全文获取类型
收费全文 | 11280篇 |
免费 | 923篇 |
国内免费 | 12篇 |
出版年
2023年 | 80篇 |
2022年 | 166篇 |
2021年 | 313篇 |
2020年 | 140篇 |
2019年 | 217篇 |
2018年 | 255篇 |
2017年 | 235篇 |
2016年 | 356篇 |
2015年 | 574篇 |
2014年 | 580篇 |
2013年 | 731篇 |
2012年 | 873篇 |
2011年 | 798篇 |
2010年 | 522篇 |
2009年 | 418篇 |
2008年 | 599篇 |
2007年 | 597篇 |
2006年 | 529篇 |
2005年 | 539篇 |
2004年 | 467篇 |
2003年 | 402篇 |
2002年 | 423篇 |
2001年 | 196篇 |
2000年 | 228篇 |
1999年 | 178篇 |
1998年 | 117篇 |
1997年 | 82篇 |
1996年 | 82篇 |
1995年 | 102篇 |
1994年 | 87篇 |
1993年 | 79篇 |
1992年 | 101篇 |
1991年 | 94篇 |
1990年 | 84篇 |
1989年 | 88篇 |
1988年 | 79篇 |
1987年 | 65篇 |
1986年 | 66篇 |
1985年 | 68篇 |
1984年 | 55篇 |
1983年 | 42篇 |
1982年 | 29篇 |
1981年 | 40篇 |
1980年 | 27篇 |
1979年 | 40篇 |
1978年 | 21篇 |
1977年 | 40篇 |
1976年 | 27篇 |
1973年 | 16篇 |
1971年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
In an attempt to question the toxic effect of heat shock and related stress, we have studied the activity of reporter enzymes during stress. Escherichia coli beta-galactosidase and Photinus pyralis luciferase were synthesized in mouse and Drosophila cells after transfection of the corresponding genes. Both enzymes are rapidly inactivated during hyperthermia. The corresponding polypeptides are not degraded but become insoluble even in the presence of non-ionic detergents. The heat inactivation is more dramatic in vivo within the living cell than in vitro, in a detergent-free crude cell lysate. The extent of enzyme inactivation at a given temperature depends on the cell type in which the enzyme is expressed. Luciferase is inactivated at lower temperatures within Drosophila cells than within mouse cells, whereas beta-galactosidase is inactivated at higher temperatures in E. coli than in mouse cells. A "priming" heat shock confers a transient increased resistance (thermotolerance) of cells against a second "challenging" heat shock. Enzyme inactivation during heat shock or exposure of the cells to ethanol is attenuated in heat shock-primed cells. A comparable thermoprotection is raised by a priming heat shock for both luciferase activity and protein synthesis. Thus, the study of reporter enzyme inactivation is a promising tool for understanding the molecular basis of the toxicity of heat shock and related stress as well as the mechanisms leading to thermotolerance. 相似文献
102.
Characterization of a human homologue of the murine peripheral lymph node homing receptor 总被引:21,自引:11,他引:10
下载免费PDF全文
![点击此处可从《The Journal of cell biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Lymphocyte trafficking is a fundamental aspect of the immune system that allows B and T lymphocytes with diverse antigen recognition specificities to be exposed to various antigenic stimuli in spatially distinct regions of an organism. A lymphocyte adhesion molecule that is involved with this trafficking phenomenon has been termed the homing receptor. Previous work (Lasky, L., T. Yednock, M. Singer, D. Dowbenko, C. Fennie, H. Rodriguez, T. Nguyen, S. Stachel, and S. Rosen. 1989. Cell. 56:1045-1055) has characterized a cDNA clone encoding a murine homing receptor that is involved in trafficking of lymphocytes to peripheral lymph nodes. This molecule was found to contain a number of protein motifs, the most intriguing of which was a carbohydrate binding domain, or lectin, that is apparently involved in the adhesive interaction between murine lymphocytes and peripheral lymph node endothelium. In this study, we have used the murine cDNA clone to isolate a human homologue of this peripheral lymph node-specific adhesion molecule. The human receptor was found to be highly homologous to the murine receptor in overall sequence, but showed no sequence similarity to another surface protein that may be involved with human lymphocyte homing, the Hermes glycoprotein. The extracellular region of the human receptor contained an NH2 terminally located carbohydrate binding domain followed by an EGF-like domain and a domain containing two repeats of a complement binding motif. Transient cell transfection assays using the human receptor cDNA showed that it encoded a surface glycoprotein that cross reacted with a polyclonal antibody directed against the murine peripheral lymph node homing receptor. Interestingly, the human receptor showed a high degree of sequence homology to another human cell adhesion glycoprotein, the endothelial cell adhesion molecule ELAM. 相似文献
103.
Storage of apple fruits ( Pyrus malus L. cv. Golden Delicious) at different temperatures (0, 12 and 35°C) markedly altered the pattern of water freezing in the seeds. Higher temperatures of storage brought about a shift from the sequential to the discontinuous type of freezing in seeds, the low temperature exotherm (LTE) being much more pronounced than the high temperature one (HTE). The occurrence of LTE was highly correlated with embryo death. Fruit storage at higher temperatures also caused a decrease in the threshold super cooling temperature of seeds and of naked embryos, removed from the seed coat and endosperm. The decrease was less pronounced in naked embryos than in intact seeds. The results presented show that the frost resistance of apple seeds relies mainly on the supercooling ability of the embryos and is increased by the presence of seed coat and endosperm. The broad peak of the LTE indicates that, in contrast with other seeds and many super cooling organs, massive ice nucleation in apple seeds occurs within the embryo tissues and that extra organ freezing seems to be of less importance. Therefore, the increased super cooling ability of apple seeds, isolated from fruits stored at higher temperatures, seems to rely on those seed properties that protect embryo cells against heterogeneous ice nucleation. 相似文献
104.
Chlorella sp. strain 3.83, a symbiotic Chlorella isolated from the heliozoan Acanthocystis turfacea, excreted between 8% and 16% of assimilated 14CO2 as maltose in the light (15000 lx), with a pH optimum around 4.8. This percentage increased when the illuminance was lowered (36% at 1700 lx). Release of [14C]maltose continued in darkness and could be inhibited by the uncoupler carbonyl cyanide p-trifluoro-methoxyphenylhydrazone and by diethylstilbestrol. Net efflux of maltose was observed even at a concentration ratio of extracellular/intracellular maltose of 7.8. Exogenous [14C]maltose (5 mM) was taken up by the cells with a rate <2% of that of simultaneous maltose release, indicating a practically unidirectional transport. It is concluded that maltose excretion is an active-transport process.Abbreviations DES
diethylstilbestrol
- FCCP
carbonyl cyanide p-trifluoromethoxyphenyl hydrazone
- p.c.
packed cells
This work was supported by the Deutsche Forschungsgemeinschaft. Thanks are due to Doris Meindl for skillful experimental help. 相似文献
105.
106.
Zora Svab Elisabeth C Harper Jonathan D. G. Jones Pal Maliga 《Plant molecular biology》1990,14(2):197-205
The bacterial gene aad A encodes the enzyme aminoglycoside-3-adenyltransferase that confers resistance to spectinomycin and streptomycin in Escherichia coli. Chimeric genes have been constructed for expression in plants, and were introduced into Nicotiana tabacum by Agrobacterium binary transformation vectors. Spectinomycin or streptomycin in selective concentrations prevent greening of N. tabacum calli. Transgenic clones, however, formed green calli on selective media containing spectinomycin, streptomycin, or both drugs. Resistance was inherited as a dominant Mendelian trait in the seed progeny. Resistance conferred by the chimeric aad A gene can be used as a color marker similar to the resistance conferred by the streptomycin phosphotransferase gene to streptomycin. 相似文献
107.
J B Gabrion H Barrière B Nguyen Than Dao M Chambard J Mauchamp F Regnouf L A Pradel 《European journal of cell biology》1990,52(2):282-290
A fodrin-like protein purified from porcine thyroid cells and characterized by its properties identical to those of pig brain spectrin (F. Regnouf et al., Eur. J. Biochem. 153, 313-319 (1985)) has been localized by immunofluorescence and electron immunocytochemistry in porcine and rat thyroid. Fodrin-like polypeptides were detected in subplasmalemmal meshworks of microfilaments attached to isolated or in situ plasma membranes. In resting cells, fodrin was found under apical and basolateral membrane domains, whereas it was always absent under the pseudopod membrane domain induced by acute TSH stimulation in vitro, using monolayers of porcine cultured cells attached to collagen permeable substrates, as well as in vivo, using rats intravenously treated with TSH. Thyroid fodrin could be involved in exocytosis and membrane stabilization which occurs during the formation of pseudopods induced by TSH stimulation. 相似文献
108.
Purified recA protein is induced by high salt concentrations to hydrolyse ATP even in the absence of DNA. By small angle neutron scattering we show that this salt activation results from a structural transition of the protein filament in the presence of ATPγS from the inactive, compact form (a helical polymer of pitch 70 Å and cross-sectional radius of gyration Rc 40 Å) to the open form (a helical filament of pitch 95 Å and Rc 35 Å, which are the same structural parameters as in the ATPase active complex with DNA and ATP), without detectable change in the degree of association. We conclude that activation of recA is due to the same structural change whether induced by the binding of DNA or by salt. Indeed, the other enzymatic activity of recA, the proteolytic cleavage of the lexA repressor, is found to be inducible by the same salt concentrations as those of the structural transition. 相似文献
109.
The physical phenomenon of clathrate hydrate formation in protein-containing reversed micelles is described. Hydrate formation in reversed micelles is a method of adjusting the water to surfactant molar ratio, wo, which influences micellar size. Lipase and alpha-chymotrypsin encapsulated in large reversed micelles of high wo show significant enhancements in activity when the micelle size is reduced through hydrate formation. Alternate methods of micelle size adjustments also show enhancements in activity. The implications for improving the activity of such encapsulated enzymes recovered from fermentation media through phase transfer into reversed micelles are discussed. 相似文献
110.
Roelof-Jan Oostra Stephan Kemp Pieter A. Bolhuis Elisabeth M. Sleeker-Wagemakers 《Human genetics》1996,97(4):500-505
Leber’s hereditary optic neuropathy (LHON) is a maternally inherited disorder of the optic nerves. It has been proposed that
the specific mutations in the mitochondrial DNA (mtDNA) that are associated with LHON require and X-chromosomally encoded
permissive factor in order to become expressed. This would explain both the preponderance of male patients and the fact that
most carriers of specific mtDNA mutations remain unaffected. Although linkage studies have been negative so far, the existence
of such a factor has not been ruled out. We investigated the genealogical data of 24 large LHON pedigrees and concluded that
the presumed X-linked factor would be recessively inherited and that at least 57% of the affected females would be heterozygous.
Therefore, these females must be the victim of nonrandom X-chromosomal inactivation (skewed lyonization). However, analysis
of X-chromosomal methylation patterns in 16 LHON-affected females revealed substantial skewing in only 15%–20% of cases, which
is not significantly different from the patterns in 49 controls. Moreover, we found the frequency of LHON in daughters of
affected heterozygous females to be twice to three times as high as in daughters of unaffected heterozygous females, which
cannot be explained by an X-chromosomally inherited factor. We conclude that the results of our investigations do not support
the hypothesis that LHON is a digenic disease with an X-linked factor being the main cause of loss of vision in the presence
of relevant mtDNA mutations.
Received: 1 June 1995 / Revised: 20 September 1995 相似文献