全文获取类型
收费全文 | 19164篇 |
免费 | 1751篇 |
国内免费 | 1682篇 |
出版年
2024年 | 51篇 |
2023年 | 275篇 |
2022年 | 611篇 |
2021年 | 1087篇 |
2020年 | 752篇 |
2019年 | 884篇 |
2018年 | 825篇 |
2017年 | 615篇 |
2016年 | 888篇 |
2015年 | 1251篇 |
2014年 | 1546篇 |
2013年 | 1489篇 |
2012年 | 1768篇 |
2011年 | 1561篇 |
2010年 | 962篇 |
2009年 | 859篇 |
2008年 | 1017篇 |
2007年 | 914篇 |
2006年 | 702篇 |
2005年 | 579篇 |
2004年 | 511篇 |
2003年 | 449篇 |
2002年 | 390篇 |
2001年 | 324篇 |
2000年 | 296篇 |
1999年 | 298篇 |
1998年 | 195篇 |
1997年 | 173篇 |
1996年 | 168篇 |
1995年 | 121篇 |
1994年 | 138篇 |
1993年 | 90篇 |
1992年 | 117篇 |
1991年 | 110篇 |
1990年 | 73篇 |
1989年 | 62篇 |
1988年 | 64篇 |
1987年 | 52篇 |
1986年 | 44篇 |
1985年 | 50篇 |
1984年 | 32篇 |
1983年 | 32篇 |
1982年 | 20篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 13篇 |
1977年 | 12篇 |
1975年 | 14篇 |
1974年 | 12篇 |
1970年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
An electrophoretic karyotype for Schizosaccharomyces pombe by pulsed field gel electrophoresis. 总被引:44,自引:25,他引:19
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
C L Smith T Matsumoto O Niwa S Klco J B Fan M Yanagida C R Cantor 《Nucleic acids research》1987,15(11):4481-4489
The three chromosomal DNAs of S. pombe have been fractionated by pulsed field gel electrophoresis. The resulting molecular karyotype will greatly speed gene mapping in this organism, and it indicates that the separation range of the technique extends to DNA molecules as large as 9,000,000 base pairs. 相似文献
32.
记辽宁东部新鳞齿鱼属一新种 总被引:1,自引:0,他引:1
本文记述了产自辽宁东部红庙子盆地下桦皮甸子组的新鳞齿鱼属—新种——Neolepidotes liaodongensis sp. nov..根据新材料,将 Neolepidotes 与 Lepidotes 等属作了补充比较,增订了新鳞齿鱼属的特征. 相似文献
33.
Tungstate, a molybdate analog inactivating nitrate reductase, deregulates the expression of the nitrate reductase structural gene 总被引:19,自引:1,他引:18
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Nitrate reductase (NR, EC 1.6.6.1) from higher plants is a homodimeric enzyme carrying a molybdenum cofactor at the catalytic site. Tungsten can be substituted for molybdenum in the cofactor structure, resulting in an inactive enzyme. When nitratefed Nicotiana tabacum plants were grown on a nutrient solution in which tungstate was substituted for molybdate, NR activity in the leaves decreased to a very low level within 24 hours while NR protein accumulated progressively to a level severalfold higher than the control after 6 days. NR mRNA level in molybdate-grown plants exhibited a considerable day-night fluctuation. However, when plants were treated with tungstate, NR mRNA level remained very high. NR activity and protein increased over a 24-hour period when nitrate was added back to N-starved molybdate-grown plants. NR mRNA level increased markedly during the first 2 hours and then decreased. In the presence of tungstate, however, the induction of NR activity by nitrate was totally abolished while high levels of NR protein and mRNA were both induced, and the high level of NR mRNA was maintained over a 10-hour period. These results suggest that the substitution of tungsten for molybdenum in NR complex leads to an overexpression of the NR structural gene. Possible mechanisms involved in this deregulation are discussed. 相似文献
34.
Level of protein kinase C activity correlates directly with resistance to adriamycin in murine fibrosarcoma cells 总被引:3,自引:0,他引:3
In this report, we demonstrate a direct correlation between protein kinase C (PKC) activity and adriamycin (ADR) resistance in mouse fibrosarcoma cells. PKC activity was measured in four murine UV-2237M fibrosarcoma cell lines that differed in the degrees to which they expressed resistance to ADR, which is an inhibitor of PKC. A comparison of the four cell lines revealed a positive correlation between the level of PKC activity and resistance to ADR. Incubation of the cells with the PKC inhibitor H-7 produced a partial reversal of ADR resistance. Taken together, these results suggest a role for PKC in the mechanism of ADR resistance. 相似文献
35.
Exon-intron organization, expression, and chromosomal localization of the human motilin gene 总被引:1,自引:0,他引:1
H Yano Y Seino J Fujita Y Yamada N Inagaki J Takeda G I Bell R L Eddy Y S Fan M G Byers 《FEBS letters》1989,249(2):248-252
The human motilin gene has been isolated and characterized. The gene spans about 9 kilobase pairs (kb) and the 0.7 kb motilin mRNA is encoded by five exons. The 22-amino-acid motilin sequence is encoded by exons 2 and 3. The human motilin gene was mapped to the p21.2----p21.3 region of chromosome 6 by hybridization of the cloned cDNA to DNAs from a panel of reduced human-mouse somatic cell hybrids and by in situ hybridization to human prometaphase chromosomes. RNA blotting using RNA prepared from various regions of the human gastrointestinal tract revealed high levels of motilin mRNA in duodenum and lower levels in the antrum of the stomach; motilin mRNA could not be detected by this procedure in the esophagus, cardia of the stomach, descending colon or gallbladder. 相似文献
36.
Classical Raman spectroscopic studies of NADH and NAD+ bound to lactate dehydrogenase by difference techniques 总被引:2,自引:0,他引:2
The binding of the coenzymes NAD+ and NADH to lactate dehydrogenase causes significant changes in the Raman spectra of both of these molecules relative to spectra obtained in the absence of enzyme. The molecular motions of the bound adenine moiety of both NAD+ and NADH as well as adenine containing analogues of these coenzymes produce Raman bands that are essentially identical, suggesting that the binding of adenine to the enzyme is the same regardless of the nicotinamide head-group nature. We also have observed that the molecular motions of the bound adenine moiety are different from both those obtained when it is in either water, various hydrophobic solvents, or various other solvent compositions. Protonation of the bound adenine ring at the 3-position is offered as a possible explanation. Significant shifts are observed in both the stretching frequency of the carboxamide carbonyl of NAD+ and the rocking motion of the carboxamide NH2 group of NADH. These shifts are probably caused by hydrogen bonding with the enzyme. The interaction energies of these hydrogen-bonding patterns are discussed. The aromatic nature of the nicotinamide moiety of NAD+ appears to be unchanged upon binding. Pronounced changes in the Raman spectrum of the nicotinamide moiety of NADH are observed upon binding; some of these changes are understood and discussed. Finally, these results are compared to analogous results that were recently reported for liver alcohol dehydrogenase [Chen et al. (1987) Biochemistry 26, 4776-4784]. In general, the coenzyme binding properties are found to be quite similar, but not identical, for the two enzymes. 相似文献
37.
38.
Production and characterization of a monoclonal antibody cross-reactive with most group A trichothecenes 总被引:1,自引:0,他引:1
A monoclonal antibody cross-reactive with most group A trichothecenes was produced by fusion of P3/NS-1/1-AG4-1 myeloma cells with spleen cells isolated from a BALB/c mouse that had been immunized with 3-acetyl-neosolaniol-hemisuccinate conjugated to bovine serum albumin. One stable clone, H159B1D5, which produced monoclonal antibody that bound with both T-2 toxin and diacetoxyscirpenol (DAS) was obtained after subcloning. Enzyme-linked immunosorbent assay (ELISA) revealed that the antibody belongs to the immunoglobulin G1 (kappa chain) isotype and had binding constants of 2.81 x 10(9), 1.05 x 10(9), and 1.57 x 10(8) liters per mole for T-2 tetraol tetraacetate, T-2 toxin, and DAS, respectively. The relative cross-reactivities of the antibody with T-2 tetraol tetraacetate, T-2 toxin, and DAS were 200, 100, and 20, respectively, with tritiated T-2 toxin as the marker ligand. The relative cross-reactivities for the above toxins were 667, 100, and 73, respectively, with tritiated DAS as the marker ligand. No cross-reaction with HT-2 and deoxynivalenol triacetate was observed in either system. By using this monoclonal antibody, an indirect ELISA for analysis of T-2 toxin was also developed. The linear portion of the standard curve for analysis of T-2 toxin in each analysis by radioimmunoassay and ELISA was in the range of 0.1 to 2 ng and 0.05 to 1.0 ng, respectively. 相似文献
39.
Wei de Zhang Fan Qing Li Xiao Yun Zhang Ying Chen 《Biological trace element research》1988,17(1):81-90
The growth of the protozoanBlepherisma is stimulated by Lanthanum (La) at concentrations as low as 0.32 ppm. In mice Yttrium (Y) and Ytterbium (Yb) are absorbed,
accumulated, and metabolized. Both rare earth elements (RE) exhibit a high affinity for teeth and bones, accumulation occurs
and metabolism is slow. In the livers of RE-exposed mice, concentrations are variable. The liver is apparently capable of
absorbing and discharging RE in a manner depending on metabolic activity. The main route of discharge for ingested REs is
the alimentary canal. Exposure of pregnant mice to RE leads to rapid placental transfer of RE; 14.1% of the total amount of
RE administered was detected in newborn mice. Young, developing organisms appear to be especially susceptible to RE accumulation. 相似文献
40.
Peptidyl-glycine alpha-amidation enzyme activity has been measured in 36 nuclei or areas in the rat CNS and pituitary using D-Tyr-Phe-Gly as the substrate. The distribution of this enzyme is highly uneven, with highest activity levels (greater than 30 pmol/mg of protein/h) in hypothalamic nuclei, substantia grisea centralis, and nucleus ruber; moderate activity levels (10-30 pmol/mg of protein/h) in globus pallidus, septum, midbrain, pons, medulla oblongata, and cervical spinal cord; and low activity levels (1-10 pmol/mg of protein/h) in other telencephalic and thalamic structures. Almost no alpha-amidation activity (less than 0.5 pmol/mg of protein/h) was detected in cerebellar cortex. The Km values in several brain regions are of the same order. 相似文献