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   2131篇
  免费   128篇
  国内免费   4篇
  2263篇
  2024年   4篇
  2023年   20篇
  2022年   33篇
  2021年   53篇
  2020年   41篇
  2019年   40篇
  2018年   51篇
  2017年   52篇
  2016年   75篇
  2015年   89篇
  2014年   138篇
  2013年   170篇
  2012年   171篇
  2011年   214篇
  2010年   131篇
  2009年   109篇
  2008年   134篇
  2007年   141篇
  2006年   120篇
  2005年   100篇
  2004年   82篇
  2003年   61篇
  2002年   84篇
  2001年   16篇
  2000年   6篇
  1999年   16篇
  1998年   16篇
  1997年   10篇
  1996年   6篇
  1995年   7篇
  1994年   13篇
  1993年   5篇
  1992年   10篇
  1990年   3篇
  1989年   2篇
  1988年   3篇
  1987年   3篇
  1986年   3篇
  1985年   8篇
  1984年   1篇
  1983年   1篇
  1982年   2篇
  1981年   2篇
  1980年   5篇
  1979年   2篇
  1978年   1篇
  1977年   5篇
  1973年   1篇
  1970年   1篇
  1966年   1篇
排序方式: 共有2263条查询结果,搜索用时 0 毫秒
11.
Abnormalities in the transport of saturated very long chain fatty acids (VLCFA; >C18:0) contribute to their toxic levels in peroxisomal disorders of fatty acid metabolism, such as adrenoleukodystrophy and adrenomyeloneuropathy. We previously showed that VLCFA desorb much slower than normal dietary fatty acids from both albumin and protein-free lipid bilayers. The important step of transbilayer movement (flip-flop) was not measured directly as a consequence of this very slow desorption from donors, and the extremely low aqueous solubility of VLCFA precludes addition of unbound VLCFA to lipid membranes. We have overcome these limitations using methyl-β-cyclodextrin to solubilize VLCFA for rapid delivery to “acceptor” phosphatidylcholine vesicles (small and large unilamellar) and to cells. VLCFA binding was monitored in real time with the fluorescent probe fluorescein-labeled phosphatidylethanolamine in the outer membrane leaflet, and entrapped pyranine was used to detect flip-flop across the membrane. The upper limit of the rate of flip-flop across the membrane was independent of temperature and media viscosity and was similar for model raft and non-raft membranes as well as living cells. We further showed that cyclodextrins can extract VLCFA rapidly (within seconds) from vesicles and cells, which have implications for the mechanism and potential alternative approaches to treat adrenoleukodystrophy. Because VLCFA diffuse through the lipid bilayer, proteins may not be required for their transport across the peroxisomal membrane.  相似文献   
12.
This study aims to establish an efficient protocol for development of seedlings of an endangered medicinally important forest tree Boswellia serrata Roxb., for mass plantation and consistent supply of salai guggul. The green mature fruits served as source of seeds. The excised green zygotic embryos were cultured on Gamborg (B5), McCown and Loyd (WPM) and Schenk and Hildebrandt (SH) media fortified with different concentration of sucrose and on Murashige and Skoog (MS) medium containing 3 % sucrose, polyvinylpyrrolidone (PVP) (0–300 mg l−l), Gibberellic acid (GA3), Indoleacetic acid (IAA), Naphthaleneacetic acid (NAA), Indole-3-Butyric acid (IBA) or 2,4-dichlorophenoxyacetic acid (2,4 D) and 6-benzylaminopurine (BA) or kinetin (Kin) individually. The highest frequency of embryo germination (96 %) and conversion into seedling was obtained on MS medium containing 3 % sucrose together with 200 mg l−l PVP; other media were either inferior or induced abnormalities in the seedlings including callus formation from the zygotic embryos. Fully developed seedlings could be successfully established in soil with about 94 % survival. The embryos from mature dry seeds did not respond for germination in any of the experiments. In conclusion, selection of zygotic embryo from green mature seeds and their in vitro germination is important for propagation of B. serrata.  相似文献   
13.

Background

Influenza is a contagious respiratory disease responsible for annual seasonal epidemics in temperate climates. An understanding of how influenza spreads geographically and temporally within regions could result in improved public health prevention programs. The purpose of this study was to summarize the spatial and temporal spread of influenza using data obtained from the Pennsylvania Department of Health''s influenza surveillance system.

Methodology and Findings

We evaluated the spatial and temporal patterns of laboratory-confirmed influenza cases in Pennsylvania, United States from six influenza seasons (2003–2009). Using a test of spatial autocorrelation, local clusters of elevated risk were identified in the South Central region of the state. Multivariable logistic regression indicated that lower monthly precipitation levels during the influenza season (OR = 0.52, 95% CI: 0.28, 0.94), fewer residents over age 64 (OR = 0.27, 95% CI: 0.10, 0.73) and fewer residents with more than a high school education (OR = 0.76, 95% CI: 0.61, 0.95) were significantly associated with membership in this cluster. In addition, time series analysis revealed a temporal lag in the peak timing of the influenza B epidemic compared to the influenza A epidemic.

Conclusions

These findings illustrate a distinct spatial cluster of cases in the South Central region of Pennsylvania. Further examination of the regional transmission dynamics within these clusters may be useful in planning public health influenza prevention programs.  相似文献   
14.
Podophyllotoxin, a well-known naturally occurring aryltetralin lignan occurs in few plant species that is used as a precursor for the chemical synthesis of the anticancer drugs like etoposide, teniposide and etopophose phosphate. The availiability of this lignan is becoming increasingly limited because of the scarce occurance of its natural sources and also because synthetic approaches for its production are still commercially unacceptable. This paper reports first time the production of podophyllotoxin by an endophytic fungus Fusarium oxysporum isolated from the medicinal plant Juniperus recurva. Further confirmation and quantification of podophyllotoxin was performed by HPLC, LC-MS, and LC-MS/MS.  相似文献   
15.
The present study was conducted to see the in vivo developmental potency of caprine parthenogenetic embryos generated in a modified way. The good quality caprine oocytes were matured in presence of cytochalasin B (CCB) and then activated by 7% ethanol followed by 2 mM 6-dimethyl amino purine (6-DMAP) and embryo development was recorded. Early stage parthenogenetic embryos (two to four cells) were surgically transferred in recipients (10). The pregnancy diagnosis was done by nonreturn to oestrus, ultrasonography (USG), and progesterone estimation. The levels of progesterone were above normal values (1 ng/ml) of pregnancy and fall below the level of pregnancy just before retuned to oestrus. Progesterone profile revealed that out of ten recipients (G1–G10), four goats (G1, G2, G3, and G5) returned to oestrus after 43?±?7.29 (Mean?±?SE) d of embryo transfer and six goats (G4, G6, G7, G8, G9, and G10) did not return to cycle even after 70 d of embryo transfer. In three recipients (G4, G5, and G6), the USG on day 40 revealed that there was fluid filled uterine body with solid fetus-like structure. These might be dead fetus and had started resorption. The progesterone profile also corroborated the assumption of pregnancy in these animals. Authors believe that this may be the first report on in vivo diploid parthenogenetic embryo development in caprine species.  相似文献   
16.
A new type of dendritic molecules Gd-DTPA-XDA-D1-Glc(OH), which work as a functionalized ligand coordinating gadolinium(III) ion at the center of their frameworks with two glucose moieties on the molecular surfaces, were readily synthesized with high yield. The structures were established by IR, 1H, 13C NMR, and mass spectral studies. Its bio-distribution patterns were evaluated on rats.  相似文献   
17.
Spot blotch caused by Bipolaris sorokiniana is a major disease of wheat in warm and humid wheat growing regions of the world including south Asian countries such as India, Nepal and Bangladesh. The CIMMYT bread wheat line Saar which carries the leaf tip necrosis (LTN)-associated rust resistance genes Lr34 and Lr46 has exhibited a low level of spot blotch disease in field trials conducted in Asia and South America. One hundred and fourteen recombinant inbred lines (RILs) of Avocet (Susceptible) × Saar, were evaluated along with parents in two dates of sowing in India for 3 years (2007–2008 to 2009–2010) to identify quantitative trait loci (QTL) associated with spot blotch resistance, and to determine the potential association of Lr34 and Lr46 with resistance to this disease. Lr34 was found to constitute the main locus for spot blotch resistance, and explained as much as 55 % of the phenotypic variation in the mean disease data across the six environments. Based on the large effect, the spot blotch resistance at this locus has been given the gene designation Sb1. Two further, minor QTL were detected in the sub-population of RILs not containing Lr34. The first of these was located about 40 cM distal to Lr34 on 7DS, and the other corresponded to Lr46 on 1BL. A major implication for wheat breeding is that Lr34 and Lr46, which are widely used in wheat breeding to improve resistance to rust diseases and powdery mildew, also have a beneficial effect on spot blotch.  相似文献   
18.
A subunit of molecular weight 18300 has been separated and isolated from seeds of Brassica campestris L. This subunit was cleaved by using cyanogen bromide, trypsin, Staphylococcus aureus V8 protease and chymotrypsin; the fragments obtained from enzymatlc and chemical cleavages were separated and isolated by polyacrylamide gel electrophoresis and gel filtration. The amino acid analyses were carried out. The complete amino acid sequence of the subunit containing 172 amino acid residues has been established by manual Edman method.  相似文献   
19.
Desiccation and starvation tolerance were measured along latitudinal transects in three Drosophilid species (Drosophila ananassae, D. melanogaster, and Zaprionus indianus) of the Indian subcontinent. In each case, significant latitudinal clines were observed; desiccation tolerance increased with latitude while starvation tolerance decreased. Such field observations suggest that desiccation and starvation tolerance are fitness related traits that are independently selected in nature and genetically independent. It was, however, difficult to relate these genetic changes with precise climatic variables, except winter temperature. The overall negative correlation between the two traits, which was evidenced in natural populations, contrasts with a positive correlation generally observed in various laboratory selection experiments and that also seems to exist between different species. These observations point to the difficulty of interpreting correlations among fitness-related traits when different evolutionary levels are compared, and also different sets of data, that is, field versus laboratory studies.  相似文献   
20.

Key message

A complete set wheat-R. ciliaris disomic addition lines (DALs) were characterized and the homoeologous groups and genome affinities of R. ciliaris chromosomes were determined.

Abstract

Wild relatives are rich gene resources for cultivated wheat. The development of alien addition chromosome lines not only greatly broadens the genetic diversity, but also provides genetic stocks for comparative genomics studies. Roegneria ciliaris (genome ScScYcYc), a tetraploid wild relative of wheat, is tolerant or resistant to many abiotic and biotic stresses. To develop a complete set of wheat-R. ciliaris disomic addition lines (DALs), we undertook a euplasmic backcrossing program to overcome allocytoplasmic effects and preferential chromosome transmission. To improve the efficiency of identifying chromosomes from Sc and Yc, we established techniques including sequential genomic in situ hybridization/fluorescence in situ hybridization (FISH) and molecular marker analysis. Fourteen DALs of wheat, each containing one pair of R. ciliaris chromosomes pairs, were characterized by FISH using four repetitive sequences [pTa794, pTa71, RcAfa and (GAA)10] as probes. One hundred and sixty-two R. ciliaris-specific markers were developed. FISH and marker analysis enabled us to assign the homoeologous groups and genome affinities of R. ciliaris chromosomes. FHB resistance evaluation in successive five growth seasons showed that the amphiploid, DA2Yc, DA5Yc and DA6Sc had improved FHB resistance, indicating their potential value in wheat improvement. The 14 DALs are likely new gene resources and will be phenotyped for more agronomic performances traits.
  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