全文获取类型
收费全文 | 498篇 |
免费 | 53篇 |
出版年
2021年 | 3篇 |
2020年 | 3篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 9篇 |
2015年 | 9篇 |
2014年 | 23篇 |
2013年 | 21篇 |
2012年 | 27篇 |
2011年 | 33篇 |
2010年 | 19篇 |
2009年 | 10篇 |
2008年 | 19篇 |
2007年 | 31篇 |
2006年 | 25篇 |
2005年 | 19篇 |
2004年 | 28篇 |
2003年 | 30篇 |
2002年 | 29篇 |
2001年 | 21篇 |
2000年 | 26篇 |
1999年 | 25篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 5篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有551条查询结果,搜索用时 31 毫秒
91.
Arbuscular mycorrhizal phosphate transport under monoxenic conditions using radio-labelled inorganic and organic phosphate 总被引:5,自引:0,他引:5
Compartmented monoxenic cultures of Ri T-DNA transformed carrot roots and a symbiotic arbuscular mycorrhizal fungus demonstrated for the first time that phosphate in an organic form (32P-labelled AMP) may be hydrolysed by extra-radical mycorrhizal hyphae in the absence of other organisms, and subsequently utilized as a mineral nutrient source by the host plant after fungal transport. 相似文献
92.
93.
Analysis of functional domain organization in DNA topoisomerase II from humans and Saccharomyces cerevisiae. 总被引:4,自引:0,他引:4 下载免费PDF全文
S Jensen A H Andersen E Kjeldsen H Biersack E H Olsen T B Andersen O Westergaard B K Jakobsen 《Molecular and cellular biology》1996,16(7):3866-3877
The functional domain structure of human DNA topoisomerase IIalpha and Saccharomyces cerevisiae DNA topoisomerase II was studied by investigating the abilities of insertion and deletion mutant enzymes to support mitotic growth and catalyze transitions in DNA topology in vitro. Alignment of the human topoisomerase IIalpha and S. cerevisiae topoisomerase II sequences defined 13 conserved regions separated by less conserved or differently spaced sequences. The spatial tolerance of the spacer regions was addressed by insertion of linkers. The importance of the conserved regions was assessed through deletion of individual domains. We found that the exact spacing between most of the conserved domains is noncritical, as insertions in the spacer regions were tolerated with no influence on complementation ability. All conserved domains, however, are essential for sustained mitotic growth of S. cerevisiae and for enzymatic activity in vitro. A series of topoisomerase II carboxy-terminal truncations were investigated with respect to the ability to support viability, cellular localization, and enzymatic properties. The analysis showed that the divergent carboxy-terminal region of human topoisomerase IIalpha is dispensable for catalytic activity but contains elements that specifically locate the protein to the nucleus. 相似文献
94.
The influence of the systemic fungicides propiconazole (Tilt 250E) and carbendazim (Bavistin) at field application rates
on the functioning of three arbuscular mycorrhizal fungi was studied. Short-term fungal 32P transport and succinate dehydrogenase (SDH) activity in external hyphae of Glomus intraradices Schenck and Smith, G. claroideum Schenck and Smith and G. invermaium Hall in symbiosis with pea (Pisum sativum L.) were measured. In the experimental system used, the hyphae grew into two root-free hyphal compartments (HCs). The fungicides
were applied to each HC 24 days after sowing and 32P was added to one HC of each pot. Four days later, the fungicide effect on fungal P transport was measured as the difference
in 32P content of treated and untreated plants. SDH activity in fungal hyphae was determined in the HCs given no 32P. Carbendazim severely inhibited 32P transport and SDH activity in external hyphae at an application rate of 0.5 μg g–1 soil. The ergosterol inhibitor propiconazole affected none of these parameters. The fungicides had similar effects on all
three fungal species, although P transport efficiency and SDH activity differed markedly between the fungi.
Accepted: 12 December 1996 相似文献
95.
The influence of temperature on nectar secretion in non-pollinatedflorets of Trifolium repens was investigated in growth chambersat 10, 14, 18 and 22°C. The effect of temperature on therate of nectar secretion was significant in all clones. Theoptimum temperature for secretion in three clones varied from10°C for a clone of Icelandic origin, to 18°C in a cloneselected from a Danish variety. Similarly, the average nectaryield varied significantly among clones of different geographicalorigin. One clone secreted two to four times more than othersat 10°C. The optimum day temperature for nectar secretionwas higher when the plants were exposed to low night temperature,presumably a result of decreased night respiration. Nectar accumulatedat the floret base until senescence. Evidence for reabsorptionof nectar was obtained in four clones. Sucrose, fructose andglucose were identified as the major sugars in the nectar. Highnight temperatures led to decreased sucrose percentage in favourof glucose and fructose. The frequency of new florets openingper day was not influenced by temperatures between 10 and 22°Cin one clone, whereas low temperatures significantly decreasedthe number of new florets in another. Few or no modified stomatawere observed in the epidermis of the nectary. The high variationwith respect to nectar secretion at low temperatures, alongwith the high heritability of this quality, suggests that breedingfor high nectar production at low temperature is plausible.The significance of nectar yield in pollination biology is discussed.Copyright1994, 1999 Academic Press Trifolium repens, white clover, nectar, temperature, floret age, flowering, nectary 相似文献
96.
DHA induces ER stress and growth arrest in human colon cancer cells: associations with cholesterol and calcium homeostasis 总被引:2,自引:0,他引:2
Jakobsen CH Størvold GL Bremseth H Follestad T Sand K Mack M Olsen KS Lundemo AG Iversen JG Krokan HE Schønberg SA 《Journal of lipid research》2008,49(10):2089-2100
Polyunsaturated fatty acids (PUFAs) are normal constituents of the diet, but have properties different from other fatty acids (e.g., through generation of signaling molecules). N-3 PUFAs reduce cancer cell growth, but no unified mechanism has been identified. We show that docosahexaenoic acid (DHA; 22:6 n-3) causes extensive changes in gene expression patterns at mRNA level in the colon cancer cell line SW620. Early changes include unfolded protein response (UPR) and increased levels of phosphorylated eIF2alpha as verified at protein level. The latter is considered a hallmark of endoplasmic reticulum (ER) stress and is abundantly present already after 3 h. It may coordinate many of the downstream changes observed, including signaling pathways for cell cycle arrest/apoptosis, calcium homeostasis, cholesterol metabolism, ubiquitination, and proteasomal degradation. Also, eicosapentaenoic acid (EPA), but not oleic acid (OA), induced key mediators of ER stress and UPR at protein level. Accumulation of esterified cholesterol was not compensated for by increased total levels of cholesterol, and mRNAs for cholesterol biosynthesis as well as de novo synthesis of cholesterol were reduced. These results suggest that cytotoxic effects of DHA are associated with signaling pathways involving lipid metabolism and ER stress. 相似文献
97.
The effect of feeding 0, 4, 8 and 16% rapeseed oil from 12-42 days of age was studied in broiler chickens on performance, digestibility of nutrients, and development of gastrointestinal tract, protein and energy metabolism. Thirty six female chickens (Ross 208) with initial body weight average 246 g were allocated to the four groups and kept pair-wise in metabolism cages. The chickens were fed similar amounts of metabolisable energy (ME) per day and similar amounts of essential amino acids relative to ME by adjusting with crystalline amino acids. The chickens were subjected to four balance periods each of five days with two 24 h measurements of gas exchange in two open-air-circuit respiration chambers inserted on the second and third day of each period. The addition of rapeseed oil increased the amount of gutfill indicating a reduced rate of passage and causing a hypertrophy of the gastrointestinal tract. There was a positive effect on feed utilisation as well as on digestibility especially of dietary fat together with higher utilisation of protein with addition of rapeseed oil. The partial fat digestibility of rapeseed oil estimated by regression was 91.1% and the partial metabolisability (ME/GE) of the rapeseed oil was estimated to 85% yielding an apparent metabolisable energy value of 34.30 MJ/kg. 相似文献
98.
Shalchian-Tabrizi K Minge MA Espelund M Orr R Ruden T Jakobsen KS Cavalier-Smith T 《PloS one》2008,3(5):e2098
Animals are evolutionarily related to fungi and to the predominantly unicellular protozoan phylum Choanozoa, together known as opisthokonts. To establish the sequence of events when animals evolved from unicellular ancestors, and understand those key evolutionary transitions, we need to establish which choanozoans are most closely related to animals and also the evolutionary position of each choanozoan group within the opisthokont phylogenetic tree. Here we focus on Ministeria vibrans, a minute bacteria-eating cell with slender radiating tentacles. Single-gene trees suggested that it is either the closest unicellular relative of animals or else sister to choanoflagellates, traditionally considered likely animal ancestors. Sequencing thousands of Ministeria protein genes now reveals about 14 with domains of key significance for animal cell biology, including several previously unknown from deeply diverging Choanozoa, e.g. domains involved in hedgehog, Notch and tyrosine kinase signaling or cell adhesion (cadherin). Phylogenetic trees using 78 proteins show that Ministeria is not sister to animals or choanoflagellates (themselves sisters to animals), but to Capsaspora, another protozoan with thread-like (filose) tentacles. The Ministeria/Capsaspora clade (new class Filasterea) is sister to animals and choanoflagellates, these three groups forming a novel clade (filozoa) whose ancestor presumably evolved filose tentacles well before they aggregated as a periciliary collar in the choanoflagellate/sponge common ancestor. Our trees show ichthyosporean choanozoans as sisters to filozoa; a fusion between ubiquitin and ribosomal small subunit S30 protein genes unifies all holozoa (filozoa plus Ichthyosporea), being absent in earlier branching eukaryotes. Thus, several successive evolutionary innovations occurred among their unicellular closest relatives prior to the origin of the multicellular body-plan of animals. 相似文献
99.
Single and dual amino acid substitutions in TCR CDRs can enhance antigen-specific T cell functions 总被引:1,自引:0,他引:1
Robbins PF Li YF El-Gamil M Zhao Y Wargo JA Zheng Z Xu H Morgan RA Feldman SA Johnson LA Bennett AD Dunn SM Mahon TM Jakobsen BK Rosenberg SA 《Journal of immunology (Baltimore, Md. : 1950)》2008,180(9):6116-6131
Single and dual amino acid substitution variants were generated in the TCR CDRs of three TCRs that recognize tumor-associated Ags. Substitutions that enhance the reactivity of TCR gene-modified T cells to the cognate Ag complex were identified using a rapid RNA-based transfection system. The screening of a panel of variants of the 1G4 TCR, that recognizes a peptide corresponding to amino acid residues 157-165 of the human cancer testis Ag NY-ESO-1 (SLLMWITQC) in the context of the HLA-A*02 class I allele, resulted in the identification of single and dual CDR3alpha and CDR2beta amino acid substitutions that dramatically enhanced the specific recognition of NY-ESO-1(+)/HLA-A*02(+) tumor cell lines by TCR gene-modified CD4(+) T cells. Within this group of improved TCRs, a dual substitution in the 1G4 TCR CDR3alpha chain was identified that enhanced Ag-specific reactivity in gene-modified CD4(+) and CD8(+) T cells. Separate experiments on two distinct TCRs that recognize the MART-1 27-35 (AAGIGILTV) peptide/HLA-A*02 Ag complex characterized single amino acid substitutions in both TCRs that enhanced CD4(+) T cell Ag-specific reactivity. These results indicate that simple TCR substitution variants that enhance T cell function can be identified by rapid transfection and assay techniques, providing the means for generating potent Ag complex-specific TCR genes for use in the study of T cell interactions and in T cell adoptive immunotherapy. 相似文献
100.
Bennert HW Horn K Kauth M Fuchs J Jakobsen IS Ollgaard B Schnittler M Steinberg M Viane R 《Annals of botany》2011,108(5):867-876