linkage disequilibrium between APOC2 (BanI) and DM, but no disequilibrium when using all other APOC2 and CKMM RFLPs; this agrees with data previously reported. In addition, we found a very strong linkage disequilibrium when using D19S63 (PstI), the + allele being associated with the DM locus. This disequilibrium in the Spanish population indicates that D19S63 is very close to the DM locus.  相似文献   
995.
The aim of this study was to evaluate the use of serum type II collagen cleavage epitope and serum hyaluronic acid as biomarkers for treatment monitoring in osteoarthritic dogs. For this purpose, a treatment model based on mesenchymal stem cells derived from adipose tissue combined with plasma rich in growth factors was used. This clinical study included 10 dogs with hip osteoarthritis. Both analytes were measured in serum at baseline, just before applying the treatment, and 1, 3, and 6 months after treatment. These results were compared with those obtained from force plate analysis using the same animals during the same study period. Levels of type II collagen cleavage epitope decreased and those of hyaluronic acid increased with clinical improvement objectively verified via force plate analysis, suggesting these two biomarkers could be effective as indicators of clinical development of joint disease in dogs.  相似文献   
996.
997.
AimTo evaluate the prevalence of alcohol consumption among university students during late adolescence and young adulthood and to identify the associated factors.ResultsThe rates of prevalence of RC were lower, but not statistically significant, in women. The age-related changes in these rates were similar in both genders, and the prevalence of RC peaked at 20 years. By contrast, the prevalence of HED was significantly lower in women and peaked at 18 years in women and at 22 years in men. Multivariate models showed that early age of onset of alcohol use (OR = 10.6 and OR = 6.9 for women; OR = 8.3 and OR = 8.2 for men) and positive expectations about alcohol (OR = 7.8 and OR = 4.5 for women; OR = 3.6 and OR = 3.3 for men) were the most important risk factors for RC and HED. Living away from the family home was also a risk factor for both consumption patterns among women (OR = 3.16 and OR = 2.34), while a high maternal education level was a risk factor for RC among both genders (OR = 1.62 for women; OR = 2.49 for men).ConclusionsAlcohol consumption decreases significantly at the end of youth, with higher rates of prevalence and a later peak among men. Prevention strategies should focus on beliefs and expectations about alcohol and on delaying the age of onset. Women are at particular risk for these consumption patterns if they live away from their parents. Belonging to a high-income family is a strong risk factor for RC.  相似文献   
998.
The neuropsychological battery used in Fundació ACE (NBACE) is a relatively brief, and easy to administer, test battery that was designed to detect cognitive impairment in the adulthood. The NBACE includes measures of cognitive information processing speed, orientation, attention, verbal learning and memory, language, visuoperception, praxis and executive functions. The aim of the present study was to establish the cut-off scores for impairment for different levels of age and education that could be useful in the cognitive assessment of Spanish subjects who are at risk for cognitive impairment, especially dementia. Data from 1018 patients with a mild dementia syndrome, and 512 cognitively healthy subjects, older than 44 years, from the Memory Clinic of Fundació ACE (Barcelona, Spain) were analyzed. In the whole sample, cut-off scores and sensitivity/specificity values were calculated for six conditions after combining 3 age ranges (44 to 64; 65 to 74; and older than 74 years old) by 2 educational levels (until Elementary school; and more than Elementary school). Moreover, general cut-offs are reported for Catalan and Spanish speakers. The results showed that most of NBACE tests reached good sensitivity and specificity values, except for Ideomotor praxis, Repetition and Verbal Comprehension tests, which had a ceiling effect. Word List Learning from the Wechsler Memory Scale-III and Semantic Verbal Fluency were the most useful tests to discriminate between cognitively healthy and demented subjects. The NBACE has been shown to be a useful tool able to detect cognitive impairment, especially dementia, in older than 44 years Spanish persons.  相似文献   
999.
Semiempirical and density functional calculations have been carried out to discuss the origin of the closed [5,6]-aza-bridged adduct obtained as a minor product in the reaction of singlet nitrenes with C60. The results indicated that the most likely source is the direct addition of singlet nitrenes to a [5,6]-bond of C60, in contradiction to the common belief that the [5,6]-bonds are not attacked in cycloaddition reactions to C60.Electronic Supplementary Material available.  相似文献   
1000.
Aggregation and fusion of lipoproteins trigger subendothelial retention of cholesterol, promoting atherosclerosis. The tendency of a lipoprotein to form fused particles is considered to be related to its atherogenic potential. We aimed to isolate and characterize aggregated and nonaggregated subfractions of LDL from human plasma, paying special attention to particle fusion mechanisms. Aggregated LDL was almost exclusively found in electronegative LDL (LDL(−)), a minor modified LDL subfraction, but not in native LDL (LDL(+)). The main difference between aggregated (agLDL(−)) and nonaggregated LDL(−) (nagLDL(−)) was a 6-fold increased phospholipase C-like activity in agLDL(−). agLDL(−) promoted the aggregation of LDL(+) and nagLDL(−). Lipoprotein fusion induced by α-chymotrypsin proteolysis was monitored by NMR and visualized by transmission electron microscopy. Particle fusion kinetics was much faster in agLDL(−) than in nagLDL(−) or LDL(+). NMR and chromatographic analysis revealed a rapid and massive phospholipid degradation in agLDL(−) but not in nagLDL(−) or LDL(+). Choline-containing phospholipids were extensively degraded, and ceramide, diacylglycerol, monoacylglycerol, and phosphorylcholine were the main products generated, suggesting the involvement of phospholipase C-like activity. The properties of agLDL(−) suggest that this subfraction plays a major role in atherogenesis by triggering lipoprotein fusion and cholesterol accumulation in the arterial wall.  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