全文获取类型
收费全文 | 4472篇 |
免费 | 437篇 |
出版年
2023年 | 22篇 |
2022年 | 27篇 |
2021年 | 63篇 |
2020年 | 47篇 |
2019年 | 52篇 |
2018年 | 56篇 |
2017年 | 60篇 |
2016年 | 105篇 |
2015年 | 194篇 |
2014年 | 219篇 |
2013年 | 212篇 |
2012年 | 315篇 |
2011年 | 306篇 |
2010年 | 232篇 |
2009年 | 179篇 |
2008年 | 267篇 |
2007年 | 232篇 |
2006年 | 237篇 |
2005年 | 218篇 |
2004年 | 220篇 |
2003年 | 208篇 |
2002年 | 229篇 |
2001年 | 43篇 |
2000年 | 33篇 |
1999年 | 64篇 |
1998年 | 63篇 |
1997年 | 57篇 |
1996年 | 48篇 |
1995年 | 43篇 |
1994年 | 42篇 |
1993年 | 58篇 |
1992年 | 41篇 |
1991年 | 51篇 |
1990年 | 33篇 |
1989年 | 73篇 |
1988年 | 47篇 |
1987年 | 48篇 |
1986年 | 37篇 |
1985年 | 40篇 |
1984年 | 38篇 |
1983年 | 30篇 |
1982年 | 31篇 |
1981年 | 28篇 |
1980年 | 28篇 |
1979年 | 20篇 |
1978年 | 28篇 |
1977年 | 19篇 |
1976年 | 17篇 |
1975年 | 17篇 |
1974年 | 22篇 |
排序方式: 共有4909条查询结果,搜索用时 15 毫秒
41.
Plasmid-borne Tn5 insertion mutation resulting in accumulation of gentisate from salicylate. 总被引:3,自引:2,他引:1
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Plasmid-borne Tn5 insertion mutants of a Pseudomonas species which accumulated 2,5-dihydroxybenzoate (gentisate) following growth on 2-hydroxybenzoate (salicylate) were obtained from a pool of mutants that were unable to grow on naphthalene. One such mutant was characterized further. The ability of this mutant to oxidize gentisate was 100-fold less than the ability of a Nah+ Sal+ strain harboring the unmutagenized plasmid, although both strains oxidized and grew on salicylate. These bacteria were presumably able to metabolize salicylate via catechol, since they possessed an inducible, plasmid-encoded catechol 2,3-dioxygenase. Our results suggest that there is an alternate, plasmid-encoded route of salicylate degradation via gentisate and that some plasmid-associated relationship between this pathway and naphthalene oxidation exists. 相似文献
42.
Ann Depicker Lieve Herman Anni Jacobs Jozef Schell Marc Van Montagu 《Molecular & general genetics : MGG》1985,201(3):477-484
Summary We investigated whether the efficiency of transformation of plant cells by Agrobacterium tumefaciens during cocultivation is limited by the properties of the plant cells or by the infecting bacteria.Therefore, tobacco protoplasts were infected by cocultivation with two different agrobacteria strains carrying Ti plasmids with distinguishable T-DNAs. These T-DNAs cotransform plant cells at a frequency equal to the product of their independent transformation frequencies, which indicates that all plant cells are equally competent. On the other hand, when these T-DNAs are located on the same Ti plasmid vector within one bacterial strain, the cotransformation frequency is significantly higher than the product of the single transformation frequencies. We interpret these results to indicate that transformation is limited more by the establishment of effective bacteria/plant cell interaction than by (i) the process of DNA integration and (ii) by the number of plant cells capable of being transformed by Agrobacterium. We found that most plant cells are transformed by only one or a few agrobacteria. Analysis of the number of T-DNA copies in these clonally transformed lines indicates amplification of the original, infecting T-region copy. 相似文献
43.
The drug resistance plasmid pKM101 makes Escherichia coli resistant to the lethal effects of ultraviolet (UV) irradiation and more susceptible to mutagenesis by a variety of agents. The plasmid operon responsible for increasing mutagenesis has been termed mucAB (Mutagenesis, UV and chemical). We have isolated a derivative of pKM101 called pGW1975 which makes cells more sensitive to killing by UV but which retains the ability of pKM101 to increase susceptibility to methyl methanesulfonate (MMS) mutagenesis. pGW1975 increases UV mutagenesis less than pKM101 in a uvrA+ strain but more than pKM101 in a uvrA− strain. muc− point and insertion mutants of pKM101 and pGW1975 complement to restore the plasmid-mediated: (i) ability to reactivate UV-irradiated phage, (ii) resistance to killing by UV, and (iii) level of susceptibility to UV mutagenesis. We have identified a 2.0 kb region of pKM101 which is responsible for the complementation and which maps counterclockwise of mucAB. 相似文献
44.
Phaeoannellomyces McGinnis et Schell gen. nov., which is based upon P. elegans McGinnis et Schell sp. nov., is proposed for dematiaceous yeasts which produce percurrently proliferating conidiogenous cells. Cladosporium werneckii Horta is transferred to Phaeoannellomyces as P. werneckii (Horta) McGinnis et Schell comb. nov. because the most stable and distinctive synanamorph produced by this fungus consists of annellidic yeast cells. The Phaeococcomycetaceae McGinnis et Schell fam. nov. is proposed in the class Blastomycetes, division Fungi Imperfecti for the dematiaceous yeast genera Phaeoannellomyces and Phaeococcomyces de Hoog. 相似文献
45.
Construction of a heat-inducible gene for plants. Demonstration of heat-inducible activity of the Drosophila hsp70 promoter in plants 总被引:8,自引:4,他引:4
下载免费PDF全文
![点击此处可从《The EMBO journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A chimeric gene containing the neomycin phosphotransferase II coding region under the control of the hsp70 promoter from Drosophila was integrated and found to be expressed, in a heat-regulated fashion, in tobacco tissue. 相似文献
46.
Douglas M. Carlson William L. Pflieger Linden Trial Pamela S. Haverland 《Environmental Biology of Fishes》1985,14(1):51-59
Synopsis
Scaphirhynchus albus and S. platorynchus were studied in Missouri during 1978–1979 to assess their distribution and abundance, to obtain information on their life
histories, and to identify existing or potential threats to their survival. S. platorynchus was collected in substantial numbers (4355 specimens) at all 12 sampling stations in the Missouri and Mississippi rivers,
while only 11 S. albus were captured from 6 stations. Twelve specimens identified in the field as hybrids between the two species were captured
from 4 stations. Morphometric and meristic comparisons of presumed hybrids with the parent species, using cluster and principal
components analyses, demonstrated intermediacy of most specimens identified in the field as hybrids. Aquatic insects comprised
most of the diet of S. platorynchus and S. albus, but S. albus and the hybrids had consumed considerable quantities of fish. S. albus grew more rapidly than S. platorynchus, while the growth of hybrids was intermediate. Hybridization appears to be a recent phenomenon, resulting from man-caused
changes in the big-river environment. Hybridization may be a threat to survival of S. albus in the study streams. 相似文献
47.
Pamela J. Weathers James F. Danielli Peter M. Bradley Diane M. Hebb Judith E. Miller Rick L. Pesano 《Physiologia plantarum》1984,61(3):441-448
We report the isolation of a cukaryotic green alga ( Chlorella , strain WPI-2) which accumulates large stores of nitrogen (N) during growth in N-free medium and seems to incorporate14 N2 , yet does not reduce acetylene to ethylene. Total N accumulation during growth on N-free medium and in gases free of combined N was measured by three methods: Kjeldahl, oxidative pyrolysis via chemiluminescence (Antek N analyzer), and Dumas (Coleman N analyzer). Increases in N ranging from 22–64%± 1% were observed. Isotope dilution studies using cells labelled with 15 NO 3 - and then shifted to 14 N2 in N-free medium showed dilution of the 15 N isotope by 14 N from 5.67 to 5.32%± 0.05%. Using a variety of conditions, we were unable to demonstrate the reduction of acctylene to ethylene by WPI-2, although diazotrophic cyanobacteria gave positive results. Although the data on WPI-2 are not conclusive in establishing this alga as a diazotroph, the data do suggest that within the Chlorophyceae there may exist a novel form of nitrogen gas metabolism. 相似文献
48.
Different hormonal requirements for androgen-independent growth of normal and tumor epithelial cells from rat prostate 总被引:4,自引:0,他引:4
Wallace L. McKeehan Pamela S. Adams Danna Fast 《In vitro cellular & developmental biology. Plant》1987,23(2):147-152
Summary The proliferation of isolated normal prostate epithelial cells from rat and man is androgen-independent and requires cholera
toxin, insulin, dexamethasone, epidermal growth factor (EGF) and one or more polypeptide factors that are concentrated in
bovine neural tissue. The active agents in the neural tissue extract are heparin-binding polypeptides (prostatropins), the
predominant form of which has a molecular weight of 17400 and an acetylalanine at the aminoterminus. Prostatropins supported
a half-maximal increase in normal prostate epithelial cell number at 50 picomolar. The proliferation of primary and serially-cultured
epithelial cells from androgen-responsive Dunning R3327 rat prostate tumors was also androgen-independent, but exhibited dramatic
alterations in response to hormones that stimulated normal cell proliferation. At low cell density, androgen-independent growth
of isolated tumor-derived epithelial cells was independent on cholera toxin, was stimulated by dexamethasone, required insulin
andeither EGFor prostatropin. The presence of either EGF or prostatropin masked the response to the other factor. In the absence of EGF,
purified prostatropins supported a half-maximal increase in tumor cell number at 7 picomolar. Endogenous production of EGF-like
and prostatropin-like factors or both was suggested by the reduced requirement for EGF and prostatropin at high prostate tumor
cell density. These results suggest that anti-hormonal therapies against prostate tumor growth should be based on intervention
with the activity of insulin (or insulin-like factors) or simultaneous intervention with both EGF and prostatropin (or their
homologues).
This work was supported by NIH grants CA 37589 and HL 33847, and grant 1718 from the Council for Tobacco Research.
Editor’s Statement This paper is the first report of the comparison of the hormone requirements of primary cultures of normal
and tumor prostate epithelial cells from the same system. 相似文献
49.
Pamela M. Reilly Dr. Palmer Rogers 《Journal of industrial microbiology & biotechnology》1987,1(5):329-334
Summary Conditions that allow regeneration of cells fromClostridium acetobutylicum strain B643 protoplasts were studied. Protoplast formation and stabilization in minimal media with 50 mM CaCl2, 50 mM MgCl2 and 0.3 M sucrose were crucial to subsequent regeneration on soft yeast extract agar containing 25 mM CaCl2 and 25 mM MgCl2. A regeneration frequency of 8–25% was consistently obtained. 相似文献
50.
K. Pawlowski P. Ratet J. Schell F. J. de Bruijn 《Molecular genetics and genomics : MGG》1987,206(2):207-219