全文获取类型
收费全文 | 4582篇 |
免费 | 357篇 |
国内免费 | 1篇 |
专业分类
4940篇 |
出版年
2023年 | 16篇 |
2022年 | 39篇 |
2021年 | 72篇 |
2020年 | 39篇 |
2019年 | 56篇 |
2018年 | 67篇 |
2017年 | 68篇 |
2016年 | 118篇 |
2015年 | 164篇 |
2014年 | 213篇 |
2013年 | 272篇 |
2012年 | 305篇 |
2011年 | 298篇 |
2010年 | 237篇 |
2009年 | 186篇 |
2008年 | 266篇 |
2007年 | 266篇 |
2006年 | 221篇 |
2005年 | 229篇 |
2004年 | 244篇 |
2003年 | 246篇 |
2002年 | 223篇 |
2001年 | 74篇 |
2000年 | 53篇 |
1999年 | 62篇 |
1998年 | 79篇 |
1997年 | 42篇 |
1996年 | 38篇 |
1995年 | 36篇 |
1994年 | 32篇 |
1993年 | 37篇 |
1992年 | 54篇 |
1991年 | 54篇 |
1990年 | 36篇 |
1989年 | 39篇 |
1988年 | 35篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 22篇 |
1984年 | 22篇 |
1983年 | 17篇 |
1982年 | 25篇 |
1981年 | 19篇 |
1980年 | 13篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1977年 | 17篇 |
1976年 | 13篇 |
1974年 | 18篇 |
1972年 | 13篇 |
排序方式: 共有4940条查询结果,搜索用时 15 毫秒
11.
Robert Cedergren B. Franz Lang Denis Gravel 《Origins of life and evolution of the biosphere》1988,18(3):299-305
Proposals that an RNA-based genetic system preceeded DNA, stem from the ability of RNA to store genetic information and to promote simple catalysis. However, to be a valid basis for the RNA world, RNA catalysis must demonstrate or be related to intrinsic chemical properties which could have existed in primordial times. We analyze this question by first classifying RNA catalysis and related processes according to their mechanism. We define: (A) thedisjunct nucleophile class which leads to 5-phosphates. These include Group I and II intron splicing, nuclear mRNA splicing and RNase P reactions. Although Group I introns and its excision mechanism is likely to have existed in primordial times, present-day examples have arisen independently in different phyla much more recently. Comparative methodology indicates that RNase P catalysis originated before the divergence of the major kingdoms. In addition, alldisjunct nucleophile reactions can be interrelated by a proposed mechanism involving a distant 2-OH nucleophile. (B) theconjunct nucleophile class leading to 3-phosphates. This class is composed of self-cleaving RNAs found in plant viruses and the newt. We propose that tRNA splicing is related to this mechanism rather than the previous one. The presence of introns in tRNA genes of eukaryotes and archaebacteria supports the idea that tRNA splicing predates the divergence of these cell types. 相似文献
12.
Pulmonary alveoler macrophages exposedto very short chrysotile asbestos fibers present a typical cytotoxic response: extracellular releases of lactate dehydrogenase and -galactosidase, and a decrease in cellular ATP content. The objective of this study was to determine if nicotinamide and 3-aminobenzamide, two inhibitors of the ADP-ribosyl transferase, could modify the in vitro toxicity of chrysotilee fibers. After 30 min of pre-exposure with each of the two inihibitors, pulmonary alveolar macrophage monolayers were concominantly exposed for 18 hours to 50g of fibers. It was observed that, in a dose-effect relationship (5 to 30 mM), nicotinamide was very effective in reducing the extracellular liberation of the marker enzymes. At 30 mM, the enzyme releases in the medium had returned to control values; the restoration of cell viability was confirmed by ATP levels. Up to 5 mM 3-aminobenzamide did not provide any protection against chrysotile cytotoxicity. Nicotinic acid, a structural analogue of nicotinamide, but not an inhibitor of the ADP-ribosyl transferase, also showed no protective effect. Nicotinamide and 3-aminobenzamide increased the intracellular NAD+ pools, respectively by 350% and 250%. However, with or without additives, the chrysotile fibers caused a constant and significant decrease in NAD+ levels (40–55 pmoles). These results suggest that the inhibition of the nuclear ADP-ribosyl transferase is not the major mechanism by which nicotinamide protects pulmonary alveolar macrophages against the chrysotile asbestos fibers.Abbreviations 3-AB
3-aminobenzamide
- ADPRT
ADP-ribosyl transferase
- -GAL
-galactosidase
- DTT
dithiothreitol
- FBS
fetal bovine serum
- FMN
flavin mononucleotide
- HEPES
N-2-hydroxyethyl piperazine-N-2-ethanesulfonic acid
- LDH
lactate dehydrogenase
- NAD+
nicotinamide adenine dinucleotide (oxidized form)
- NADH
nicotimide adenine dinucleotide (reduced forms)
- NADPH
nicotimide adenine dinucleotide phosphate (reduced form)
- NAM
nicotinamide
- NIC
nicotinic acid
- ORS
oxygen radical species
- PAM
pulmonary alveolar macrophages
- S.E.
standard error of the mean
- TAPS
tris (hydroxymethyl) methylamino-propane sulfonic acid
- TRIS
tris (hydroxymethyl) aminomethane
- VSF
very short chrysotile fibers 相似文献
13.
14.
15.
Synopsis Sea surface temperatures across Cabot Strait (Gulf of St. Lawrence) ranged from 6 to 9°C on June 3, 1989, but only from 3 to 5°C on June 2, 1990. Periods of peak commercial landings of mackerel in eastern Cape Breton Island extended from May 22 to June 3 in 1989, and from May 28 to June 2 in 1990. In late May 1990, Atlantic mackerel were captured with a purse-seiner during exploratory fishing in Cabot Strait; commercial quantities were caught in water as cold as 2.8°C. The presence of mackerel in water a full 4°C colder than its reported lower tolerance limit indicates that the development of the 7°C isotherm is not a requirement for the vernal appearing of mackerel. The overlap of the periods of peak commercial landings between 1989 and 1990, despite: marked differences in warming chronology, suggests that the movements of mackerel are not as closely linked to water temperature as previously reported. The fish's thermal preferences could be subordinate to their reproductive requirements at this stage of their spawning migration. 相似文献
16.
Denis Roy Pierre Chevalier Pierre Ward Luc Savoie 《Applied microbiology and biotechnology》1991,34(5):653-655
Summary The ability of Bifidobacterium infantis ATCC 27 920 to ferment glucose, galactose, lactose, melibiose and raffinose was investigated with respect to -galactosidase (-d-galactoside galactohydrolase, E.C. 3.2.1.22). The sugars were tested at three concentrations: 0.5, 1.0 and 2.0%. The growth of B. infantis was slower on glucose compared with the other sugars. The highest specific growth rate was observed on melibiose followed by lactose. High cell numbers could be rapidly obtained on galactose-containing sugars. For each carbohydrate, enzyme activity was maximal at the end of the exponential phase and the highest specific -galactosidase activities were recorded on the two -1,6 galactosaccharides (melibiose and raffinose: 3.0 and 4.5 nkat · 109 colony-forming units, respectively).Contribution no. 186 from the Food Research and Development Centre
Offprint requests to: D. Roy 相似文献
17.
18.
Isolation of pituitary fibroblast growth factor by fast protein liquid chromatography (FPLC): partial chemical and biological characterization 总被引:14,自引:0,他引:14
Denis Gospodarowicz Sharon Massoglia Jannie Cheng Ge-Ming Lui Peter Bhlen 《Journal of cellular physiology》1985,122(2):323-332
Bovine pituitary fibroblast growth factor has been purified 222,000-fold to homogeneity by a combination of differential salt extraction, gel filtration, and ion exchange chromatography on Mono S column. Pituitary FGF is a single-chain polypeptide with an apparent molecular mass of 15,800 and an isoelectric point of 9.6. It is highly active in triggering the proliferation of bovine and human vascular endothelial cell [half-maximal stimulation at 23-40 pg/ml (1.5-2.6 pM) and saturation between 140 and 280 pg/ml (9.3-18.6 pM)]. It displays a similar activity on bovine vascular smooth muscle cells, corneal endothelial cells, granulosa and adrenal cortex cells, and rabbit costal chondrocytes. 相似文献
19.
S Bresson D Vuitton J L Bresson E Racadot D Fellmann J P Miguet 《Comptes rendus des séances de la Société de biologie et de ses filiales》1985,179(3):327-330
Phenotypic analysis with monoclonal antibodies showed that OKT8+ T cells represent the main cell population of the periparasitic mononuclear cell infiltrate in the liver of 7 patients with alveolar echinococcosis. A significant decrease of the OKT8+ subpopulation had been previously demonstrated in the peripheral blood mononuclear cells of these patients. The results obtained in this study suggest an intrahepatic homing for these particular T cells. However, functional analysis of these cells is required in order to assess the physiopathological significance of these observations. 相似文献
20.
Antisera have been developed against the wholecell antigens of Desulfovibrio africanus Benghazi and Walvis Bay, D. vulgaris Hildenborough, D. salexigens British Guiana, D. gigas, and D. desulfuricans Essex 6. An enzymelinked immunoadsorption assay (ELISA) was developed to measure the reaction of these antisera with the homologous and heterologous antigens. The ELISA method demonstrated a reaction between pre-immune sera and cells of D. africanus, D. gigas and D. desulfuricans, suggesting the presence of a lectin-like substance on these cell surfaces. Extensive cross-reactions were seen between the antisera and heterologous cells, suggesting the sharing of a number of surface antigens amongst the Desulfovibrio. However, the pattern of these cross-reactions was different from that observed for an ELISA reaction developed for the cytochrome c3 from various Desulfovibrio.Abbreviation ELISA
enzyme-linked immunoadsorption assay 相似文献