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   280篇
  免费   25篇
  2023年   2篇
  2022年   2篇
  2021年   7篇
  2020年   4篇
  2019年   2篇
  2018年   7篇
  2017年   8篇
  2016年   10篇
  2015年   27篇
  2014年   21篇
  2013年   33篇
  2012年   30篇
  2011年   24篇
  2010年   21篇
  2009年   12篇
  2008年   13篇
  2007年   13篇
  2006年   13篇
  2005年   11篇
  2004年   11篇
  2003年   5篇
  2002年   7篇
  2001年   6篇
  2000年   2篇
  1999年   1篇
  1998年   2篇
  1997年   2篇
  1996年   1篇
  1994年   1篇
  1989年   1篇
  1988年   2篇
  1986年   1篇
  1985年   1篇
  1980年   1篇
  1978年   1篇
排序方式: 共有305条查询结果,搜索用时 31 毫秒
1.
A yeast artificial chromosome (YAC) genomic library of Beta vulgaris was constructed in the pYAC4 vector. High-molecular-weight DNA was prepared from agarose-embedded leaf protoplasts from a triploid cultivar. The library was found to contain 33,500 clones in an ordered array of microtiter plates. Mean size of the inserts was estimated to be 135 kb, and the library should therefore represent the equivalent of five haploid genomes. The library was characterised for the presence of highly repetitive, chloroplast and single-copy sequences. In order to isolate single-copy sequences, 18 pools of DNA, each from 1920 individual YAC clones, were prepared for rapid screening of the library by the polymerase chain reaction. The results of these screenings showed that the number of isolated clones was at or near the frequency expected.  相似文献   
2.
A highly-efficient protocol for the large-scale isolation ofguard cell protoplasts from sugar beet (Beta vulgaris L.) hasbeen developed. Optimization of conditions for culturing theseprotoplasts resulted in extensive cell division and colony formation,at frequencies exceeding 50%. Plants can subsequently be regeneratedfrom these guard cell-derived colonies. This provides definitiveconfirmation that, in sugar beet leaf protoplast populations,only guard cells are the source of totipotent protoplasts. Thesefindings are the outcome of a directed, non-empirical approachto overcoming plant cell recalcitrance which was initiated byexploiting computer-assisted microscopy to couple in vitro responseto cell origin. The results reaffirm the conclusion that, inplants, extreme degrees of cytodifferentiation need not entailterminal specialization. The responsive nature of this systemcan be ascribed to the unique use of cultures essentially comprisinga single in vivo cell type. A uniform model system has thusbeen created with potential for widespread application. Theirdistinct morphological (and mechanical) features make guardcells a valuable choice for studying various fundamental aspects,not only of stomatal physiology, but also of plant cell (de)differentiation,differential gene expression etc. Furthermore, an applied valuefor such a system can also be envisaged. Results indicate thatthese cells are highly amenable to genetic manipulation techniques.The importance of these observations to our understanding ofplant cell function and behaviour is discussed. Key words: Beta, guard cells, stomatal physiology, totipotency, transformation  相似文献   
3.
Short rotation coppice (SRC) of willow and poplar might be a promising phytoremediation option since it uses fast growing, high biomass producing tree species with often a sufficient metal uptake. This study evaluates growth, metal uptake and extraction potentials of eight willow clones (Belders, Belgisch Rood, Christina, Inger, Jorr, Loden, Tora and Zwarte Driebast) on a metal-contaminated agricultural soil, with total cadmium (Cd) and zinc (Zn) concentrations of 6.5 ± 0.8 and 377 ± 69 mg kg?1 soil, respectively. Although, during the first cycle, on average generally low productivity levels (3.7 ton DM (dry matter) ha?1 y?1) were obtained on this sandy soil, certain clones exhibited quite acceptable productivity levels (e.g. Zwarte Driebast 12.5 ton DM ha?1 y?1). Even at low biomass productivity levels, SRC of willow showed promising removal potentials of 72 g Cd and 2.0 kg Zn ha?1 y?1, which is much higher than e.g. energy maize or rapeseed grown on the same soil. Cd and Zn removal can be increased by 40% if leaves are harvested as well. Nevertheless, nowadays the wood price remains the most critical factor in order to implement SRC as an acceptable, economically feasible alternative crop on metal-contaminated agricultural soils.  相似文献   
4.

Background

While evaluation of liver function in preclinical animal studies is commonly performed at selected time-points by invasive determination of the liver/body weight ratio and histological analyses, the validation of longitudinal measurement tools for monitoring liver function are of major interest.

Aims

To longitudinally evaluate serum cholinesterase (CHE) and total serum bilirubin (TSB) levels as non-invasive markers to determine injury- and partial hepatectomy (PHx)-induced alterations of liver function in rats.

Methods

Male and female Lewis rats were subjected to either methionine/choline deficient (MCD) diet or treatment with FOLFOX chemotherapy prior to PHx. Body weight and CHE/TSB levels are determined weekly. Following PHx and at the study end, histological analyses of liver tissue are performed.

Results

Following MCD diet, but not after FOLFOX chemotherapy treatment, results indicate gender-specific alterations in serum CHE levels and gender-independent alterations in TSB levels. Likewise, histological analyses of resected liver parts indicate significant liver injury following MCD-diet, but not following FOLFOX treatment. While TSB levels rapidly recover following MCD diet/FOLFOX treatment combined with a PHx, serum CHE levels are subject to significant model- and gender-specific differences, despite full histopathological recovery of liver tissue.

Conclusions

Longitudinal measurements of serum CHE levels and TSB levels in rats are highly complementary as non-invasive parameters for evaluation of liver injury and/or recovery.  相似文献   
5.

Background

Schistosoma mansoni and S. haematobium are co-endemic in many areas in Africa. Yet, little is known about the micro-geographical distribution of these two infections or associated disease within such foci. Such knowledge could give important insights into the drivers of infection and disease and as such better tailor schistosomiasis control and elimination efforts.

Methodology

In a co-endemic farming community in northern Senegal (346 children (0–19 y) and 253 adults (20–85 y); n = 599 in total), we studied the spatial distribution of S. mansoni and S. haematobium single and mixed infections (by microscopy), S. mansoni-specific hepatic fibrosis, S. haematobium-specific urinary tract morbidity (by ultrasound) and water contact behavior (by questionnaire). The Kulldorff''s scan statistic was used to detect spatial clusters of infection and morbidity, adjusted for the spatial distribution of gender and age.

Principal Findings

Schistosoma mansoni and S. haematobium infection densities clustered in different sections of the community (p = 0.002 and p = 0.023, respectively), possibly related to heterogeneities in the use of different water contact sites. While the distribution of urinary tract morbidity was homogeneous, a strong geospatial cluster was found for severe hepatic fibrosis (p = 0.001). Particularly those people living adjacent to the most frequently used water contact site were more at risk for more advanced morbidity (RR = 6.3; p = 0.043).

Conclusions/Significance

Schistosoma infection and associated disease showed important micro-geographical heterogeneities with divergent patterns for S. mansoni and S. haematobium in this Senegalese community. Further in depth investigations are needed to confirm and explain our observations. The present study indicates that local geospatial patterns should be taken into account in both research and control of schistosomiasis. The observed extreme focality of schistosomiasis even at community level, suggests that current strategies may not suffice to move from morbidity control to elimination of schistosomiasis, and calls for less uniform measures at a finer scale.  相似文献   
6.
Robotic surgery is an attractive, minimally invasive and high precision alternative to conventional surgical procedures. However, it lacks the natural touch and force feedback that allows the surgeon to control safe tissue manipulation. This is an important problem in standard surgical procedures such as clamping, which might induce severe tissue damage. In complex, heterogeneous, large deformation scenarios, the limits of the safe loading regime beyond which tissue damage occurs are unknown. Here, we show that a continuum damage model for arteries, implemented in a finite element setting, can help to predict arterial stiffness degradation and to identify critical loading regimes. The model consists of the main mechanical constituents of arterial tissue: extracellular matrix, collagen fibres and smooth muscle cells. All constituents are allowed to degrade independently in response to mechanical overload. To demonstrate the modularity and portability of the proposed model, we implement it in a commercial finite element programme, which allows to keep track of damage progression via internal variables. The loading history during arterial clamping is simulated through four successive steps, incorporating residual strains. The results of our first prototype simulation demonstrate significant regional variations in smooth muscle cell damage. In three additional steps, this damage is evaluated by simulating an isometric contraction experiment. The entire finite element simulation is finally compared with actual in vivo experiments. In the short term, our computational simulation tool can be useful to optimise surgical tools with the goal to minimise tissue damage. In the long term, it can potentially be used to inform computer-assisted surgery and identify safe loading regimes, in real time, to minimise tissue damage during robotic tissue manipulation.  相似文献   
7.

Background

Mental disorders may be reducible to sets of symptoms, connected through systems of causal relations. A clinical staging model predicts that in earlier stages of illness, symptom expression is both non-specific and diffuse. With illness progression, more specific syndromes emerge. This paper addressed the hypothesis that connection strength and connection variability between mental states differ in the hypothesized direction across different stages of psychopathology.

Methods

In a general population sample of female siblings (mostly twins), the Experience Sampling Method was used to collect repeated measures of three momentary mental states (positive affect, negative affect and paranoia). Staging was operationalized across four levels of increasing severity of psychopathology, based on the total score of the Symptom Check List. Multilevel random regression was used to calculate inter- and intra-mental state connection strength and connection variability over time by modelling each momentary mental state at t as a function of the three momentary states at t-1, and by examining moderation by SCL-severity.

Results

Mental states impacted dynamically on each other over time, in interaction with SCL-severity groups. Thus, SCL-90 severity groups were characterized by progressively greater inter- and intra-mental state connection strength, and greater inter- and intra-mental state connection variability.

Conclusion

Diagnosis in psychiatry can be described as stages of growing dynamic causal impact of mental states over time. This system achieves a mode of psychiatric diagnosis that combines nomothetic (group-based classification across stages) and idiographic (individual-specific psychopathological profiles) components of psychopathology at the level of momentary mental states impacting on each other over time.  相似文献   
8.

Background and aims

Microbe-assisted phytoremediation is particularly effective for organic pollutants. The leguminous shrub Cytisus striatus (Hill) Rothm. has been proposed as a candidate species for the rhizoremediation of hexachlorocyclohexane (HCH)-contaminated sites. The aim of this study was to improve the performance of this species using microbial inoculants.

Methods

C. striatus was grown in substrates contaminated with 0, 10 and 35 mg HCH kg?1 for 8 weeks. Plants were either not inoculated (NI), or inoculated with the endophyte Rhodococcus erythropolis ET54b and the HCH-degrader Sphingomonas sp. D4 (isolated from a HCH-contaminated soil) on their own or in combination (ET, D4 and ETD4).

Results

Inoculation with both bacterial strains (ETD4) resulted in decreased HCH phytotoxicity and improved plant growth. HCH-exposed plants inoculated with ETD4 presented a 120–160 % increase in root, and 140–160 % increase in shoot biomass, and led to a decrease in the activities of enzymes involved in anti-oxidative defence. APOD activity was reduced by up to 37 % in shoot tissues and 25 % in root tissues, and corresponding activities of SOD were reduced by up to 35 % and 30 %. HCH dissipation was enhanced in the presence of C. striatus but no significant effect of microbial inoculants was observed.

Conclusions

Inoculating C. striatus with this combination of bacterial strains is a promising approach for the remediation of HCH-contaminated sites.  相似文献   
9.
10.
A phospholipid flippase activity from the endoplasmic reticulum (ER) of the model organism Saccharomyces cerevisiae has been characterized and functionally reconstituted into proteoliposomes. Analysis of the transbilayer movement of acyl-7-nitrobenz-2-oxa-1,3-diazol-4-yl (acyl-NBD)-labeled phosphatidylcholine in yeast microsomes using a fluorescence stopped-flow back exchange assay revealed a rapid, ATP-independent flip-flop (half-time, <2 min). Proteoliposomes prepared from a Triton X-100 extract of yeast microsomal membranes were also capable of flipping NBD-labeled phospholipid analogues rapidly in an ATP-independent fashion. Flippase activity was sensitive to the protein modification reagents N-ethylmaleimide and diethylpyrocarbonate. Resolution of the Triton X-100 extract by velocity gradient centrifugation resulted in the identification of a approximately 4S protein fraction enriched in flippase activity as well as of other fractions where flippase activity was depleted or undetectable. We estimate that flippase activity is due to a protein(s) representing approximately 2% (wt/wt) of proteins in the Triton X-100 extract. These results indicate that specific proteins are required to facilitate ATP-independent phospholipid flip-flop in the ER and that their identification is feasible. The architecture of the ER protein translocon suggests that it could account for the flippase activity in the ER. We tested this hypothesis using microsomes prepared from a temperature-sensitive yeast mutant in which the major translocon component, Sec61p, was quantitatively depleted. We found that the protein translocon is not required for transbilayer movement of phospholipids across the ER. Our work defines yeast as a promising model system for future attempts to identify the ER phospholipid flippase and to test and purify candidate flippases.  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