全文获取类型
收费全文 | 5966篇 |
免费 | 507篇 |
国内免费 | 2篇 |
专业分类
6475篇 |
出版年
2022年 | 45篇 |
2021年 | 70篇 |
2020年 | 72篇 |
2019年 | 77篇 |
2018年 | 108篇 |
2017年 | 87篇 |
2016年 | 127篇 |
2015年 | 204篇 |
2014年 | 225篇 |
2013年 | 332篇 |
2012年 | 353篇 |
2011年 | 359篇 |
2010年 | 252篇 |
2009年 | 205篇 |
2008年 | 286篇 |
2007年 | 295篇 |
2006年 | 304篇 |
2005年 | 256篇 |
2004年 | 259篇 |
2003年 | 266篇 |
2002年 | 259篇 |
2001年 | 86篇 |
2000年 | 73篇 |
1999年 | 107篇 |
1998年 | 82篇 |
1997年 | 54篇 |
1996年 | 59篇 |
1995年 | 62篇 |
1994年 | 58篇 |
1993年 | 60篇 |
1992年 | 65篇 |
1991年 | 43篇 |
1990年 | 49篇 |
1989年 | 52篇 |
1988年 | 45篇 |
1987年 | 46篇 |
1986年 | 42篇 |
1985年 | 55篇 |
1984年 | 67篇 |
1983年 | 40篇 |
1982年 | 62篇 |
1981年 | 45篇 |
1980年 | 64篇 |
1979年 | 49篇 |
1978年 | 40篇 |
1977年 | 37篇 |
1976年 | 37篇 |
1975年 | 31篇 |
1974年 | 31篇 |
1973年 | 31篇 |
排序方式: 共有6475条查询结果,搜索用时 0 毫秒
11.
12.
The photosynthetic membranes of Anacystis nidulans R2 were examined electrophoretically following solubilization with lithium dodecyl sulfate. Electrophoresis yielded six prominent chlorophyll-containing bands. In addition, five polypeptides were observed which possessed heme-dependent peroxidase activity, monitored by incubating gels with 3,3′,5,5′-tetramethylbenzidine plus hydrogen peroxide. One such polypeptide, at 105 kdaltons, was removed by repeated washing of the membranes. Four remaining peroxidase-active polypeptides were observed at 7.2, 13.5, 18.5 and 33 kdaltons. Further examination of these four polypeptides yielded the following results. (1) The mobility of the 33 kdalton polypeptide was altered from 29 to 33 kdaltons upon heating (70°C) during membrane solubilization. (2) All four polypeptides showed stable heme-protein associations in the presence of 8 M urea; however, in the presence of urea, alterations in protein mobility were observed for each poly-peptide and only two (at 13.5 and 33 kdaltons) showed peroxidase activity following heating (70°C) during membrane solubilization. (3) The presence of thiols during membrane solubilization at 0°C was required to observe peroxidase activity at 7.2 kdaltons. These results, when compared to known properties of isolated cytochromes, suggest that the four polypeptides characterized here correspond to the subunits of photosynthetic cytochromes. Electrophoretic assessment of maize mutants lacking cytochrome f and b6 activity supports this suggestion. 相似文献
13.
14.
Thomas K. C. Leung Christine Hall Clinton Monfries Louis Lim 《Journal of neurochemistry》1987,49(1):232-238
Neurone-specific enolase (NSE) and the brain form of creatine phosphokinase (CPK-BB) were previously found to be present in rat synaptosomal plasma membranes (SPM) using two-dimensional gel (2-D gel) and peptide analysis; enzymatic activities of these and of pyruvate kinase (PK), all involved in ATP generation, were shown to be "cryptic" unless the SPM were treated with Triton X-100. We now show that enzymatic activation also occurs when the SPM are treated with trifluoperazine (TFP). TFP activation occurred even when the enzymes were membrane associated, showing that solubilization was not responsible for "unmasking" the enzyme activities. When TFP treatment was performed at alkaline instead of neutral pH, NSE and CPK-BB were released as well as PK, nonneuronal enolase, and aldolase which were identified by 2-D gel and tryptic peptide analysis. Other proteins released included calmodulin, actin, and the 70-kilodalton heat-shock cognate protein. Tubulin, synapsin I, and a 35-kilodalton basic protein were largely unaffected. The latter was identified as the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase on the basis of 2-D gel and peptide analyses and subsequent partial sequencing of a rat brain cDNA coding for the same protein. TFP treatment is thus useful for activating latent enzymes as well as for distinguishing enzymes that have a different disposition on the membrane. 相似文献
15.
Since translational start codons also occur internally, more-complex features within mRNA must determine initiation. We compare the potential secondary structure of 123 prokaryotic mRNA start regions to that of regions coding for internal methionines. The latter display an unexpectedly-uniform, almost-periodic pattern of pairing potential. In contrast, sequences 5' to start codons have little self-pairing, and do not pair extensively with the proximal coding region. Pairing potential surrounding start codons was found to be less than half of that found near internal AUGs. In groups of random sequences where the distribution of nucleotides at each position, or of trinucleotides at each in-frame codon position, matched the observed natural distribution, there was no periodicity in the pairing potential of the internal sequences. Randomized internal sequences had less pairing: the ratio of pairing intensity between internals and starts was reduced from 2.0 to 1.6 by randomization. We propose that the transition from the relatively-unstructured start domains to the highly-structured internal sequences may be an important determinant of translational start-site recognition. 相似文献
16.
Localization and cloning of Xp21 deletion breakpoints involved in muscular dystrophy 总被引:26,自引:2,他引:24
Anthony P. Monaco Corlee J. Bertelson Chris Colletti-Feener Louis M. Kunkel 《Human genetics》1987,75(3):221-227
Summary Twenty-nine deletion breakpoints were mapped in 220 kb of the DXS164 locus relative to potential exons of the Duchenne and Becker muscular dystrophy gene. Four deletion junction fragments were isolated to acquire outlying Xp21 loci on both the terminal and centromere side of the DXS164 locus. The junction loci were used for chromosome walking, searches for DNA polymorphisms, and mapping against deletion and translocation breakpoints. Forty-four unrelated deletions were analyzed using the junction loci as hybridization probes to map the endpoints between cloned Xp21 loci. DNA polymorphisms from the DXS164 and junction loci were used to follow the segregation of a mutation in a family that represents a recombinant. Both the physical and genetic data point to a very large size for this X-linked muscular dystrophy locus. 相似文献
17.
Louis Liljedahl 《Pal?ontologische Zeitschrift》1989,63(3-4):229-240
Two new micromorphic bivalve species from the Silurian of Gotland are described and assigned to the new genusKenzieana, viz.Kenzieana lata andKenzieana angusta. Kenzieana exhibits morphological features characteristic of some unrelated extant species; above all a conspicuous antero-posterior flattening of the shell. Probably it was a siphonate suspension feeder with a reclining mode of life. 相似文献
18.
A general method is described for the assay of glycosyltransferase activity, which makes use of synthetic glycoside acceptors attached to hydrophobic aglycones. The products formed by incubation of an enzyme with acceptor and radiolabelled sugarnucleotide can then be rapidly (one minute) separated from interfering radioactivity by adsorption on to reverse-phase C-18 cartridges. After aqueous washing, products are easily isolated by elution with methanol. The utility of the method for the assay of (1–4)galactosyltransferase, (1–2)fucosyltransferase andN-acetylglucosaminyltransferase I and V is demonstrated. 相似文献
19.
Studies on the Tissue Distribution of the Puromycin-Sensitive Enkephalin-Degrading Aminopeptidases 总被引:3,自引:0,他引:3
Stacey McLellan Simon H. Dyer Gerry Rodriguez Louis B. Hersh 《Journal of neurochemistry》1988,51(5):1552-1559
An antiserum generated to the soluble form of the rat brain puromycin-sensitive enkephalin-degrading aminopeptidase was used to determine the tissue distribution of the soluble and membrane-associated forms of this enzyme. All tissues examined contained significant levels of the soluble enzyme form, with this enzyme accounting for greater than 90% of the arylamidase activity in brain, heart, and skeletal muscle. Native gel electrophoresis coupled with activity staining as well as inhibition studies were used to confirm the presence of this enzyme in various tissues. Serum was found not to contain this particular aminopeptidase. In contrast to the results obtained with the soluble enzyme form, brain was the only tissue found to contain the membrane-associated enzyme form. Although all tissues contained membrane-associated aminopeptidase activity only the brain enzyme could be maintained in solution in the absence of detergent. In addition, the brain membrane-associated enzyme could be distinguished from the membrane-associated aminopeptidase activity in other tissues on the basis of its sensitivity to inhibition by puromycin. 相似文献
20.
Influence of maize root mucilage on soil aggregate stability 总被引:9,自引:0,他引:9
This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected
from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg−1 dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting
cycle after 21 d. Evolved CO2 was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil.
In mucilage-amended soils CO2 evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase
in evolved CO2 up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation
of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent
of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate
stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work
supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the
newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of
these established organo-mineral bounds. 相似文献