全文获取类型
收费全文 | 7825篇 |
免费 | 604篇 |
国内免费 | 1篇 |
专业分类
8430篇 |
出版年
2023年 | 55篇 |
2022年 | 67篇 |
2021年 | 134篇 |
2020年 | 97篇 |
2019年 | 123篇 |
2018年 | 253篇 |
2017年 | 241篇 |
2016年 | 298篇 |
2015年 | 468篇 |
2014年 | 373篇 |
2013年 | 525篇 |
2012年 | 676篇 |
2011年 | 616篇 |
2010年 | 365篇 |
2009年 | 266篇 |
2008年 | 417篇 |
2007年 | 358篇 |
2006年 | 353篇 |
2005年 | 334篇 |
2004年 | 328篇 |
2003年 | 276篇 |
2002年 | 232篇 |
2001年 | 209篇 |
2000年 | 206篇 |
1999年 | 153篇 |
1998年 | 63篇 |
1997年 | 33篇 |
1996年 | 39篇 |
1995年 | 55篇 |
1994年 | 25篇 |
1993年 | 33篇 |
1992年 | 88篇 |
1991年 | 53篇 |
1990年 | 56篇 |
1989年 | 51篇 |
1988年 | 48篇 |
1987年 | 60篇 |
1986年 | 43篇 |
1985年 | 43篇 |
1984年 | 47篇 |
1983年 | 20篇 |
1982年 | 23篇 |
1981年 | 12篇 |
1979年 | 17篇 |
1977年 | 16篇 |
1975年 | 19篇 |
1974年 | 30篇 |
1973年 | 19篇 |
1968年 | 15篇 |
1967年 | 11篇 |
排序方式: 共有8430条查询结果,搜索用时 15 毫秒
1.
In Escherichia coli, two enzymes catalyze the synthesis of methionine from homocysteine using methyltetrahydrofolate as the donor of the required methyl group: cobalamin-dependent and cobalamin-independent methionine synthases. Comparison of the mechanisms of these two enzymes offers the opportunity to examine two different solutions to the same chemical problem. We initiated the research described here to determine whether the two enzymes were evolutionarily related by comparing the deduced amino acid sequences of the two proteins. We have determined the nucleotide sequence for the metE gene, encoding the cobalamin-independent methionine synthase. Our results reveal an absence of similarity between the deduced amino acid sequences of the cobalamin-dependent and cobalamin-independent proteins and suggest that the two have arisen by convergent evolution. We have developed a rapid one-step purification of the recombinant cobalamin-independent methionine synthase (MetE) that yields homogeneous protein in high yield for mechanistic and structural studies. In the course of these studies, we identified a highly reactive thiol in MetE that is alkylated by chloromethyl ketones and by iodoacetamide. We demonstrated that alkylation of this residue, shown to be cysteine 726, results in complete loss of activity. While we are unable to deduce the role of cysteine 726 in catalysis at this time, the identification of this reactive residue suggests the possibility that this thiol functions as an intermediate methyl acceptor in catalysis, analogous to the role of cobalamin in the reaction catalyzed by the cobalamin-dependent enzyme. 相似文献
2.
A González-Fernández J Sans P Aller C de la Torre 《Cell biology international reports》1985,9(3):237-243
Bromosubstitution for most of the S period in synchronous populations of Allium cepa L. meristematic cells resulted in a delay in the late S-G2 transition point where protein synthesis is needed for later mitotic entrance to occur. This retardation in the position of the transition point was not accompanied by the expected delay in the entrance into mitosis, suggesting that such protein synthesis is a requisite, but not a timer for prophase triggering. 相似文献
3.
Alba Minelli Carmela Conte Silvia Grottelli Ilaria Bellezza Carla Emiliani† Juan P. Bolaños‡ 《Journal of neurochemistry》2009,111(4):956-966
Paraquat (1,1'-dimethyl-4,4'-bipyridinium), a widely used non-selective herbicide, is a redox cycling agent with adverse effects on dopamine systems. Epidemiological data have shown that exposure to paraquat is one of the several risk factors for Parkinson's disease. We have already shown that cyclo(His-Pro), an endogenous cyclic dipeptide produced by the cleavage of the thyrotropin releasing hormone, has a cytoprotective effect through a mechanism involving Nrf2 activation that decreases production of reactive oxygen species and increases glutathione synthesis. Using primary neuronal cultures and PC12 cells as targets of paraquat neurotoxicity, we addressed whether and how cyclo(His-Pro) causes cellular protective response against paraquat-mediated cell death. We found that cyclo(His-Pro) attenuated reactive oxygen species production, and prevented glutathione depletion by up-regulating Nrf2 gene expression, triggering its nuclear accumulation and activating the expression of heme oxygenase1. These protective effects were abolished by RNA interference-mediated Nrf2 knock down whereas were unaffected by RNA interference-mediated Keap1 knock down. Inhibition of heme oxygenase activity decreased cyclo(His-Pro)-induced neuroprotection. These results suggest that cyclo(His-Pro), acting as a selective activator of the brain modulable Nrf2 pathway, may be a promising candidate as neuroprotective agent that act through induction of phase II genes. 相似文献
4.
5.
Berta Fiszer-Szafarz David Szafarz Alba Guevara de Murillo 《Analytical biochemistry》1981,110(1):165-170
A fluorometric method using 3,5-diaminobenzoic acid for DNA determination in tissues, cultured cells, nucleated blood cells, and yeast cells is described. The method is general, simple, and rapid, and does not require prior DNA extraction, since tissue is directly solubilized in Triton X-100 and ammonia. The procedure is highly sensitive, and is able to measure rather accurately as little as 10 ng of DNA. It is applicable to all types of DNA structure. The DNA content determined in various tissues and cells was: 2.50 mg/g fresh rat liver, 3.32 mg/g rat diethylnitrosamine-induced hepatoma, 2.49 mg/g fresh mouse liver, 8.76 μg/106 human leukocytes, 3.37 μg/106 chicken fibroblasts, 2.97 μg/108 haploid yeast cells, and 2.84 μg/108 haploid yeast protoplasts. 相似文献
6.
J. Alba Tercedor 《水生昆虫》2013,35(3)
Ephemera hispanica Navás, 1903 has been considered a doubtful species until recently. It is shown to be a long established synonym of E. vulgata Linnaeus, 1758. 相似文献
7.
8.
C González-Bosch V J Miralles J Hernández-Yago S Grisolía 《Biochemical and biophysical research communications》1987,146(3):1318-1323
Apocytochrome c, the cytosolic precursor of cytochrome c, competes with the precursor of ornithine carbamoyltransferase (OCT) for entry into isolated rat liver mitochondria. 相似文献
9.
Spermidine and spermine stimulate the transport of the precursor of ornithine carbamoyltransferase into rat liver mitochondria 总被引:1,自引:0,他引:1
C González-Bosch V J Miralles J Hernández-Yago S Grisolia 《Biochemical and biophysical research communications》1987,149(1):21-26
We have examined the effect of low molecular weight components of the transport mixture generally used for the import of rat liver pre-ornithine carbamoyltransferase by isolated rat liver mitochondria. These studies revealed that spermidine and spermine, at physiological concentrations, stimulate the transport of the precursor of ornithine carbamoyltransferase into mitochondria. This stimulatory effect of spermidine and spermine is concentration-dependent and is completely inhibited at higher than physiological concentrations (20 mM for spermidine and 4 mM for spermine). Magnesium ions, which also have a stimulatory effect, inhibit the stimulatory effect of spermidine. 相似文献
10.