全文获取类型
收费全文 | 4133篇 |
免费 | 251篇 |
国内免费 | 4篇 |
专业分类
4388篇 |
出版年
2021年 | 38篇 |
2020年 | 44篇 |
2019年 | 31篇 |
2018年 | 46篇 |
2017年 | 41篇 |
2016年 | 63篇 |
2015年 | 122篇 |
2014年 | 147篇 |
2013年 | 210篇 |
2012年 | 262篇 |
2011年 | 236篇 |
2010年 | 177篇 |
2009年 | 173篇 |
2008年 | 196篇 |
2007年 | 247篇 |
2006年 | 254篇 |
2005年 | 259篇 |
2004年 | 223篇 |
2003年 | 194篇 |
2002年 | 226篇 |
2001年 | 57篇 |
2000年 | 38篇 |
1999年 | 65篇 |
1998年 | 80篇 |
1997年 | 38篇 |
1996年 | 45篇 |
1995年 | 47篇 |
1994年 | 57篇 |
1993年 | 35篇 |
1992年 | 40篇 |
1991年 | 36篇 |
1990年 | 44篇 |
1989年 | 21篇 |
1988年 | 29篇 |
1987年 | 25篇 |
1986年 | 25篇 |
1985年 | 31篇 |
1984年 | 40篇 |
1983年 | 29篇 |
1982年 | 34篇 |
1981年 | 29篇 |
1980年 | 33篇 |
1979年 | 16篇 |
1978年 | 27篇 |
1977年 | 19篇 |
1976年 | 23篇 |
1975年 | 20篇 |
1974年 | 23篇 |
1973年 | 25篇 |
1967年 | 16篇 |
排序方式: 共有4388条查询结果,搜索用时 0 毫秒
51.
Jean- -François Laliberté Olivier Nicolas Serge Durand Rolf Morosoli 《Plant molecular biology》1992,18(3):447-451
The xylanase gene from Cryptococcus albidus contains seven introns. Genomic and cDNA clones under the control of the CaMV 35S promoter were transferred into tobacco plants using Agrobacterium-mediated cell transformation. The genes were transcribed and the mRNAs were amplified by the polymerase chain reaction using primers on each side of the intron region. About 90% of the amplification products from plants transformed with the genomic clone corresponded to the size of the pre-mRNA (1.2 kb) and 10% represented the spliced product (0.85 kb). The 0.85 kb fragment was cloned and sequenced and the result indicated that the introns from the xylanase gene were accurately spliced by the plant cells. 相似文献
52.
Manfred Braun Jong Min Kim Rolf D. Schmid 《Applied microbiology and biotechnology》1992,37(5):594-598
Summary The soil isolate Cellulomonas cellulans AM8 produces an extracellular l-amino acid oxidase (L-AAO) with broad substrate specificity. The strain produced up to 0.35 unit (U)/ml of the extracellular L-AAO in a simple medium containing glycerol and yeast extract. The enzyme was easily purified up to 30 U/mg protein using Phenyl-Sepharose fast flow. The purified enzyme migrated as single band on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) with a molecular mass of 55 kDa. On native PAGE the molecular mass was approx. 300 000 kDa, which may be due to aggregation. With the exception of glycine, proline, and threonine, all the amino acids normally constituting proteins were oxidized. The V
max values from 0.7 to 35.2 U/mg for aspartic acid and lysine, respectively, and the K
m values from 0.007 to 7.1 mm for cysteine and valine, respectively, were obtained at 25° C and pH 7.0 in oxygen-saturated solutions. The L-AAO had a pH optimum of 6.5–7.5. It was stable for several months at — 30° C and for some days at 35° C. Ferricyanide served as an electron acceptor with a V
max of 50 U/mg and K
m for 0.3 mm with phenylalanine as the substrate.
Correspondence to: R. D. Schmid 相似文献
53.
Summary A metamorphosis-inducing factor was isolated from medium conditioned by either metamorphosing larvae or 3-day postmetamorphic primary polyps. The factor has a molecular weight 8 kDa and is heatlabile. It does not induce metamorphosis of isolated posterior fragments and is therefore not identical to the internal signal described by Schwoerer-Böhning et al. (1990). The biological significance of the substance is currently unclear, therefore its inducing activity may be a side effect. 相似文献
54.
55.
Pathological morphogenesis on leaves of Fraxinus ornus (ash) and Solanum lycopersicum (tomato) under the influence of mites (Aceria fraxinivora and Eriophyes cladophthirus respectively) leads to a range of structures whose morphology and development cannot be reduced to the classical categories of plant morphology, but present a heterogeneous continuum which links fundamental structural categories. These findings support the pyramid model of plant construction. 相似文献
56.
Dr. med. Peter W. Vaupel Johannes Otte Rolf Manz 《Radiation and environmental biophysics》1982,20(4):289-300
Summary The oxyhemoglobin saturation (HbO2) of single red blood cells within tumor microvessels (diameter: 3–12 µm) of DS-Carcinosarcoma was studied using a cryophotometric micromethod. In untreated control tumors (mean tissue temperature approx. 35° C) the measured values scattered over the whole saturation range from zero to 100 sat.%, the mean being 51 sat.%. Upon heating at 40° C for 30 min, the oxygenation of the tumor tissue significantly improved as compared with control conditions. After 40° C-hyperthermia a mean oxyhemoglobin saturation of 66 sat.% was obtained. In contradistinction to this, after 43° C-hyperthermia the tumor oxygenation was significantly lower and reached a mean HbO2 saturation value of 47 sat.%. A further temperature rise to 45° C caused the oxygenation to drop drastically (mean oxyhemoglobin saturation value: 24 sat.%). This is due to a severe restriction of nutritive blood flow.The changes in tumor oxygenation after hyperthermia seem to be predominantly mediated through changes in tumor blood flow, including tumor microcirculation, which showed a similar temperature dependence. Metabolic effects probably play a minor role in the oxyhemoglobin saturation distribution within tumor microvessels.Supported by the Deutsche Forschungsgemeinschaft (Va 57/2-1). Presented in part at the International Symposium on Biomedical Thermology, June 30 to July 4, 1981, Strasbourg, France 相似文献
57.
Rolf Eiben 《Development genes and evolution》1982,191(4):270-276
Summary The formation of tentacles and stolons during metamorphosis is severely disturbed if inhibitors of mRNA metabolism are applied during certain phases of development. The periods of sensitivity to -amanitin are late gastrulation and the disk stage of metamorphosis. A cordycepin sensitive phase exists during the first hour of metamorphosis. In all drug sensitive phases an enhanced poly(A) synthesis is found indicating increased mRNA metabolism in these stages. Pulse-chase experiments show that planula larvae store a poly(A)-rich RNA population sedimenting between 28–18s. These long living molecules are of embryonic origin, are located in RNP particles and are degraded during metamorphosis. The particles in question appear to be stored mainly in interstitial cells. In early metamorphosis no uridine is incorporated but labelled poly(A) is added to preexisting molecules. 相似文献
58.
The three-dimensional structure of the sulfhydryl protease calotropin DI from the madar plant, Calotropis gigantea, has been determined at 3·2 Å resolution using the multiple isomorphous replacement method with five heavy atom derivatives. A Fourier synthesis based on protein phases with a mean figure of merit of 0·857 was used for model building. The polypeptide backbone of calotropin DI is folded to form two distinct lobes, one of which is comprised mainly of α-helices, while the other is characterized by a system of all antiparallel pleated sheets. The overall molecular architecture closely resembles those found in the sulfhydryl proteases papain and actinidin.Despite the unknown amino acid sequence of calotropin DI a number of residues around its active center could be identified. These amino acid side-chains were found in a similar arrangement as the corresponding ones in papain and actinidin. The polypeptide chain between residues 1 and 18 of calotropin DI folds in a unique manner, providing a possible explanation for the unusual inability of calotropin DI to hydrolyze those synthetic substrates that papain and actinidin act upon. 相似文献
59.
60.
Dieter Haas John Watson Rolf Krieg Thomas Leisinger 《Molecular & general genetics : MGG》1981,182(2):240-244
Summary A derivative of the IncP-1 plasmid RP1, temperature-sensitive for maintenance, was inserted into the Pseudomonas aeruginosa chromosome by selection for a plasmid marker (carbenicillin resistance) at nonppermissive temperature. In one strain, PAO 1000, the plasmid was stably integrated in the trpA, B gene cluster mapped at 27 min, as shown by the following evidence. (i) Trp+ transductants lost all plasmid markers. (ii) Cleared lysates of PAO 1000 showed no plasmid band typical of the autonomous RP1 in agarose gel electrophoresis. (iii) No transfer of carbenicillin resistance by PAO 1000 was detectable. (iv) PAO 1000 mobilised the chromosome from an origin at, or very near, the plasmid insertion site with high frequency (recovery of proximal markers 10–3 per donor). Matings on the plate with and without interruption of conjugation showed that chromosome transfer was unidirectional. (v) Recombinants from PAO 1000-mediated crosses did not inherit plasmid markers or the trpA, B mutation. A derivative of PAO 1000 was obtained which had lost the Hfr property and all plasmid markers except carbenicillin resistance. This strain (PAO 1001), when carrying the autonomous RP1 plasmid, was capable of unidirectional chromosome mobilisation like PAO 1000, but with 50-fold lower efficiency. We propose that integration of the temperature-sensitive RP1 plasmid in PAO 1000 occurred via transposition of Tnl, the element specifying carbenicillin resistance. 相似文献