全文获取类型
收费全文 | 9653篇 |
免费 | 717篇 |
国内免费 | 4篇 |
出版年
2023年 | 53篇 |
2022年 | 103篇 |
2021年 | 203篇 |
2020年 | 173篇 |
2019年 | 215篇 |
2018年 | 271篇 |
2017年 | 223篇 |
2016年 | 325篇 |
2015年 | 554篇 |
2014年 | 551篇 |
2013年 | 676篇 |
2012年 | 831篇 |
2011年 | 766篇 |
2010年 | 512篇 |
2009年 | 461篇 |
2008年 | 560篇 |
2007年 | 532篇 |
2006年 | 520篇 |
2005年 | 472篇 |
2004年 | 438篇 |
2003年 | 359篇 |
2002年 | 340篇 |
2001年 | 85篇 |
2000年 | 66篇 |
1999年 | 86篇 |
1998年 | 118篇 |
1997年 | 91篇 |
1996年 | 66篇 |
1995年 | 61篇 |
1994年 | 65篇 |
1993年 | 63篇 |
1992年 | 40篇 |
1991年 | 40篇 |
1990年 | 43篇 |
1989年 | 38篇 |
1988年 | 31篇 |
1987年 | 26篇 |
1986年 | 23篇 |
1985年 | 29篇 |
1984年 | 39篇 |
1983年 | 22篇 |
1982年 | 27篇 |
1981年 | 25篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1976年 | 13篇 |
1975年 | 16篇 |
1974年 | 10篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
31.
gltF, a member of the gltBDF operon of Escherichia coli, is involved in nitrogen-regulated gene expression 总被引:4,自引:0,他引:4
Irene Castaño † Noemi Flores Fernando Valle Alejandra A. Covarrubias Francisco Bolivar 《Molecular microbiology》1992,6(18):2733-2741
We report here the construction and analysis of insertional mutations in each of the three genes of the gltBDF operon and the nucleotide sequence of the region downstream from gltD. Two open reading frames were identified, the first of which corresponds to gltF. The gltB and gltD genes code for the large and small subunits, respectively, of the enzyme glutamate synthase (GOGAT). gltF codes for a protein, with a molecular mass of 26,350 Da, which is required for Ntr induction. Histidase synthesis was determined as a measure of Ntr function. First, insertions in gltB, gltD or gltF all prevent Ntr induction. Second, complementation analysis indicates that high-level expression of both the gltD and gltF genes is required for the induction of the Ntr enzymes under nitrogen-limiting conditions, indicating that the phenotype of the gltB insertion probably results from polarity on gltD and gltF. Third, glutamate-dependent repression of the glt operon appears to be mediated by the product of the gltF gene. Thus, the gltBDF operon of Escherichia coli is involved in induction of the so-called Ntr enzymes in response to nitrogen deprivation, as well as in glutamate biosynthesis. 相似文献
32.
33.
A gibberellin-regulated gene from wheat with sequence homology to cathepsin B of mammalian cells 总被引:5,自引:0,他引:5
Francisco J. Cejudo George Murphy Catherine Chinoy David C. Baulcombe 《The Plant journal : for cell and molecular biology》1992,2(6):937-948
A previous report described several cDNAs corresponding to mRNAs which accumulated in wheat aleurone layers treated with gibberellic acid (GA) (Baulcombe and Buffard, 1983). The protein sequence deduced from one of these clones (2529) has extensive similarity to the thiol protease, cathepsin B from mammalian cells. Southern analysis of wheat DNA has shown that the 2529 mRNA is encoded by a small family of genes carried on the group 4 chromosome. The nucleotide sequence of a member of the gene family expressed at a low level in aleurone layers and the use of a primer extension assay to identify a clone of a member of the gene family producing an abundant mRNA are reported. The 2529 mRNA accumulates in the scutellum and the aleurone layer of germinating grains where its expression is regulated by GA. In the scutellum the expression was restricted to the parenchyma, suggesting that the 2529 product may have a role other than for mobilization of the endosperm. 相似文献
34.
Francisco Yegres Nicole Richard-Yegres Kazuko Nishimura Makoto Miyaji 《Mycopathologia》1991,114(2):71-76
Three strains of Cladosporium carrionii, two human isolates and one from a xerophilous plant, were used to study the effect of culture conditions in 106 newborn ddY mice. Growth in a complex medium (YPG) and a basal synthetic medium (BSM) was compared. Filamentous forms developed during static incubation while conidia were readily formed with shaking. Mice inoculated intraperitoneally were sacrified and autopsied after 4 weeks. Mortality was related only to sporulated exponential phase growing cells. Invasiveness ability was preserved in all experimental conditions. BSM medium that inhibited exopigment formation appeared more suitable than YPG to obtain intact cells for further studies.Biochemical and physiological alteration associated with shape changes during differentiation of vegetative cells into spores could play an important role in virulence of C. carrionii 相似文献
35.
Pigment patterns in mutants affecting the biosynthesis of pteridines and xanthommatin in Drosophila melanogaster 总被引:3,自引:0,他引:3
Juan Ferré Francisco J. Silva M. Dolores Real José L. Ménsua 《Biochemical genetics》1986,24(7-8):545-569
Eye-color mutants of Drosophila melanogaster have been analyzed for their pigment content and related metabolites. Xanthommatin and dihydroxanthommatin (pigments causing brown eye color) were measured after selective extraction in acidified butanol. Pteridines (pigments causing red eye color) were quantitated after separation of 28 spots by thin-layer chromatography, most of which are pteridines and a few of which are fluorescent metabolites from the xanthommatin pathway. Pigment patterns have been studied in 45 loci. The pteridine pathway ramifies into two double branches giving rise to isoxanthopterin, drosopterins, and biopterin as final products. The regulatory relationship among the branches and the metabolic blockage of the mutants are discussed. The Hn locus is proposed to regulate pteridine synthesis in a step between pyruvoyltetrahydropterin and dihydropterin. The results also indicate that the synthesis and accumulation of xanthommatin in the eyes might be related to the synthesis of pteridines.Support for this work was provided to J.F. in part by a grant from the Ministerio de Universidades e Investigación (Spain) and to F.J.S. by a grant from the Ministerio de Educación y Ciencia (Spain). 相似文献
36.
Interactions in Syntrophic Associations of Endospore-Forming, Butyrate-Degrading Bacteria and H2-Consuming Bacteria 总被引:5,自引:4,他引:1 下载免费PDF全文
Butyrate is an important intermediate in the anaerobic degradation of organic matter. In sulfate-depleted environments butyrate is oxidized to acetate and hydrogen by obligate proton reducers, in syntrophic association with hydrogen-consuming methanogens. This paper describes two enrichments of endospore-forming bacteria degrading butyrate in consortia with methanogens. The isolates are readily established in coculture with H2-consuming, sulfate-reducing bacteria by pasteurizing the culture. The two original enrichments differed in that one grew to an optically dense culture while the second grew in clumps. Examination by scanning electron microscopy showed that clumping resulted from the production of large amounts of extracellular polymer. Several H2-consuming methanogens were identified in the enrichments. Some of them grew closely associated to the butyrate degraders. This attachment to the hydrogen producer may permit some methanogens to compete for the growth substrate against other bacteria having higher substrate affinity. 相似文献
37.
Natural Selection VS. Random Drift: Evidence from Temporal Variation in Allele Frequencies in Nature 总被引:1,自引:1,他引:0 下载免费PDF全文
We have obtained monthly samples of two species, Drosophila pseudoobscura and Drosophila persimilis, in a natural population from Napa County, California. In each species, about 300 genes have been assayed by electrophoresis for each of seven enzyme loci in each monthly sample from March 1972 to June 1975. Using statistical methods developed for the purpose, we have examined whether the allele frequencies at different loci vary in a correlated fashion. The methods used do not detect natural selection when it is deterministic (e.g., overdominance or directional selection), but only when alleles at different loci vary simultaneously in response to the same environmental variations. Moreover, only relatively large fitness differences (of the order of 15%) are detectable. We have found strong evidence of correlated allele frequency variation in 13-20% of the cases examined. We interpret this as evidence that natural selection plays a major role in the evolution of protein polymorphisms in nature. 相似文献
38.
Effect of prolactin and glucocorticoids on P-enolpyruvate carboxykinase activity in liver and mammary gland from diabetic and lactating rats 总被引:1,自引:0,他引:1
María F. Lobato Mertxe Careche Manuel Ros Francisco J. Moreno Josefa P. García-Ruíz 《Molecular and cellular biochemistry》1985,67(1):19-23
Summary The administration of 2 bromo--ergocryptine, to reduce serum prolactin decreased the activity of cytosolic P-enolpyruvatc carboxykinase (GTP) (EC4.1.1.32) about 50% in both liver and mammary gland of lactating animals. Adrenalectomy had similar effects to those of bromo-a-ergocryptine. In contrast, there was a 50% increase in enzyme activity in the mammary gland of diabetic, lactating rats and a 10-fold increase in liver as compared with normal rats. P-enolpyruvate carboxykinase activity in mammary gland as liver is coordinately regulated by prolactin, glucocorticoids and insulin. 相似文献
39.
Francisco Romero Francisco Javier Caballero Francisco Castillo José Manuel Roldán 《Archives of microbiology》1985,143(2):111-116
Anti-glutamine synthetase serum was raised in rabbits by injecting purified glutamine synthetase (GS) of the phototrophic bacterium Rhodopseudomonas capsulata E1F1. The antibodies were purified to monospecificity by immunoaffinity chromatography in GS-sepharose gel. These anti-GS antibodies were used to measure the antigen levels in crude extracts from bacteria, grown phototrophically with dinitrogen, nitrate, nitrite, ammonia, glutamate, glutamine or alanine as nitrogen sources. The amount of GS detected by rocket immunoelectrophoresis was proportional to Mn2+-dependent transferase activity measured in the crude extracts. Addition of GS inhibitor l-methionine-d,l-sulfoximine (MSX) to the actively growing cells promoted increased antigen levels, that were not found in the presence of glutamine or chloramphenicol. The ammonia-induced decrease in GS relative levels was reverted by MSX. GS levels remained constant when phototrophically growing cells were kept in the dark.Abbreviations GS
glutamine synthetase
- MOPS
2-(N-morpholine) propane sulfonate
- MSX
l-methionine-d,l-sulfoximine 相似文献
40.
Dr. Manuel Megias Miguel A. Caviedes Francisco Rodriguez-Quiñones Antonio J. Palomares Francisco Ruiz-Berraquero 《Current microbiology》1985,12(6):325-328
Rhizobium trifolii was highly resistant to the lethal effect ofN-methyl-N-nitro-N-nitrosoguanidine (MNNG), but it was sensitive to the mutagenic action of this chemical. A concentration of 500g/ml yields a survival of between 1% and 10%, which allows us to obtain a higher number of mutants than lower concentrations that yield higher survival rates. Lethal damage produced by nitrosoguanidine was repaired, and repair is inhibited by acriflavine. 相似文献