全文获取类型
收费全文 | 590篇 |
免费 | 72篇 |
国内免费 | 32篇 |
出版年
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 7篇 |
2016年 | 10篇 |
2015年 | 19篇 |
2014年 | 19篇 |
2013年 | 26篇 |
2012年 | 34篇 |
2011年 | 40篇 |
2010年 | 28篇 |
2009年 | 28篇 |
2008年 | 26篇 |
2007年 | 26篇 |
2006年 | 35篇 |
2005年 | 27篇 |
2004年 | 19篇 |
2003年 | 12篇 |
2002年 | 14篇 |
2001年 | 17篇 |
2000年 | 11篇 |
1999年 | 11篇 |
1998年 | 15篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 11篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 10篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1973年 | 6篇 |
1970年 | 4篇 |
1967年 | 3篇 |
1964年 | 3篇 |
1958年 | 5篇 |
1955年 | 3篇 |
1953年 | 3篇 |
1949年 | 6篇 |
1948年 | 4篇 |
排序方式: 共有694条查询结果,搜索用时 15 毫秒
1.
Payant V; Abukashawa S; Sasseville M; Benkel BF; Hickey DA; David J 《Molecular biology and evolution》1988,5(5):560-567
Nuclear DNA was extracted from each of the eight species comprising the
Drosophila melanogaster species subgroup. Southern hybridization of this
DNA by using a molecular probe specific for the alpha-amylase coding region
showed that the duplicated structure of the amylase locus, first found in
D. melanogaster, is conserved among all species of the melanogaster
subgroup. Evidence is also presented for the concerted evolution of the
duplicated genes within each species. In addition, it is shown that the
glucose repression of amylase gene expression, which has been extensively
studied in D. melanogaster, is not confined to this species but occurs in
all eight members of the species subgroup. Thus, both the duplicated gene
structure and the glucose repression of Drosophila amylase gene activity
are stable over extended periods of evolutionary time.
相似文献
2.
Results from three experiments on basic learning and transfer in rhesus monkeys (Macaca mulatta) are reported in which fully automated testing paradigms, afforded by the Language Research Center's Computerized Test System
(LRC-CTS), were employed. Performance levels for discrimination learning set, transfer index, and mediational-learning testing
were uniformly higher than was predicted from the literature, in contrast to previous reports of compromised learning under
similar conditions (automated apparatus, planimetric stimuli, spatial discontiguity between stimuli and response loci). Analyses
reveal relatively advanced learning set performance, transfer-index ratios, and positive transfer of learning even at stringent
criterion levels. Moreover, the data suggest that rhesus monkeys tested in these experiments exhibit mediational instead of
associative learning strategies, as do great apes and in contrast to previous reports of rhesus learning. We argue that the
LRC-CTS enhances learning by nonhuman primate subjects, obviating those factors, reported in the literature from experiments
in which manual or other automated systems were employed, that compromise learning. 相似文献
3.
K. Bondari K. W. Washburn G. O. Ware 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,71(2):153-158
Summary Two experiments were conducted simultaneously to determine (1) if fast-growing fingerlings of channel catfish, Ictalurus punctatus, could be identified by simple visual selection of body size and (2) if initial size advantages influenced subsequent growth and carcass traits of divergently selected channel catfish. Exp. 1 included large (L), medium (M), and small (S) fingerling sizes from each of the control (C), selected upward (+) and selected downward (–) lines for body weight. Exp. 2 included all fmgerlings of the same size (25±5 g) from the 3 lines. Catfish from the L size-class, within each full-sib family in each line, were consistently heavier and longer than M and S size-classes throughout the 53-week experimental period. Fingerlings from the M size-class were also superior in growth to those from the S size-class. Catfish from the + line exceeded those from the C and –lines in body weight and total length at the conclusion of Exp. 1 but not in Exp. 2. This was attributed to the selection of equal size fmgerlings in Exp. 2 which may have excluded fingerlings with the best growth potential from the + body weight line. Results of the two experiments combined indicated that one generation of divergent selection has created genetic differences among lines of channel catfish.Supported by State and Hatch funds allocated to the Georgia Agricultural Experiment Station 相似文献
4.
5.
JOÃO BATISTA TAVARES DA SILVA ISAAC ROITMAN 《The Journal of eukaryotic microbiology》1990,37(6):521-523
ABSTRACT. Three strains of Phytomonas serpens two from tomatoes, Lycopersicon esculentum one from the insect Phtia picta (Hemiptera, Coreidae), were cultivated in a chemically defined medium developed from a defined medium for cultivating insect flagellates. Besides organic growth factors required by other insect trypanosomatids this flagellate requires, serine and inositol. Glutamine stimulates growth, and, surprisingly, does not require heme. 相似文献
6.
Borrelia burgdorferi is a spirochete pathogen transmitted among warm-
blooded hosts by ixodid ticks. Frequency-dependent selection for variant
outer-surface proteins might be expected to arise in this species, since
rare variants are more likely to avoid immune surveillance in previously
infected hosts. We sequenced the OspA and OspB genes of nine North American
strains and compared them with nine strains previously described. For each
gene, the mean number of synonymous substitutions per synonymous site and
the mean number of nonsynonymous substitutions per nonsynonymous site show
only a twofold excess of silent mutations. Synonymous rates vary widely
along the OspB protein. Some regions show a significant excess of silent
substitutions, while divergence in other regions is constrained by biased
base composition or selection. The presence, in antigenically important
regions of the protein, of significant variation among strains, as well as
evidence for recombination among strains, should be considered in attempts
to develop vaccines against this disease.
相似文献
7.
Characterization of DNA helicase II from a uvrD252 mutant of Escherichia coli. 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The loss of DNA helicase II (UvrD) in Escherichia coli results in sensitivity to UV light and increased levels of spontaneous mutagenesis. While the effects of various uvrD alleles have been analyzed in vivo, the proteins produced by these alleles have not been examined in any detail. We have cloned one of these alleles, uvrD252, and determined the site of the mutation conferring the phenotype. In addition, the protein it encodes has been purified to homogeneity and characterized in vitro. The mutation responsible for the phenotype was identified as a glycine-to-aspartic-acid change in the putative ATP-binding domain. In comparison to wild-type DNA helicase II, the UvrD252 enzyme exhibited reduced levels of ATPase activity and a large increase in the Km for ATP. The ability of UvrD252 to unwind DNA containing single-stranded regions, as well as DNA containing only nicks, was reduced in comparison to that of the wild-type enzyme. Possible interpretations of these results in relation to the phenotypes of the uvrD252 mutant are discussed. This represents the first detailed analysis of the biochemical properties of a mutant DNA helicase II protein. 相似文献
8.
M P ROBINSON G BUTCHER R H CURTIS K G DA VIES K EVANS 《The Annals of applied biology》1993,123(2):337-347
Two monoclonal antibodies, which differentially recognise the two species of potato cyst nematodes (PCN), Globodera pallida and G. rostochiensis, are described. They have been shown to have potential for quantification of these two species, recognising proteins of the same molecular weight (34 kD) in both species. Further investigation showed these proteins to have isoelectric points at pH values of 5.7 in G. pallida and 5.9 in G. rostochiensis, in common with the proteins used by Fleming & Marks (1983) to differentiate the species of PCN. They are likely to be structurally very similar, with the same physiological function (and therefore similar concentrations) in the two species. In cross-reactivity tests with a wide range of soil nematode species, the antibodies reacted strongly only with species of the genus Globodera, and thereby confirmed their potential as the basis of a quantitative immunoassay likely to be useful in management of PCN populations. 相似文献
9.
10.