全文获取类型
收费全文 | 4301篇 |
免费 | 454篇 |
国内免费 | 1篇 |
专业分类
4756篇 |
出版年
2022年 | 36篇 |
2021年 | 81篇 |
2020年 | 47篇 |
2019年 | 72篇 |
2018年 | 54篇 |
2017年 | 64篇 |
2016年 | 107篇 |
2015年 | 163篇 |
2014年 | 202篇 |
2013年 | 230篇 |
2012年 | 312篇 |
2011年 | 267篇 |
2010年 | 153篇 |
2009年 | 113篇 |
2008年 | 170篇 |
2007年 | 200篇 |
2006年 | 160篇 |
2005年 | 163篇 |
2004年 | 145篇 |
2003年 | 134篇 |
2002年 | 123篇 |
2001年 | 110篇 |
2000年 | 118篇 |
1999年 | 95篇 |
1998年 | 59篇 |
1997年 | 49篇 |
1996年 | 46篇 |
1995年 | 32篇 |
1994年 | 40篇 |
1993年 | 36篇 |
1992年 | 69篇 |
1991年 | 58篇 |
1990年 | 54篇 |
1989年 | 57篇 |
1988年 | 58篇 |
1987年 | 47篇 |
1986年 | 49篇 |
1985年 | 62篇 |
1984年 | 33篇 |
1982年 | 26篇 |
1981年 | 25篇 |
1979年 | 37篇 |
1978年 | 30篇 |
1977年 | 46篇 |
1976年 | 34篇 |
1974年 | 37篇 |
1973年 | 41篇 |
1972年 | 35篇 |
1971年 | 25篇 |
1970年 | 28篇 |
排序方式: 共有4756条查询结果,搜索用时 0 毫秒
41.
Amino- and carboxy-terminal domains of the yeast Rab escort protein are both required for binding of Ypt small G proteins. 下载免费PDF全文
B E Bauer S Lorenzetti M Miaczynska D M Bui R J Schweyen A Ragnini 《Molecular biology of the cell》1996,7(10):1521-1533
The Rab escort protein (REP) is an essential component of the heterotrimeric enzyme Rab geranylgeranyl transferase that modifies the carboxy-terminal cysteines of the Ras-like small G proteins belonging to the Rab/Ypt family. Deletions in the human CHM locus, encoding one of the two REPs known in humans, result in a retinal degenerative syndrome called choroideremia. The only known yeast homologue of the choroideremia gene product is encoded by an essential gene called MRS6. Besides three structurally conserved regions (SCRs) previously detected in the amino-terminal half of REPs and RabGDIs, three other regions in the carboxy-terminal domain (RCR 1-3) are here identified as being characteristic of REPs alone. We have performed the first mutational analysis of a REP protein to experimentally define the regions functionally important for Rab/Ypt protein binding, making use of the genetic system of the yeast Saccharomyces cerevisiae. This analysis has shown that the SCRs are necessary but not sufficient for Ypt1p binding by the yeast REP, the carboxy-terminal region also being required. 相似文献
42.
Cloning and characterization of senC, a gene involved in both aerobic respiration and photosynthesis gene expression in Rhodobacter capsulatus. 总被引:2,自引:1,他引:1 下载免费PDF全文
The purple nonsulfur photosynthetic eubacterium Rhodobacter capsulatus is a versatile organism that can obtain cellular energy by several means, including the capture of light energy for photosynthesis as well as the use of light-independent respiration, in which molecular oxygen serves as a terminal electron acceptor. In this study, we have identified and characterized a novel gene, senC, mutations in which affect respiration as well as the induction of photosynthesis gene expression. The protein coded by senC exhibits 33% sequence identity to the yeast nucleus-encoded protein SCO1, which is thought to be a mitochondrion-associated cytochrome c oxidase assembly factor. Like yeast SCO1, SenC is required for optimal cytochrome c oxidase activity in aerobically grown R. capsulatus cells. We further show that senC is required for maximal induction from the puf and puh operons, which encode the structural polypeptides of the light-harvesting and reaction center complexes. 相似文献
43.
The nonisothermal transitions of lyophilized recombinant bovine somatotropin (rbSt) as seen via differential scanning calorimetry were evaluated with respect to moisture content. The transition peak temperature of rbSt decreased with increasing moisture from 161°C in the dry state to a plateau of 65°C at 28% moisture, which is similar to that of rbSt in solution. Using high performance liquid chromatography, this irreversible endothermic transition consisted primarily of unfolding, hydrophobic aggregation, and some covalent modifications. In the dry state, covalent modifications, including polymerization into compounds of higher molecular weight, were more prominent, while in the presence of moisture, hydrophobic aggregation was most prominent. The irreversibility and scan rate dependence of the endothermic phenomena supports the kinetic nature of the transition rather than a simple equilibrium between globular and unfolded states. The apparent activation energy, for the net transition (i.e., unfolding, hydrophobic aggregation, and covalent modifications) was 57 kcal/mol for rbSt at 9.9% moisture. The observed enthalpy of the transition increased, decreased, then approximately leveled off as a function of increasing moisture content. This can be explained by the increasingly significant contribution of the exothermic aggregation at higher moisture contents. © 1995 John Wiley & Sons, Inc. 相似文献
44.
45.
46.
Chemosensory and photosensory perception in purple photosynthetic bacteria utilize common signal transduction components. 总被引:9,自引:6,他引:3 下载免费PDF全文
The chemotaxis gene cluster from the photosynthetic bacterium Rhodospirillum centenum contains five open reading frames (ORFs) that have significant sequence homology to chemotaxis genes from other bacteria. To elucidate the functions of each ORF, we have made various mutations in the gene cluster and analyzed their phenotypic defects. Deletion of the entire che operon (delta che), as well as nonpolar disruptions of cheAY, cheW, and cheR, resulted in a smooth-swimming phenotype, whereas disruption of cheB resulted in a locked tumbly phenotype. Each of these mutants was defective in chemotactic response. Interestingly, disruption of cheY resulted in a slight increase in the frequency of tumbling/reversal with no obvious defects in chemotactic response. In contrast to observations with Escherichia coli and several other bacteria, we found that all of the che mutant cells were capable of differentiating into hyperflagellated swarmer cells when plated on a solid agar surface. When viewed microscopically, the smooth-swimming che mutants exhibited active surface motility but were unable to respond to a step-down in light intensity. Both positive and negative phototactic responses were abolished in all che mutants, including the cheY mutant. These results indicate that eubacterial photosensory perception is mediated by light-generated signals that are transmitted through the chemotaxis signal transduction cascade. 相似文献
47.
Activation of phospholipase C and prostaglandin synthesis by [arginine]vasopressin in cultures. 总被引:6,自引:4,他引:2 下载免费PDF全文
[Arginine]vasopressin (AVP) stimulates maximal prostaglandin E2 production in cultured rat renal mesangial cells within 2 min. As early as 10s after addition of AVP (10(-6)M) a significant loss of radioactivity from phosphatidylinositol 4,5-bisphosphate but not from phosphatidylinositol 4-phosphate and phosphatidylinositol was observed in cells prelabelled with 32Pi. Cells labelled with [14C]arachidonic acid showed an increase of label in 1,2-diacylglycerol after 15 s and in phosphatidic acid after 30 s upon stimulation with AVP. Pretreatment of the cells with indomethacin (10(-5)M) did not abolish the effect of AVP on the increased labelling of phosphatidic acid. 相似文献
48.
Preservation of Rhizobium viability and symbiotic infectivity by suspension in water 总被引:2,自引:0,他引:2
D K Crist R E Wyza K K Mills W D Bauer W R Evans 《Applied and environmental microbiology》1984,47(5):895-900
Three Rhizobium japonicum strains and two slow-growing cowpea-type Rhizobium strains were found to remain viable and able to rapidly modulate their respective hosts after being stored in purified water at ambient temperatures for periods of 1 year and longer. Three fast-growing Rhizobium species did not remain viable under the same water storage conditions. After dilution of slow-growing Rhizobium strains with water to 10(3) to 10(5) cells ml-1, the bacteria multiplied until the viable cell count reached levels of between 10(6) and 10(7) cells ml-1. The viable cell count subsequently remained fairly constant. When the rhizobia were diluted to 10(7) cells ml-1, they did not multiply, but full viability was maintained. If the rhizobia were washed and suspended at 10(9) cells ml-1, viability slowly declined to 10(7) cells ml-1 during 9 months of storage. Scanning electron microscopy showed that no major morphological changes took place during storage. Preservation of slow-growing rhizobia in water suspensions could provide a simple and inexpensive alternative to current methods for the preservation of rhizobia for legume inoculation. 相似文献
49.
Peter Singer Manfred Wirth Ute Gerike Wolfgang Gdicke Vera Moritz 《Prostaglandins & other lipid mediators》1984,27(3):375-390
The lipid content as well as the fatty acid pattern of triglycerides, free fatty acids (FFA), phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were estimated in renal cortex and medulla of spontaneously hypertensive rats (SHR) and normotensive Wistar rats (WR) at 4,8,26 and 52 weeks of age. In general, the level of triglycerides in renal medulla appeared higher when compared with the cortex. On the other hand, PC and PE, increasing with age, were usually higher in the cortex. A decreased percentage of linoleic acid (LA) in triglycerides, of arachidonic acid (AA) in PC and of eicosapentaenoic acid (EPA) in triglycerides, FFA, PC and PE could be found in the kidneys of SHR at 8 weeks of age, i.e. during the development of hypertension. This was accompanied with a rise of AA in FFA of SHR at 8 weeks of age, which occurred with delay in WR (at 26 weeks of age). From the data presented it can be concluded that systematic alterations in the availability of individual polyunsaturated fatty acids (PUFA) in various renal lipids might be related to the onset of hypertension in SHR which should be elucidated in more detail. 相似文献
50.
Aspergillus flavus grown on yeast extract-sucrose medium produced higher amounts of aflatoxin B1 in the presence of 0.025% sorbic acid than without this chemical with a maximum at 17 days of incubation. Addition of 0.05 to 0.0125% sorbic acid stimulated T-2 toxin production of Fusarium acuminatum cultures grown on maize meal. The highest amounts of the mycotoxin were detected in 14-day-old cultures containing 0.025% sorbic acid. It is assumed that certain amounts of sorbic acid near the minimal inhibitory concentration reduce the activity of the tricarboxylic acid cycle; this may lead to an accumulation of acetyl coenzyme A, which is an essential intermediate in the biosynthesis of aflatoxin B1 and T-2 toxin. 相似文献