全文获取类型
收费全文 | 6239篇 |
免费 | 600篇 |
出版年
2022年 | 44篇 |
2021年 | 102篇 |
2020年 | 48篇 |
2019年 | 62篇 |
2018年 | 83篇 |
2017年 | 69篇 |
2016年 | 148篇 |
2015年 | 269篇 |
2014年 | 291篇 |
2013年 | 365篇 |
2012年 | 429篇 |
2011年 | 374篇 |
2010年 | 265篇 |
2009年 | 220篇 |
2008年 | 323篇 |
2007年 | 308篇 |
2006年 | 309篇 |
2005年 | 302篇 |
2004年 | 297篇 |
2003年 | 269篇 |
2002年 | 252篇 |
2001年 | 101篇 |
2000年 | 82篇 |
1999年 | 97篇 |
1998年 | 88篇 |
1997年 | 77篇 |
1996年 | 72篇 |
1995年 | 71篇 |
1994年 | 69篇 |
1993年 | 61篇 |
1992年 | 76篇 |
1991年 | 60篇 |
1990年 | 64篇 |
1989年 | 60篇 |
1988年 | 62篇 |
1987年 | 56篇 |
1986年 | 43篇 |
1985年 | 52篇 |
1984年 | 57篇 |
1983年 | 43篇 |
1982年 | 32篇 |
1981年 | 38篇 |
1980年 | 34篇 |
1979年 | 44篇 |
1978年 | 31篇 |
1977年 | 41篇 |
1976年 | 46篇 |
1975年 | 34篇 |
1973年 | 30篇 |
1971年 | 28篇 |
排序方式: 共有6839条查询结果,搜索用时 31 毫秒
41.
A coiling-inducing factor was isolated from tendrils of Bryonia dioica Jacq. and identified by infrared, 1H-, 13C-nuclear magnetic resonance and mass spectrometry as -linolenic acid. When applied to detached tendrils, exogenous -linolenic acid, but not linoleic acid or oleic acid, induced tendril coiling. Further investigations showed that metabolites of -linolenic acid, jasmonic acid and, even more so, methyljasmonate, are highly effective inducers of tendril coiling in B. dioica. Methyljasmonate was most active when administered by air and, in atmospheric concentrations as low as 40–80 nM, induced a full free-coiling response with kinetics similar to mechanical stimulation. Even atmospheric levels as low as 4–5 nM methyljasmonate were still found to be significantly active. Methyljasmonate could be one of the endogenous chemical signals produced in mechanically stimulated parts of a tendril and, being highly volatile, act as a diffusible gaseous mediator spreading through the intracellular spaces to trigger free coiling of tendrils.Abbreviations EI-MS
electron impact-mass spectrometry
- HPLC
high-performance liquid chromatography
- IAA
indole-3-acetic acid
- NMR
nuclear magnetic resonance
- TFA
trifluoroacetic acid
We are indebted to the Deutsche Forschungsgemeinschaft, Bonn and the Fonds der Chemischen Industrie, Frankfurt (literature provision) for their support and to Dr. C. Brückner, Halle, for jasmonic-acid determinations. 相似文献
42.
Marie-Geneviève Mattei Agnès Moreau Marie-Claude Gesnel Elisabeth Houssaint Richard Breathnach 《Human genetics》1991,87(1):84-86
Summary A 2.3-kb cDNA probe for the human bek fibroblast growth factor receptor was used to determine the chromosomal localization of the corresponding gene by in situ hybridization. The results show that this gene, a form of which is amplified in some poorly differentiated stomach cancers, is localized on chromosome region 10q26. The two previously identified fibroblast growth factor receptor genes are thus not on the same chromosome, as the related fig (fms-like gene) fibrovblast growth factor receptor gene has previously been mapped to human chromosome region 8p12. 相似文献
43.
Simian virus 40 large T antigen DNA helicase. Characterization of the ATPase-dependent DNA unwinding activity and its substrate requirements 总被引:31,自引:0,他引:31
The ATPase of SV40 large T antigen (T antigen) which is essential for the replication of SV40 minichromosomes was recently shown to be functionally related to a newly discovered DNA helicase activity. The T antigen helicase unwinds DNA duplices of several kilobase pairs in a reaction depending on the presence of hydrolyzable ribo- or deoxyribonucleoside triphosphates. The in vitro rate of movement through duplex DNA was found to be about 100 base pairs/min at 37 degrees C. For DNA unwinding, T antigen requires a 3'-single strand extension of a partially double-stranded substrate and invades the double strand section processively, in a 3' to 5' direction. The minimum length of the single-stranded tail was determined to be less than 5 nucleotides. Unwinding studies in the presence of the Escherichia coli single strand-specific DNA-binding protein and competition experiments indicate that T antigen helicase binds preferentially at the single-stranded/double-stranded DNA junction. This DNA structure is therefore proposed to serve as an entry site for the T antigen helicase. Previously reported data suggest that T antigen is the replicative helicase of the SV40 minichromosome. The results presented here are consistent with these findings and imply that T antigen migrates actively and processively along the template for the leading strand. 相似文献
44.
Cathepsin D is membrane-associated in macrophage endosomes 总被引:27,自引:0,他引:27
Previously we identified an acid protease activity which was located in the endosomes of rabbit alveolar macrophages (Diment, S., and Stahl, P.D. (1985) J. Biol. Chem. 260, 15311-15317). In this study, the endosomal protease is identified as cathepsin D by immunoprecipitation with polyclonal antibodies raised against rabbit cathepsin D and by NH2-terminal sequence. In order to elucidate the mechanism for targeting of cathepsin D to endosomes, we first examined the membrane association of cathepsin D with light (rho = 1.05 g/ml) and heavy density (rho = 1.1 g/ml) vesicles from Percoll density gradients. After sequential washes, 8.4 and 21.9% of cathepsin D activity remained associated with heavy and light density vesicles, respectively. This membrane-associated cathepsin D could not be solubilized in either buffer at pH 5.0 containing mannose 6-phosphate and EDTA or in buffer at pH 10.6. Solubilization required the detergent Triton X-100. To determine whether membrane-associated cathepsin D was found in endosomes, the enzyme was radioiodinated within endosomes and lysosomes with internalized lactoperoxidase. The membrane-associated form was detected in endosomes, but much less in lysosomes. Biosynthetic studies combined with the same extraction procedure revealed that macrophage cathepsin D is first synthesized as an inactive membrane-associated precursor. The precursor is processed to an active, membrane-associated form and then to the active soluble form found in lysosomes. Our studies provide evidence that 1) cathepsin D is in endosomes of macrophages; 2) cathepsin D is transported to endosomes as a membrane-associated form; and 3) the membrane-associated form is a biosynthetic precursor for the soluble form found in endosomes and lysosomes. 相似文献
45.
Lepidoptera males bear two kinds of meiotic divisions. One is regular (eupyrene) and leads to nucleate, fertilizing spermatozoa. The other (apyrene) shows metaphase I chromosomes clumping together into irregular masses which later split forming daughter cells with unbalanced sets of chromosomes which are eventually extruded from the cells; hence, the spermatids develop into anucleate spermatozoa of unknown function. The apyrene divisions are induced by a haemolymph factor which becomes functional towards pupation. Using incorporation of tritiated thymidine at the premeiotic S-phase as a marker for timing, it was found that the prophase of the apyrene spermatocyte is shorter than that of the eupyrene spermatocyte. It is proposed that meiosis-specific proteins cannot be synthesized during the shortened apyrene prophase and that this is correlated with the irregular chromosome behaviour during the subsequent metaphase-telophase of these spermatocytes. 相似文献
46.
Monique Breton Eliane Berrou M. -Christine Brahimi-Horn Elisabeth Deudon Jacques Picard 《Experimental cell research》1986,166(2)
Cultured smooth muscle cells from pig aorta arrested in G0 phase by serum deprivation were stimulated to proliferate by replacing the medium with one containing 10% serum. Studies in DNA replication and proliferation of cells showed a relatively good synchrony: 90% of the cells were in G1 phase for 16 h after addition of serum; they entered S phase between 18 and 24 h, completed S phase and traversed G2 phase between 24 and 30–32 h; 75% of these cells multiplied after 30–32 h and the remainder were blocked at the end of G2 phase. The synthesis and secretion of sulfated proteoglycans were examined throughout a full cell cycle using metabolic labelling with [35S]sulfate. Smooth muscle cells in G1 or G2 phase synthesized and secreted sulfated proteoglycans with a possible pause at the end of the G2 phase but at the beginning of the S phase and during mitosis the incorporation of [35S]sulfate into these macromolecules stopped entirely. Structural characteristics of sulfated proteoglycans secreted into the medium during G1 phase and an entire cell cycle were investigated. The proportion of proteoglycan complexes and the relative hydrodynamic size of monomers and of constituent subunits of complexes were determined after chromatography on Sepharose CL-2B and CL-6B columns run under both associative and dissociative conditions. No significant differences were observed for the periods of the cell cycle that were studied:
- 1. 1. [35S]Proteoglycan complexes represented at the end of G1 phase and of the cell cycle respectively 19 and 16% of the total [35S]proteoglycans secreted into the medium.
- 2. 2. More than 90% of the subunits, obtained after dissociation of complexes, were characterized by a similar kav after chromatography on Sepharose CL-2B columns eluted under dissociative conditions (kav 0.68 at the end of G1 phase and 0.65 at the end of full cell cycle).
- 3. 3. About 95% of monomers synthesized at the two stages of the cell cycle were eluted at kav 0.25 after chromatography on Sepharose CL-6B column run under associative conditions and were characterized by a similar glycosaminoglycan distribution. These results suggest that smooth muscle cells in culture liberate similar populations of proteoglycans into the medium during the G1 and G2 phases.
47.
Enhancement of Oxygen-Evolution in Photosystem II-Phycobilisome Particles from Porphyridium cruentum
Oxygen-evolving photosystem II-phycobilisome particles froma red alga were inhibited 5080% by aging, dilution, lowpH and salt-washing. Bovine serum albumin, and dithiothreitolwere found to stimulate activity in all but salt-washed particles.CaCl2 and MnCl2 partially restored activity lost after agingor dilution.
1Current address: Waksman Institute of Microbiology, RutgersUniversity, Piscataway, New Jersey 08854-0759, U.S.A. (Received October 5, 1985; Accepted March 31, 1986) 相似文献
48.
Synthesis and phosphorylation of cytoskeleton components in foetal,regenerating and adult normal rat hepatocytes during culture 总被引:1,自引:0,他引:1
Georges Baffet Annie Ruelland Bruno Clement Elisabeth Le Rumeur Siegmund Fischer 《Molecular and cellular biochemistry》1985,68(2):97-105
Summary Detergent insoluble material (DIM) was prepared by gentle treatment with detergent from foetal, regenerating and adult normal rat hepatocytes cultured for various times. It retained to some degree the morphology of the cells. After incubation of intact cells with 35S-methionine, most of the labelled DIM proteins were found to be components of the cytoskeleton. They included several cytokeratins, vimentin and actin. The synthesis rate varied with the age of animals and culture conditions. The high synthetic rate of vimentin in foetal and regenerating hepatocytes could be associated with cell proliferation. No correlation was found between cytokeratin synthesis and hepatocyte growth. Most of the cytoskeleton proteins could be phosphorylated in intact cells and in DIM from cultured hepatocytes. However the degree of phosphorylation of these proteins was not related to their synthetic rate. The decreased phosphorylation level in cultured adult rat hepatocytes could be related to the rapid loss of specific functions. 相似文献
49.
Lidia Sautebin Hans Kindahl Maria Kumlin Elisabeth Granstrm 《Prostaglandins & other lipid mediators》1985,30(3):435-456
Conjugates of prostaglandins and thromboxanes with tritium labeled amino acids were prepared and employed as labeled ligands in porstaglandin and thromboxane radioimmunoassays. Assays for PGF2α, 15-keto-13, 14-dihydro-PGF2α, TXB2 and 15-keto-13, 14-dihydro-TXB2 were evaluated in comparative studies using either these heterologous ligands or the corresponding homologus tritiated eicosanoid as tracers. Binding properties for the respective antibodies were found to be similar using either tracer.Three biological studies were also conducted, viz. study of the release of TXB2 during collagen induced platelet aggregation, of 15-keto-13, 14-dihydro-TXB2 during guinea pig pulmonary anaphylaxis, and of PGF2α (measured as 15-keto-13, 14-dihydro-PGF2α in peripheral plasma) during bovine luteolysis. The analyses gave comparable results using either the heterologous or the homologous assay.Thus, this type of labeled prostanoid conjugates may serve as a convenient alternative to homologous tracers in radioimmunoassay. Heterologous tracers may even in certain cases provide the only simple solution to the problem of preparing a labeled ligand of high specific activity. 相似文献
50.
Jean-Claude Wissocq Catherine Heurteaux Elisabeth Hennequin Michel Thellier 《Development genes and evolution》1985,194(7):433-435
Summary The quantitative imaging of lithium distribution, in histological sections of 15-days old mouse embryos (whose mother had been submitted to Li-treatment), was performed using6Li isotope as tracer,6Li(n,)3H nuclear reaction for detection, and dielectric track detectors. Despite the particular difficulties of cryosectioning the embryos without disturbing the lithium distribution, the Li regionalization appeared to be very clear-cut. The ectomesodermic tissues were significantly more loaded with lithium than the endodermic ones. This is probably related to the ectomesodermic tissues being also those most sensitive to the teratogenic effect of lithium. The Li-distribution in the embryo brain was almost homogeneous, instead of being heterogeneous as in adult brain. The mean Li-concentration in the embryo brain was not much below the Li concentration in the grey matter of the mother brain, but it was significantly larger than that in the white matter of the mother brain. Our results are discussed in the context of teratogenic effects observed in situ during mammalian development. 相似文献