全文获取类型
收费全文 | 1717篇 |
免费 | 184篇 |
出版年
2022年 | 13篇 |
2021年 | 15篇 |
2020年 | 17篇 |
2019年 | 13篇 |
2018年 | 20篇 |
2017年 | 20篇 |
2016年 | 29篇 |
2015年 | 53篇 |
2014年 | 69篇 |
2013年 | 87篇 |
2012年 | 106篇 |
2011年 | 96篇 |
2010年 | 75篇 |
2009年 | 70篇 |
2008年 | 97篇 |
2007年 | 89篇 |
2006年 | 98篇 |
2005年 | 82篇 |
2004年 | 96篇 |
2003年 | 72篇 |
2002年 | 72篇 |
2001年 | 27篇 |
2000年 | 10篇 |
1999年 | 26篇 |
1998年 | 34篇 |
1997年 | 19篇 |
1996年 | 26篇 |
1995年 | 22篇 |
1994年 | 15篇 |
1993年 | 26篇 |
1992年 | 19篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 27篇 |
1988年 | 28篇 |
1987年 | 16篇 |
1986年 | 16篇 |
1985年 | 14篇 |
1984年 | 24篇 |
1983年 | 23篇 |
1982年 | 19篇 |
1981年 | 14篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1975年 | 10篇 |
1973年 | 17篇 |
1970年 | 8篇 |
1969年 | 8篇 |
排序方式: 共有1901条查询结果,搜索用时 15 毫秒
1.
Juvenile material with the main focus on the upper jaw of the fossil predator Hyaenodon was evaluated to study the tooth eruption sequence and to examine the ontogeny of its dentition in detail. The comparison in size of milk to permanent teeth indicates a growth rate of 12–16 % in Hyaenodon. The thin section of a deciduous canine of a North American taxon shows four dental rings. Based on the knowledge of recent carnivores, this implies an age of 3–4 years in the last stage of tooth eruption and thus a long juvenile phase. The mandibles ascertained the most recent established tooth eruption sequence for North American and European species. For the first time ever, juvenile material from Asia is documented and interpreted. This study likewise shows a difference in the sequence of the upper jaw: the first upper premolar erupts before the first upper molar in North American species, whereas the European taxa show an earlier eruption of the first upper molar. This fact further confirms the divergence between the Hyaenodon lineages from North America and Europe. 相似文献
2.
3.
4.
5.
6.
Harry J. Klee Yvonne M. Muskopf Charles S. Gasser 《Molecular & general genetics : MGG》1987,210(3):437-442
Summary 5-enolpyruvylshikimate-3-phosphate synthase (EPSPs), the target of the herbicide glyphosate, catalyzes an essential step in the shikimate pathway common to aromatic amino acid biosynthesis. We have cloned an EPSP synthase gene from Arabidopsis thaliana by hybridization with a petunia cDNA probe. The Arabidopsis gene is highly homologous to the petunia gene within the mature enzyme but is only 23% homologous in the chloroplast transit peptide portion. The Arabidopsis gene contains seven introns in exactly the same positions as those in the petunia gene. The introns are, however, significantly smaller in the Arabidopsis gene. This reduction accounts for the significantly smaller size of the gene as compared to the petunia gene. We have fused the gene to the cauliflower mosaic virus 35 S promoter and reintroduced the chimeric gene into Arabidopsis. The resultant overproduction of EPSPs leads to glyphosate tolerance in transformed callus and plants. 相似文献
7.
Harry J. Klee Maria B. Hayford Stephen G. Rogers 《Molecular & general genetics : MGG》1987,210(2):282-287
Summary Cloning of genes by transformation with genomic banks and rescue of a phenotype has been extensively used in bacterial systems. This approach has not been possible in plant systems because of the large genome sizes and poor transformation frequencies of most plant species. Recent advances in plant transformation permit the generation of large numbers of transformants in petunia. We have used this system to rescue a model gene encoding resistance to kanamycin by shotgun cloning. The gene encoding neomycin phosphotransferase (NPTII) was introduced into the genome of Arabidopsis thaliana by Agrobacterium tumefaciens-mediated transformation. A genomic bank of DNA from this tissue was constructed in a cosmid vector containing features which would allow its use in plant transformation. The unselected genomic bank was mobilized from Escherichia coli to A. tumefaciens and used to retransform petunia leaf discs. The rescued gene was identified by its ability to confer a kanamycin-resistant phenotype in petunia tissue. The presence of the NPTII gene was confirmed by nopaline assay and Southern blot analysis. This experiment demonstrates the feasibility of gene rescue, in certain circumstances, in plants. 相似文献
8.
Factors Involved in Hydrolysis of Microcrystalline Cellulose by Acetivibrio cellulolyticus 总被引:1,自引:1,他引:0 下载免费PDF全文
Acetivibrio cellulolyticus cellulase obtained by the water elution of residual cellulose from the growth medium was compared with the cellulase activity present in culture supernatants. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated that water elution released most of the protein bands which adhered to undigested cellulose from the culture medium. The enzyme in the culture supernatant and that eluted from residual cellulose had specific activities for Avicel hydrolysis that were 20- to 40-fold greater than that of Trichoderma reesei cellulase. However, Ca2+ and a reducing agent such as dithiothreitol were required for maximum Avicel hydrolysis rates by these A. cellulolyticus enzyme preparations. The effect of these agents on p-nitrophenyl lactopyranoside hydrolysis suggested that they were required by an exoglucanase component. Supernatant enzyme preparations contained large amounts of carbohydrate which was separated from most of the cellulase protein by phenyl-Sepharose chromatography. Removal of this carbohydrate, which interfered with protein fractionations, allowed for an activity stain analysis of the supernatant enzyme. 相似文献
9.
Chlorella sorokiniana strain 211-40c, a symbiotic Chlorella isolated from a freshwater sponge, excreted between 3% and 5% of assimilated 14CO2 as glucose in the light, with a pH optimum around 5. This percentage increased when the illuminance was lowered (to 15% at 20 lx). Release of [14C]glucose continued in the dark and could be inhibited by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). Net efflux of glucose occurred even at a concentration ratio of extracellular/intracellular glucose of 4. This, together with the sensitivity to FCCP, is taken as evidence for active transport. Exogenous [14C]glucose was taken up by the cells under conditions of net glucose efflux, showing uptake and excretion to take place simultaneously.Abbreviations FCCP
carbonyl cyanide p-trifluoromethoxyphenylhydrazone
- p.c.
packed cells 相似文献
10.
Karl Hagspiel Doris Haab Christian P. Kubicek 《Applied microbiology and biotechnology》1989,32(1):61-67
Summary The secretion of multiple forms of cellulolytic enzymes by a Trichoderma reesei QM 9414 selectant exhibiting high protease activity (T. reesei QM 9414/A 30) was investigated using monoclonal, domain-specific antibodies against cellobiohydrolase (CBH) I, CBH II and -glucosidase, and a polyclonal antibody against endoglucanase I. The pattern of appearance of these proteins was followed during growth of the fungus on Avicel cellulose, using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE)/Western blotting/immunostaining. Evidence was obtained that, at late cultivation stages, CBH I and II became partially modified to lower molecular weight components, whereas -glucosidase and endoglucanase I appeared to remain largely intact. Modification of CBH I appeared to commence from the carboxy-terminal AB region, whereas CBH II appeared to become modified both from the amino- (ABB') and the carboxy-terminal. Evidence for a protease activity that modifies the already truncated cellobiohydrolases in the culture filtrate was obtained. These results show that proteolysis at late culture stages may contribute to the multiplicity of cellulases found in T. reesei culture fluids. Initial proteolytic cleavage of CBH I and II may, however, involve an unusual protease not detectable by the azocasein method.Offprint requests to: C. P. Kubicek 相似文献