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   12200篇
  免费   1157篇
  国内免费   1363篇
  14720篇
  2024年   57篇
  2023年   271篇
  2022年   553篇
  2021年   824篇
  2020年   592篇
  2019年   666篇
  2018年   646篇
  2017年   430篇
  2016年   527篇
  2015年   847篇
  2014年   870篇
  2013年   992篇
  2012年   1136篇
  2011年   1022篇
  2010年   591篇
  2009年   540篇
  2008年   629篇
  2007年   515篇
  2006年   435篇
  2005年   356篇
  2004年   319篇
  2003年   294篇
  2002年   250篇
  2001年   222篇
  2000年   169篇
  1999年   192篇
  1998年   106篇
  1997年   89篇
  1996年   64篇
  1995年   63篇
  1994年   55篇
  1993年   44篇
  1992年   66篇
  1991年   45篇
  1990年   34篇
  1989年   40篇
  1988年   30篇
  1987年   25篇
  1986年   28篇
  1985年   25篇
  1984年   11篇
  1983年   15篇
  1982年   9篇
  1981年   3篇
  1979年   4篇
  1976年   2篇
  1974年   2篇
  1973年   3篇
  1969年   2篇
  1967年   2篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
The mechanism of mass collaboration in risk management was studied during the Sichuan earthquake under a Web-based “PeopleFinder” project, where information is contributed and shared among mass contributors. The case study is provided by a great earthquake that happened in Wenchuan County, Sichuan Province, of southwestern China at 2:28 p.m. on May 12, 2008. We witnessed and experienced the rescue and relief efforts for the great earthquake. In this article, two fundamental frameworks are developed to study the mechanism of mass collaboration. Mass collaboration is proven to be effective in a big public crisis such as the Sichuan earthquake.  相似文献   
2.
3.
Background: Little is known about geographic variations in liver cancer at high incident regions. We aimed to identify spatial variation of hepatocellular carcinoma (HCC) at a high-risk area in China and determine its association with socioeconomic status (SES). Methods: Based on 2299 liver cancer cases diagnosed in Haimen from 2003 to 2006, we calculated age–sex standardized incidence ratios (SIRs) and used two spatial scan statistics to determine the geographic variations in HCC. Bayesian hierarchical model was used to explore the association between HCC incidence and SES. Results: Age and sex SIRs for HCC varied from 0.54 to 1.97 for 24 townships. The eastern region of Haimen was identified to have a significantly increased risk of HCC. Fitting of a Bayesian hierarchical model linking per-capita fiscal revenue with SIRs of HCC indicated that the area with a lower revenue had a significantly higher incidence of HCC [βlog(revenue) = ?0.179, posterior 95% Bayesian credible interval (CI) = (?0.326, ?0.04)]. Conclusions: This study demonstrated substantial geographic variation in the incidence of HCC within a high-risk region, which was associated with SES. HCC control and intervention should focus on disadvantaged areas to reduce the HCC disparities.  相似文献   
4.
We assessed the effects of topographic heterogeneity and stem density on species composition between grains of different sizes (20 × 20, 50 × 50, and 100 × 100 m), based on partial Mantel tests. Similarity in species composition was measured by the abundance-based Jaccard index (C_J) and by an index that incorporates phylogenetic information into C_J (pC_J). Plants were divided into five groups, arbor, subarbor, and shrub according to life form and two other groups: species that produce dry fruits (PDF) and that produce fleshy fruits (PFF). C_J and pC_J between any two grains at each grain size were calculated separately for these groups and for all species combined. In order to examine what influences C_J and pC_J, we analyzed their correlations with topographic heterogeneity variables and two dispersal limitation-related variables (stem and topographic resistance). Our data indicate that at all three grain sizes, C_J and pC_J decrease with increasing distance for all plant groups. Dispersal limitation and topographic heterogeneity were both important at 20 × 20 and 50 × 50 m grain sizes for C_J and pC_J of all plant groups; and at 100 × 100 m grain size, topographic heterogeneity dominates over dispersal limitation for some plant groups. C_J and pC_J of PDFs are less negatively correlated with stem resistance than those of PFFs. We conclude that both beta diversity and phylobetadiversity are dependent on plant groups and grain sizes.  相似文献   
5.
Social responsibility links personal behavior with societal expectations and plays a key role in affecting an agent’s emotional state following a decision. However, the neural basis of responsibility attribution remains unclear. In two previous event-related brain potential (ERP) studies we found that personal responsibility modulated outcome evaluation in gambling tasks. Here we conducted a functional magnetic resonance imaging (fMRI) study to identify particular brain regions that mediate responsibility attribution. In a context involving team cooperation, participants completed a task with their teammates and on each trial received feedback about team success and individual success sequentially. We found that brain activity differed between conditions involving team success vs. team failure. Further, different brain regions were associated with reinforcement of behavior by social praise vs. monetary reward. Specifically, right temporoparietal junction (RTPJ) was associated with social pride whereas dorsal striatum and dorsal anterior cingulate cortex (ACC) were related to reinforcement of behaviors leading to personal gain. The present study provides evidence that the RTPJ is an important region for determining whether self-generated behaviors are deserving of praise in a social context.  相似文献   
6.
Protein malnutrition and drug metabolism   总被引:3,自引:0,他引:3  
T L Yue 《生理科学进展》1987,18(3):246-251
  相似文献   
7.
D Chen  K T Yue  C Martin  K W Rhee  D Sloan  R Callender 《Biochemistry》1987,26(15):4776-4784
We report the Raman spectra of reduced and oxidized nicotinamide adenine dinucleotide (NADH and NAD+, respectively) and adenosine 5'-diphosphate ribose (ADPR) when bound to the coenzyme site of liver alcohol dehydrogenase (LADH). The bound NADH spectrum is calculated by taking the classical Raman difference spectrum of the binary complex, LADH/NADH, with that of LADH. We have investigated how the bound NADH spectrum is affected when the ternary complexes with inhibitors are formed with dimethyl sulfoxide (Me2SO) or isobutyramide (IBA), i.e., LADH/NADH/Me2SO or LADH/NADH/IBA. Similarly, the difference spectra of LADH/NAD+/pyrazole or LADH/ADPR with LADH are calculated. The magnitude of these difference spectra is on the order of a few percent of the protein Raman spectrum. We report and discuss the experimental configuration and control procedures we use in reliably calculating such small difference signals. These sensitive difference techniques could be applied to a large number of problems where the classical Raman spectrum of a "small" molecule, like adenine, bound to the active site of a protein is of interest. The spectrum of bound ADPR allows an assignment of the bands of the bound NADH and NAD+ spectra to normal coordinates located primarily on either the nicotinamide or the adenine moiety. By comparing the spectra of the bound coenzymes with model compound data and through the use of deuterated compounds, we confirm and characterize how the adenine moiety is involved in coenzyme binding and discuss the validity of the suggestion that the adenine ring is protonated upon binding. The nicotinamide moiety of NADH shows significant molecular changes upon binding.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
8.
J Aimi  H Qiu  J Williams  H Zalkin    J E Dixon 《Nucleic acids research》1990,18(22):6665-6672
The trifunctional enzyme encoding glycinamide ribonucleotide synthetase (GARS)-aminoimidazole ribonucleotide synthetase (AIRS)-glycinamide ribonucleotide transformylase (GART) was cloned by functional complementation of an E. coli mutant using an avian liver cDNA expression library. In E. coli, genes encoding these separate activities (purD, purM, and purN, respectively) produce three proteins. The avian cDNA, in contrast, encodes a single polypeptide with all three enzyme activities. Using the avian DNA as a probe, a cDNA encoding the complete coding sequence of the trifunctional human enzyme was also isolated and sequenced. The deduced amino acid sequence of the human and avian polyproteins show extensive sequence homologies to the bacterial purD, purM, and purN encoded proteins. Avian and human liver RNAs appear to encode both a trifunctional enzyme (G-ARS-AIRS-GART) as well as an RNA which encodes only GARS. The trifunctional protein has been implicated in the pathology of Downs Syndrome and molecular tools are now available to explore this hypothesis. Initial efforts to compare the expression of GARS-AIRS-GART between a normal fibroblast cell line and a Downs Syndrome cell line indicate that the levels of RNA are similar.  相似文献   
9.
人体小卫星DNA探针的制备   总被引:3,自引:2,他引:1  
郭光明  蒋左庶 《遗传学报》1990,17(3):226-229
根据人体小卫星DNA核心顺序,化学合成长23碱基寡核苷酸探针,筛选人体基因组文库,旨在获得能用作遗传分析探针的小卫星顺序。结果得到15个含小卫星的阳性重组子。随机取其一(C_(35.9))作探针,试做群体分析。所有个体均可检出多条杂交带。其中某些带具有多态性。在一定检测条件下,检出的DNA图谱在有限的个体内具有个体特异性。结果表明筛选文库得到的小卫星顺序可用于小卫星多态性的检测。其它小卫星探针的筛选和应用性研究正在进行。  相似文献   
10.
Resonance Raman, NMR, and visible spectroscopies, as well as viscosity and equilibrium dialysis studies were used to assess the effect of the N-alkyl substituent of meso-tetrakis(4-N-alkylpyridinium-4-yl)porphyrin cations on DNA binding. The DNAs studied include the native DNA, calf thymus DNA (CT DNA), the synthetic polynucleotides [poly(dGdC)]2 and [poly(dAdT)]2, and the oligonucleotide d(TATACGTATA)2. Both the porphyrins and the metalloporphyrins containing Ni(II) were examined with the N-alkyl = propyl (TPrpyP(4) and NiTPrpyP(4)) and 2-hydroxyethyl (TEtOHpyP(4) and NiTEtOHpyP(4)). The results were compared to those from the parent porphyrins with the N-methyl substituent (TMpyP(4) and NiTMpyP(4)). For almost all the comparisons made, the new porphyrin cations gave results very similar to those for the TMpyP(4) species. The resonance Raman study indicated that for the three DNA polymers all the Ni species were in the four-coordinate form when bound to all three polymers. It is suggested that both TPrpyP(4) and TEtOHpyP(4) bind to GC regions of DNA in the same intercalative manner as TMpyP(4) with the N-alkyl substituent extended into the solvent. For AT regions of DNA, the binding of TPrpyP(4) and TEtOHpyP(4) is nonintercalative, as found previously for TMpyP(4). The NiPrpy(4) and NiTEtOHpyP(4) cations bind to these polymers in a similar manner to the apo-porphyrins. The similar Raman spectral changes for the three Ni porphyrins upon addition of [poly(dAdT)]2 suggest that partial intercalation is not occurring because models indicate that it would be difficult to accommodate the bulkier N-alkyl substituents.  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