全文获取类型
收费全文 | 9708篇 |
免费 | 719篇 |
国内免费 | 4篇 |
专业分类
10431篇 |
出版年
2023年 | 59篇 |
2022年 | 111篇 |
2021年 | 205篇 |
2020年 | 174篇 |
2019年 | 215篇 |
2018年 | 272篇 |
2017年 | 225篇 |
2016年 | 326篇 |
2015年 | 554篇 |
2014年 | 553篇 |
2013年 | 681篇 |
2012年 | 832篇 |
2011年 | 768篇 |
2010年 | 512篇 |
2009年 | 462篇 |
2008年 | 560篇 |
2007年 | 533篇 |
2006年 | 521篇 |
2005年 | 473篇 |
2004年 | 439篇 |
2003年 | 359篇 |
2002年 | 341篇 |
2001年 | 87篇 |
2000年 | 67篇 |
1999年 | 86篇 |
1998年 | 118篇 |
1997年 | 91篇 |
1996年 | 66篇 |
1995年 | 61篇 |
1994年 | 66篇 |
1993年 | 65篇 |
1992年 | 42篇 |
1991年 | 41篇 |
1990年 | 45篇 |
1989年 | 39篇 |
1988年 | 34篇 |
1987年 | 26篇 |
1986年 | 24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 0 毫秒
91.
92.
We have tested the use of firefly luciferase for monitoring regulated symbiotic nitrogen fixation gene expression. Broad-host-range plasmids carrying translational fusions of Rhizobium meliloti nifH, fixA and nifA promoters were constructed. Despite low levels of promoter activity the absence of Escherichia coli endogenous luminescence and the high sensitivity of the bioluminescent assay for firefly luciferase allowed rapid screening for functional luciferase expression. Plasmids containing symbiotic promoter-luc fusions were established in R. meliloti. Luciferase activity was detected and measured in both vegetative and symbiotic cells giving comparable results with those obtained by beta-galactosidase assays. In addition, the luciferase assay was quicker, more sensitive and could be carried out with unrestricted cells. Furthermore, bioluminescence was high enough in alfalfa nodules containing nifH—luc fusion to be observed by a dark-adapted eye and photographed. 相似文献
93.
Teresa J. Arrowsmith Francisco Malpartida David H. Sherman Ashley Birch David A. Hopwood John A. Robinson 《Molecular & general genetics : MGG》1992,234(2):254-264
Summary Cloned DNA encoding polyketide synthase (PKS) genes from one Streptomyces species was previously shown to serve as a useful hybridisation probe for the isolation of other PKS gene clusters from the same or different species. In this work, the actI and actIII genes, encoding components of the actinorhodin PKS of Streptomyces coelicolor, were used to identify and clone a region of homologous DNA from the monensin-producing organism S. cinnamonensis. A 4799 by fragment containing the S. cinnamonensis act-homologous DNA was sequenced. Five open reading frames (ORFs 1–5) were identified on one strand of this DNA. The five ORFs show high sequence similarities to ORFs that were previously identified in the granaticin, actinorhodin, tetracenomycin and whiE PKS gene clusters. This allowed the assignment of the following putative functions to these five ORFS : a heterodimeric -ketoacyl synthase (ORF1 and ORF2), an acyl carrier protein (ORF3), a -ketoacyl reductase (ORF5), and a bifunctional cyclase/dehydrase (ORF4). The ORFs are encoded in the order ORFl-ORF2-ORF3-ORF5-ORF4, and ORFs-1 and -2 show evidence for translational coupling. This act-homologous region therefore appears to encode a PKS gene cluster. A gene disruption experiment using the vector pGM 160, and other evidence, suggests that this cluster is not essential for monensin biosynthesis but rather is involved in the biosynthesis of a cryptic aromatic polyketide in S. cinnamonensis. An efficient plasmid transformation system for S. cinnamonensis has been established, using the multicopy plasmids pWOR120 and pWOR125. 相似文献
94.
In a large experiment, using nearly 200 population cages, we have measured the fitness of Drosophila melanogaster homozygous (1) for the second chromosome, (2) for the third chromosome, and (3) for both chromosomes. Twentyfour second chromosomes and 24 third chromosomes sampled from a natural population were tested. The mean fitness of the homozygous flies is 0.081 ± 0.014 for the second chromosome, 0.080 ± 0.017 for the third chromosome, and 0.079 ± 0.024 for both chromosomes simultaneously. Assuming that fitnesses are multiplicative (the additive fitness model makes no sense in the present case because of the large selection coefficients involved), the expected mean fitness of the homozygotes for both chromosomes is 0.0066; their observed fitness is more than ten times greater. Thus, it appears that synergistic interactions between loci are considerable; and that, consequently, the fitness function substantially departs from linearity. Two models are tentatively suggested for the fitness function: a "threshold" model and a "synergistic" model.—The experiments reported here confirm previous results showing that the concealed genetic load present in natural populations of Drosophila is sufficient to account for the selective maintenance of numerous polymorphisms (of the order of 1000). 相似文献
95.
Louis Piovetti Christian Francisco Ginette Pauly Otmane Benchabane Colette Bernard-Dagan Anne Diara 《Phytochemistry》1981,20(6):1299-1302
Analysis of wood essential oil of Cupressus dupreziana revealed 26 components: 13 monoterpenes and 13 sesquiterpenes. The main components were carv 相似文献
96.
JoséLuis Avila Antonio Bretaña María Argelia Casanova Angela Avila Francisco Rodríguez 《Experimental parasitology》1979,48(1):27-35
A liquid medium was developed for the continuous cultivation of Trypanosoma cruzi. Among the several highly purified macromolecules tested only bovine liver catalase, horseradish peroxidase, lactoperoxidase, and bovine hemoglobin supported the continuous growth, at high yield, of mice-virulent Trypanosoma cruzi; other hemoproteins were inactive. Bovine liver catalase showed optimal Trypanosoma cruzi growth-promoting activity, parasites reaching 20 × 106 parasites/ml (95% epimastigotes) at about 10 days in most of the 45 subpassages to date. Furthermore, this protein in the incubation medium provided all the amino acid requirements of actively growing parasites, thus eliminating the need for exogeneous free amino acids. Additional experiments revealed that the hemoprotein's growth-promoting activity was independent of any enzymatic activity and that reconstituting the exact protein composition by means of exogeneous amino acids did not support parasite multiplication, suggesting the importance of the primary structure of the active proteins for growth-promoting activity. These active macromolecules supported the multiplication of five different strains of Trypanosoma cruzi, but did not support Leishmania brasiliensis or Leishmania mexicana proliferation, suggesting species specificity. 相似文献
97.
98.
99.
Fitness of Allozyme Variants in DROSOPHILA PSEUDOOBSCURA. I. Selection at the PGM-1 and ME-2 Loci 总被引:4,自引:3,他引:1
下载免费PDF全文
![点击此处可从《Genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We have studied in Drosophila pseudoobscura the effect of allozyme variation on seven fitness components: female fecundity, egg hatchability, egg-to-adult survival under near-optimal and under competitive conditions, rate of development under near-optimal and under competitive conditions, and mating capacity of males. Three genotypes at each of two loci, Pgm-1 and Me-2, have been studied in various combinations. These two loci are highly polymorphic in natural populations of D. pseudoobscura. Statistically significant differences involving one or more genotypes exist for all components of fitness. No single genotype is best for all fitness components; rather the relative fitnesses of genotypes are reversed when different parameters are considered, or when they are studied in different environmental conditions. Also, the average egg-to-adult survival and rate of development are better when different genotypes are reared together than when they occur in pure culture. Four different modes of selection have been uncovered by our experiments. These forms of selection may account for the persistence of the two allozyme polymorphisms in nature, and for previously observed seasonal fluctuations of the allelic frequencies in natural populations. 相似文献
100.