全文获取类型
收费全文 | 98篇 |
免费 | 9篇 |
出版年
2023年 | 3篇 |
2022年 | 1篇 |
2021年 | 3篇 |
2020年 | 1篇 |
2019年 | 3篇 |
2018年 | 1篇 |
2017年 | 3篇 |
2016年 | 3篇 |
2015年 | 3篇 |
2014年 | 4篇 |
2013年 | 6篇 |
2012年 | 9篇 |
2011年 | 6篇 |
2010年 | 5篇 |
2009年 | 1篇 |
2007年 | 4篇 |
2006年 | 3篇 |
2005年 | 5篇 |
2004年 | 3篇 |
2003年 | 2篇 |
2002年 | 2篇 |
2001年 | 1篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1980年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有107条查询结果,搜索用时 969 毫秒
91.
An exception to the generally conservative nature of plastid gene evolution
is the gene coding for the beta" subunit of RNA polymerase, rpoC2. Previous
work by others has shown that maize and rice have an insertion in the
coding region of rpoC2, relative to spinach and tobacco. To assess the
distribution of this extra coding sequence, we surveyed a broad
phylogenetic sample comprising 55 species from 17 angiosperm families by
using Southern hybridization. The extra coding sequence is restricted to
the grasses (Poaceae). DNA sequence analysis of 11 species from all five
subfamilies within the grass family demonstrates that the extra sequence in
the coding region of rpoC2 is a repetitive array that exhibits more than a
twofold increase in nucleotide substitution, as well as a large number of
insertion/deletion events, relative to the adjacent flanking sequences. The
structure of the array suggests that slipped-strand mispairing causes the
repeated motifs and adds to the mechanisms through which the coding
sequence of plastid genes are known to evolve. Phylogenetic analyses based
on the sequence data from grass species support several relationships
previously suggested by morphological work, but they are ambiguous about
broad relationships within the family.
相似文献
92.
Nucleolar organizer regions are nucleolar components that contain proteins that are stained selectively by silver methods; they can be identified as black dots throughout the nucleolus and are known as silver binding nucleolar organizer regions (AgNOR). The number of AgNOR is related to the cell cycle and the proliferative activity of the cells. We investigated AgNOR using exfoliative cytology smears of potentially malignant oral lesions. Eighty individuals were divided into four equal groups: healthy controls, oral leukoplakia, oral submucous fibrosis and oral squamous cell carcinoma. The mean number of AgNOR in each study group gradually increased from control to oral leukoplakia to oral submucous fibrosis to oral squamous cell carcinoma. The proliferative index was increased in the oral premalignant and malignant patients compared to normal subjects. The mean AgNOR size gradually increased from control to oral leukoplakia to oral submucous fibrosis to oral squamous cell carcinoma. Spherical shaped AgNOR were most common in controls, whereas large, clustered and kidney shapes were most common in oral squamous cell carcinoma. Multiparameter analysis of AgNOR in oral exfoliative smears is a simple, sensitive and cost-effective method for differentiating premalignant from malignant lesions and can be used in conjunction with routine cytomorphological evaluation. 相似文献
93.
Metabolic engineering of Escherichia coli for industrial production of glucosamine and N-acetylglucosamine 总被引:1,自引:0,他引:1
Deng MD Severson DK Grund AD Wassink SL Burlingame RP Berry A Running JA Kunesh CA Song L Jerrell TA Rosson RA 《Metabolic engineering》2005,7(3):201-214
Glucosamine and N-acetylglucosamine are currently produced by extraction and acid hydrolysis of chitin from shellfish waste. Production could be limited by the amount of raw material available and the product potentially carries the risk of shellfish protein contamination. Escherichia coli was modified by metabolic engineering to develop a fermentation process. Over-expression of glucosamine synthase (GlmS) and inactivation of catabolic genes increased glucosamine production by 15 fold, reaching 60 mg l(-1). Since GlmS is strongly inhibited by glucosamine-6-P, GlmS variants were generated via error-prone PCR and screened. Over-expression of an improved enzyme led to a glucosamine titer of 17 g l(-1). Rapid degradation of glucosamine and inhibitory effects of glucosamine and its degradation products on host cells limited further improvement. An alternative fermentation product, N-acetylglucosamine, is stable, non-inhibitory to the host and readily hydrolyzed to glucosamine under acidic conditions. Therefore, the glucosamine pathway was extended to N-acetylglucosamine by over-expressing a heterologous glucosamine-6-P N-acetyltransferase. Using a simple and low-cost fermentation process developed for this strain, over 110 g l(-1) of N-acetylglucosamine was produced. 相似文献
94.
Angélica MS Firmo Mônica MP Tognella Saulo R Silva Raynner RRD Barboza Rômulo RN Alves 《Journal of ethnobiology and ethnomedicine》2012,8(1):1-12
Background
Blue Land Crab (Cardisoma guanhumi) is one of the most important crustacean species captured and commercialized in Brazil. Although this species is not considered to be threatened with extinction, populations of C. guanhumi are known to be rapidly diminishing due to heavy harvesting pressures and degradation of their natural habitats, highlighting the necessity of developing and implanting management and protection strategies for their populations. There have been no ethnozoological publications that have focused specifically on C. guanhumi, in spite of importance of this type of information for developing efficient management plans of resource utilization. So, the present work describes the ethnoecological aspects of the capture and commercialization of C. guanhumi by a fishing community in northeastern Brazil.Methods
Field work was carried out in the municipality of Mucuri, Bahia in Brazil, between the months of January and March/2011 through the use of open semi-structured interviews with all of the crustacean harvesters in city who acknowledged their work in capturing this species, totaling 12 interviewees. The informants were identified through the use of the "snowball" sampling technique. In addition to the interviews themselves, the "guided tour" technique and direct observations was employed.Results
According all the interviewees, the C. guanhumi is popularly called "guaiamum" and is collected in "apicum" zones. They recognize sexual dimorphism in the species based on three morphological characteristics and the harvesters also pointed two stages in the reproductive cycle during the year and another phase mentioned by the interviewees was ecdysis. All of the interviewed affirmed that the size and the quantities C. guanhumi stocks in Mucuri have been diminishing. All of the interviewees agreed that the species and other mangrove resources constituted their principal source of income. The harvesters dedicated three to five days a week to collect Blue Land Crabs and the principal technique utilized for capturing is a trap called a "ratoeira" (rat-trap).Conclusions
The results of the present work demonstrated that the community retains a vast and important volume of knowledge about C. guanhumi that could subsidize both scientific studies and the elaboration of viable management and conservation strategies for this species. 相似文献95.
Julie M. Thole Pierre‐Francois Perroud Ralph S. Quatrano Mark P. Running 《The Plant journal : for cell and molecular biology》2014,78(3):441-451
Protein prenylation is required for a variety of growth and developmental processes in flowering plants. Here we report the consequences of loss of function of all known prenylation subunits in the moss Physcomitrella patens. As in Arabidopsis, protein farnesyltransferase and protein geranylgeranyltransferase type I are not required for viability. However, protein geranylgeranyltransferase type I activity is required for cell adhesion, polar cell elongation, and cell differentiation. Loss of protein geranylgeranyltransferase activity results in colonies of round, single‐celled organisms that resemble unicellular algae. The loss of protein farnesylation is not as severe but also results in polar cell elongation and differentiation defects. The complete loss of Rab geranylgeranyltransferase activity appears to be lethal in P. patens. Labeling with antibodies to cell wall components support the lack of polarity establishment and the undifferentiated state of geranylgeranyltransferase type I mutant plants. Our results show that prenylated proteins play key roles in P. patens development and differentiation processes. 相似文献
96.
Champica K. Bodinayake Ajith DeS Nagahawatte Vasantha Devasiri Niroshana J. Dahanayake Gaya B. Wijayaratne Nayani P. Weerasinghe Madureka Premamali Tianchen Sheng Bradley P. Nicholson Harshanie A. Ubeysekera Ruvini MP Kurukulasooriya Aruna D. de Silva Truls
stbye Christopher W. Woods L. Gayani Tillekeratne 《PLoS neglected tropical diseases》2022,16(5)
97.
Michael G. Ryan Stith T. Gower Robert M. Hubbard Richard H. Waring Henry L. Gholz Wendell P. Cropper Jr Steven W. Running 《Oecologia》1995,101(2):133-140
We estimate maintenance respiration for boles of four temperate conifers (ponderosa pine, western hemlock, red pine, and slash pine) from CO2 efflux measurements in autumn, when construction respiration is low or negligible. Maintenance respiration of stems was linearly related to sapwood volume for all species; at 10°C, respiration per unit sapwood volume ranged from 4.8 to 8.3 mol CO2 m–3 s–1. For all sites combined, respiration increased exponentially with temperature (Q
10
=1.7, r
2=0.78). We estimate that maintenance respiration of aboveground woody tissues of these conifers consumes 52–162 g C m–2 y–1, or 5–13% of net daytime carbon assimilation annually. The fraction of annual net daytime carbon fixation used for stem maintenance respiration increased linearly with the average annual temperature of the site. 相似文献
98.
Contrasting Climatic Controls on the Estimated Productivity of Global Terrestrial Biomes 总被引:15,自引:2,他引:15
Net primary productivity (NPP) represents the greatest annual carbon flux from the atmosphere to the biosphere, is an important
component of seasonal fluctuations in atmospheric CO2 concentrations, and is the most critical biotic component of the global carbon cycle. NPP measures products of major economic
and social importance, such as crop yield and forest production. Given that global NPP can not be measured directly, model
simulations must provide understanding of its global spatial and temporal dynamics. In this study, we used the biogeochemical
model BIOME-BGC to simulate global terrestrial NPP and assessed relative importance of climatic controls (temperature, water
availability, and radiation) in limiting NPP in the array of climatic combinations found globally. The degree of limitation
on NPP by climatic controls was defined by using an empirical membership function. Results showed that temperature or water
availability limited NPP over larger land areas (31% and 52%, respectively) than did radiation limitation (5%). Climatic controls
appeared to be important in limiting productivity in most vegetation biomes, except for evergreen broadleaf forests. Nevertheless,
there were areas of the globe (12%) where none of the climatic factors appeared to limit NPP. Our research has suggested that other environmental controls, such as nutrient
availability or biological constraints, should then be considered. The wide distribution of NPP between zero and the upper
boundary values in the correlation plots indicated that multivariate environmental balances, not single limiting factors,
controlled biospheric productivity.
Received 27 August 1997; accepted 19 November 1997. 相似文献
99.
100.