全文获取类型
收费全文 | 3920篇 |
免费 | 335篇 |
国内免费 | 1篇 |
专业分类
4256篇 |
出版年
2021年 | 39篇 |
2020年 | 21篇 |
2019年 | 30篇 |
2018年 | 34篇 |
2017年 | 46篇 |
2016年 | 70篇 |
2015年 | 117篇 |
2014年 | 148篇 |
2013年 | 187篇 |
2012年 | 203篇 |
2011年 | 194篇 |
2010年 | 150篇 |
2009年 | 148篇 |
2008年 | 210篇 |
2007年 | 227篇 |
2006年 | 208篇 |
2005年 | 213篇 |
2004年 | 228篇 |
2003年 | 221篇 |
2002年 | 240篇 |
2001年 | 57篇 |
2000年 | 43篇 |
1999年 | 50篇 |
1998年 | 81篇 |
1997年 | 58篇 |
1996年 | 47篇 |
1995年 | 67篇 |
1994年 | 50篇 |
1993年 | 52篇 |
1992年 | 49篇 |
1991年 | 47篇 |
1990年 | 31篇 |
1989年 | 47篇 |
1988年 | 36篇 |
1987年 | 36篇 |
1986年 | 23篇 |
1985年 | 47篇 |
1984年 | 46篇 |
1983年 | 28篇 |
1982年 | 39篇 |
1981年 | 35篇 |
1979年 | 19篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 33篇 |
1975年 | 28篇 |
1974年 | 23篇 |
1973年 | 20篇 |
1971年 | 22篇 |
1968年 | 20篇 |
排序方式: 共有4256条查询结果,搜索用时 15 毫秒
31.
32.
Dictyosiphon foeniculaceus from Sweden and Newfoundland was studied in laboratory culture. Zoids from unilocular sporangia developed into dioecious microscopic filamentous gametophytes which produced uniseriate plurilocular gametangia in low temperatures (0 to 8 °C). Zygotes and unfused isogametes gave rise to filamentous protonemata on which parenchymatous macroscopic sporophytes were formed. Isolates from Sweden and Newfoundland were interfertile. Although formed in culture, genetically unisexual sporophytes were not detected in nature. Female gametes ofD. foeniculaceus produced a sexual pheromone. It was identified as finavarrene, which is also known as the sperm attractant inAscophyllum nodosum. 相似文献
33.
Dieter C. Wasshausen 《Brittonia》1984,36(1):1-7
34.
Electrorotation—the spin of cells in rotating high frequency electric fields—has been used to investigate properties of human peripheral blood lymphocytes. The rotation spectra of lymphocytes deviate from those of single shell spheres. The deviations are caused by the electrical properties of the nucleus in the cell interior.Electrorotation allows the distinction between successfully stimulated lymphocytes and unstimulated cells after application of concanavalin A. Notwithstanding the fact that only a proportion of the cells will be mitogenically stimulated we detected an enhanced cell membrane conductivity for the whole cell population immediately after the addition of mitogen. 相似文献
35.
Heinrich Reichert Wulf Dieter Krenz 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1986,158(5):625-637
Summary A simple method for the in vivo visualization of dye filled cells by laser illumination is used to characterize neurons in situ in the segmentai ganglia of the locust and the crayfish (Fig. 1). Neuron visualization provides the structural information necessary for identification of cells during an ongoing physiological experiment (Figs. 2, 3). Sequential penetrations of soma and neuropil as well as simultaneous double neuropil penetrations of spiking and nonspiking cells are facilitated by the visual control afforded by neuron visualization (Figs. 4, 5, 6). Furthermore, neuron visualization allows the sampling of cellular properties at multiple, predetermined sites in the dendritic and axonal arbors of identified neurons (Fig. 7) and aids in establishing synaptic connectivity through double neuropil recordings (Fig. 8). 相似文献
36.
The kinetic properties of active Ca2+ transport into mitochondria and microsomal membrane vesicles prepared from coleoptiles of dark-and light-grown corn seedlings have been studied. The apparent values for K m and V max for Ca2+ of the mitochondrial transport system from dark-grown plants are about one order of magnitude higher than those from the microsomal transport system. Calmodulin has no effect on the Ca2+ accumulation into mitochondria whereas the apparent maximum transport velocity and affinity for Ca2+ of the microsomal Ca2+-transport system are both increased by calmodulin. When intact corn seedlings are irradiated with far-red light, the calmodulin-induced increase of the apparent maximum transport velocity and affinity for Ca2+ can no longer be observed. From these data it can be concluded that the low cytoplasmic Ca2+ concentration in the cytoplasm of coleoptile cells from dark-grown corn is maintained by a calmodulin-regulated Ca2+ pump. Irradiation with photomorphogenically active far-red light lowers the Ca2+-transport activity and thus causes an increase of the cytoplasmic, free-Ca2+ concentration. The physiological implications will be discussed. 相似文献
37.
38.
Genetic analysis of the high responder-non-(or low) responder differences between WF and ACI rats to the synthetic copolymers GT and GA established singleIr-genes for both antigens and dominance for high responder status.Ir-GT andIr-GA are linked to the major histocompatibility locus. It could be demonstrated that only T cells carrying the high responderIr-GT gene undergo in vitro blast transformation to GT. The advantages of the rat systems for further studies of the regulatory role ofIr-genes on the cellular level are discussed.Abbreviations cpm
counts per minute
- DHR
delayed hypersensitivity reaction
- GA
random synthetic copolymer of L-glutamic acid50, L-alanine50, M.W. 45,000
- GT
random synthetic copolymer of L-glutamic acid50, L-tyrosine50, M.W. 22,000
-
Ir gene
immune response gene
- MLC
mixed lymphocyte cultures
- LDH
lactic dehydrogenase
- (T,G)-A-L
branched chain synthetic polymer of poly-L (tyrosine, glutamic acid)-poly-D, L-alanine-poly-L-lysine
Rat Strains ACI
ACI/MaI
- WF
Wistar Furth
- AUG
August 28807/Cr 相似文献
39.
Enzymatic methylations: III. Cadaverine-induced conformational changes of E.coli tRNAfMet as evidenced by the availability of a specific adenosine and a specific cytidine residue for methylation+ 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A partially purified tRNA methylase fraction from rat liver, containing m(2)G- m(1)A- and m(5)C-methylase, was used to study the influence of Mg(++) and of the biogenic polyamine cadaverine on the enzymatic methylation of E.coli tRNA(fMet)in vitro. In presence of 1 or 10 mM Mg(++), guanosine no. 27 was methylated to m(2)G. In 1 mM Mg(++) plus 30 mM cadaverine, guanosine in position 27 and adenosine in position 59 were methylated. In presence of 30 mM cadaverine alone tRNA(fMet) accepted three methyl groups: in addition to guanosine no. 27 and adenosine no. 59 cytidine no. 49 was methylated. In order to correlate tRNA(fMet) tertiary structure changes with the methylation patterns, differentiated melting curves of tRNA(fMet) were measured under the methylation conditions. It was shown that the thermodynamic stability of tRNA(fMet) tertiary structure is different in presence of Mg(++), or Mg(++) plus cadaverine, or cadaverine alone. From the differentiated melting curves and from the methylation experiments one can conclude that at 37 degrees in the presence of Mg(++) tRNA(fMet) has a compact structure with the extra loop and the TpsiC-loop protected by tertiary structure interactions. In Mg(++) plus cadaverine, the TpsiC-loop is available, while the extra loop is yet engaged in teritary structure (G-15: C-49) interactions. In cadaverine alone, the TpsiC-loop and the extra loop are free; hence under these conditions the open tRNA(fMet) clover leaf may be the substrate for methylation. In general, cadaverine destabilizes tRNA tertiary structure in the presence of Mg(++), and stabilizes tRNA(fMet) tertiary structure in the absence of Mg(++). This may be explained by a competition of cadaverine with Mg(++) for specific binding sites on the tRNA. On the basis of these experiments a possible role of biogenic polyamines in vivo may be discussed: as essential components of procaryotic and eucaryotic ribosomes they may together with ribosomal factors facilitate tRNA-ribosome binding during protein biosynthesis by opening the tRNA tertiary structure, thus making the tRNA's TpsiC-loop available for interaction with the complementary sequence of the ribosomal 5S RNA. 相似文献
40.