全文获取类型
收费全文 | 1072篇 |
免费 | 101篇 |
国内免费 | 1篇 |
专业分类
1174篇 |
出版年
2022年 | 8篇 |
2021年 | 18篇 |
2020年 | 14篇 |
2019年 | 4篇 |
2018年 | 9篇 |
2016年 | 16篇 |
2015年 | 28篇 |
2014年 | 33篇 |
2013年 | 47篇 |
2012年 | 58篇 |
2011年 | 60篇 |
2010年 | 35篇 |
2009年 | 35篇 |
2008年 | 59篇 |
2007年 | 51篇 |
2006年 | 70篇 |
2005年 | 53篇 |
2004年 | 69篇 |
2003年 | 58篇 |
2002年 | 49篇 |
2001年 | 15篇 |
2000年 | 18篇 |
1999年 | 24篇 |
1998年 | 15篇 |
1997年 | 32篇 |
1996年 | 11篇 |
1995年 | 19篇 |
1994年 | 15篇 |
1993年 | 12篇 |
1992年 | 15篇 |
1991年 | 12篇 |
1990年 | 10篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 16篇 |
1984年 | 6篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 17篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1974年 | 7篇 |
1973年 | 4篇 |
1969年 | 4篇 |
1965年 | 4篇 |
1964年 | 6篇 |
排序方式: 共有1174条查询结果,搜索用时 14 毫秒
81.
Loss of HR6B ubiquitin-conjugating activity results in damaged synaptonemal complex structure and increased crossing-over frequency during the male meiotic prophase 总被引:7,自引:0,他引:7 下载免费PDF全文
Baarends WM Wassenaar E Hoogerbrugge JW van Cappellen G Roest HP Vreeburg J Ooms M Hoeijmakers JH Grootegoed JA 《Molecular and cellular biology》2003,23(4):1151-1162
The ubiquitin-conjugating enzymes HR6A and HR6B are the two mammalian homologs of Saccharomyces cerevisiae RAD6. In yeast, RAD6 plays an important role in postreplication DNA repair and in sporulation. HR6B knockout mice are viable, but spermatogenesis is markedly affected during postmeiotic steps, leading to male infertility. In the present study, increased apoptosis of HR6B knockout primary spermatocytes was detected during the first wave of spermatogenesis, indicating that HR6B performs a primary role during the meiotic prophase. Detailed analysis of HR6B knockout pachytene nuclei showed major changes in the synaptonemal complexes. These complexes were found to be longer. In addition, we often found depletion of synaptonemal complex proteins from near telomeric regions in the HR6B knockout pachytene nuclei. Finally, we detected an increased number of foci containing the mismatch DNA repair protein MLH1 in these nuclei, reflecting a remarkable and consistent increase (20 to 25%) in crossing-over frequency. The present findings reveal a specific requirement for the ubiquitin-conjugating activity of HR6B in relation to dynamic aspects of the synaptonemal complex and meiotic recombination in spermatocytes. 相似文献
82.
Song Y Clizbe L Bhakta C Teng W Wong P Huang B Tran K Sinha U Park G Reed A Scarborough RM Zhu BY 《Bioorganic & medicinal chemistry letters》2003,13(2):297-300
In addition to our previously reported fluoro acrylamides Xa inhibitors 2 and 3, a series of potent and novel cyclic diimide amidine compounds has been identified. In efforts to improve their oral bioavailability, replacement of the amidine group with methyl amidrazone gives compounds of moderate potency (14, IC(50)=0.028 microM). In the amidoxime prodrug approach, the amidoxime compounds show good oral bioavailability in rats and dogs. High plasma level of prodrug 26 and significant concentration of active drug 26a were obtained upon oral administration of prodrug 26 in rats. 相似文献
83.
Alexander F. Arendsen Jonathan Hadden Graeme Card Alan S. McAlpine Susan Bailey Vjacheslav Zaitsev Elizabeth H. M. Duke P. F. Lindley Monika Kröckel Alfred X. Trautwein Martinus C. Feiters John M. Charnock C. David Garner Sophie J. Marritt Andrew J. Thomson Ingeborg M. Kooter Michael K. Johnson Willy A. M. van den Berg Walter M. A. M. van Dongen W. R. Hagen 《Journal of biological inorganic chemistry》1998,3(1):81-95
The three-dimensional structure of the native "putative prismane" protein from Desulfovibrio vulgaris (Hildenborough) has been solved by X-ray crystallography to a resolution of 1.72?Å. The molecule does not contain a [6Fe-6S] prismane cluster, but rather two 4Fe clusters some 12?Å apart and situated close to the interfaces formed by the three domains of the protein. Cluster 1 is a conventional [4Fe-4S] cubane bound, however, near the N-terminus by an unusual, sequential arrangement of four cysteine residues (Cys 3, 6, 15, 21). Cluster 2 is a novel 4Fe structure with two μ2-sulfido bridges, two μ2-oxo bridges, and a partially occupied, unidentified μ2 bridge X. The protein ligands of cluster 2 are widely scattered through the second half of the sequence and include three cysteine residues (Cys 312, 434, 459), one persulfido-cysteine (Cys 406), two glutamates (Glu 268, 494), and one histidine (His 244). With this unusual mixture of bridging and external type of ligands, cluster 2 is named the "hybrid" cluster, and its asymmetric, open structure suggests that it could be the site of a catalytic activity. X-ray absorption spectroscopy at the Fe K-edge is readily interpretable in terms of the crystallographic model when allowance is made for volume contraction at 10?K; no Fe··Fe distances beyond 3.1?Å could be identified. EPR, Mössbauer and MCD spectroscopy have been used to define the oxidation states and the magnetism of the clusters in relation to the crystallographic structure. Reduced cluster 1 is a [4Fe-4S]1+ cubane with S?=?3/2; it is the first biological example of a "spin-admixed" iron-sulfur cluster. The hybrid cluster 2 has four oxidation states from (formally) all FeIII to three FeII plus one FeIII. The four iron ions are exchange coupled resulting in the system spins S?=?0, 9/2, 0 (and 4), 1/2, respectively, for the four redox states. Resonance Raman spectroscopy suggests that the bridging ligand X which could not be identified unambiguously in the crystal structure is a solvent-exchangeable oxygen. 相似文献
84.
Targeted Inactivation of Mouse RAD52 Reduces Homologous Recombination but Not Resistance to Ionizing Radiation 总被引:6,自引:1,他引:5 下载免费PDF全文
Tonnie Rijkers Jody Van Den Ouweland Bruno Morolli Anton G. Rolink Willy M. Baarends Petra P. H. Van Sloun Paul H. M. Lohman Albert Pastink 《Molecular and cellular biology》1998,18(11):6423-6429
The RAD52 epistasis group is required for recombinational repair of double-strand breaks (DSBs) and shows strong evolutionary conservation. In Saccharomyces cerevisiae, RAD52 is one of the key members in this pathway. Strains with mutations in this gene show strong hypersensitivity to DNA-damaging agents and defects in recombination. Inactivation of the mouse homologue of RAD52 in embryonic stem (ES) cells resulted in a reduced frequency of homologous recombination. Unlike the yeast Scrad52 mutant, MmRAD52−/− ES cells were not hypersensitive to agents that induce DSBs. MmRAD52 null mutant mice showed no abnormalities in viability, fertility, and the immune system. These results show that, as in S. cerevisiae, MmRAD52 is involved in recombination, although the repair of DNA damage is not affected upon inactivation, indicating that MmRAD52 may be involved in certain types of DSB repair processes and not in others. The effect of inactivating MmRAD52 suggests the presence of genes functionally related to MmRAD52, which can partly compensate for the absence of MmRad52 protein. 相似文献
85.
86.
Lydia Herrmann Caspar Wiegmann Annika Arsalan-Werner Isabel Hilbrich Carsten J?ger Katharina Flach Anne Suttkus Ingolf Lachmann Thomas Arendt Max Holzer 《PloS one》2015,10(3)
Defects in intracellular transport are implicated in the pathogenesis of Alzheimer’s disease (AD). Hook proteins are a family of cytoplasmic linker proteins that participate in endosomal transport. In this study we show that Hook1 and Hook3 are expressed in neurons while Hook2 is predominantly expressed in astrocytes. Furthermore, Hook proteins are associated with pathological hallmarks in AD; Hook1 and Hook3 are localized to tau aggregates and Hook2 to glial components within amyloid plaques. Additionally, the expression of Hook3 is reduced in AD. Modelling of Hook3 deficiency in cultured cells leads to slowing of endosomal transport and increases β-amyloid production. We propose that Hook3 plays a role in pathogenic events exacerbating AD. 相似文献
87.
Tania van Laer Peter de Smedt Frederik Ronsse Greet Ruysschaert Pascal Boeckx Willy Verstraete Jeroen Buysse Luc J. Lavrysen 《Global Change Biology Bioenergy》2015,7(1):14-24
This article addresses biochar from a legal point of view. It analyses different policies and regulations from a European (Flemish) point of view and provides a first and general insight in what potential legal constraints the development of a biochar industry might face and what opportunities lie ahead. This is due to the fact that biochar is a recent product and a lot of scientific uncertainty still exists regarding the consequences of its application. From the analysis it appears a multitude of policies and legislative measures influence the development of the biochar industry. Hence, it is important that all these policies and legislative measures are analyzed in an appropriate manner. Moreover, considerable lobbying, negotiating and cooperation between different disciplines (legal, scientific, economical, etc.) will be required so as to develop a feasible and safe biochar framework. 相似文献
88.
Willem De Muynck Kim Verbeken Nele De Belie Willy Verstraete 《Ecological Engineering》2010,36(2):99-111
Bacterially induced carbonate precipitation has been explored for the protection and consolidation of ornamental stone. Attempts to improve the efficiency of this biodeposition process were primarily focused on the microbial aspects, i.e. type of microorganism and metabolic pathway. In this study, the influence of the chemical parameters, i.e. concentration of calcium salts and urea, on the effectiveness of the biodeposition treatment has been examined. The amount of calcium carbonate that can be precipitated in the stone is conditioned both by the amount of cells retained in the stone and the concentration of urea and calcium used. From sonication experiments, a good consolidation was observed for limestone prisms treated with a calcium dosage of 17 g Ca2+ m?2 with no improvement at higher concentrations. For limestone prisms of 4 cm × 2 cm × 1 cm, the biodeposition treatment resulted in a 63% lower weight loss upon sonication compared to untreated specimens. The waterproofing effect was observed to increase with increasing calcium dosages. While for a calcium dosage of 17 g Ca2+ m?2 the water absorption was similar to that of untreated specimens, concentrations of 67 g Ca2+ m?2 resulted in a 50% decrease of the rate of water absorption. For calcium dosages higher than 34 g Ca2+ m?2 a significant change in the visual aspect (ΔE > 6) of the treated stones could be observed. Overall, the urea/calcium chloride-based biodeposition treatment attained a protective performance comparable with that of the commonly used ethylsilicates. 相似文献
89.
Aggregate Size and Architecture Determine Microbial Activity Balance for One-Stage Partial Nitritation and Anammox 总被引:1,自引:0,他引:1
Siegfried E. Vlaeminck Akihiko Terada Barth F. Smets Haydée De Clippeleir Thomas Schaubroeck Selin Bolca Lien Demeestere Jan Mast Nico Boon Marta Carballa Willy Verstraete 《Applied and environmental microbiology》2010,76(3):900-909
Aerobic ammonium-oxidizing bacteria (AerAOB) and anoxic ammonium-oxidizing bacteria (AnAOB) cooperate in partial nitritation/anammox systems to remove ammonium from wastewater. In this process, large granular microbial aggregates enhance the performance, but little is known about granulation so far. In this study, three suspended-growth oxygen-limited autotrophic nitrification-denitrification (OLAND) reactors with different inoculation and operation (mixing and aeration) conditions, designated reactors A, B, and C, were used. The test objectives were (i) to quantify the AerAOB and AnAOB abundance and the activity balance for the different aggregate sizes and (ii) to relate aggregate morphology, size distribution, and architecture putatively to the inoculation and operation of the three reactors. A nitrite accumulation rate ratio (NARR) was defined as the net aerobic nitrite production rate divided by the anoxic nitrite consumption rate. The smallest reactor A, B, and C aggregates were nitrite sources (NARR, >1.7). Large reactor A and C aggregates were granules capable of autonomous nitrogen removal (NARR, 0.6 to 1.1) with internal AnAOB zones surrounded by an AerAOB rim. Around 50% of the autotrophic space in these granules consisted of AerAOB- and AnAOB-specific extracellular polymeric substances. Large reactor B aggregates were thin film-like nitrite sinks (NARR, <0.5) in which AnAOB were not shielded by an AerAOB layer. Voids and channels occupied 13 to 17% of the anoxic zone of AnAOB-rich aggregates (reactors B and C). The hypothesized granulation pathways include granule replication by division and budding and are driven by growth and/or decay based on species-specific physiology and by hydrodynamic shear and mixing.In the last few years, autotrophic nitrogen removal via partial nitritation and anoxic ammonium oxidation (anammox) has evolved from lab- to full-scale treatment of nitrogenous wastewaters with a low biodegradable organic compound content, and this evolution has been driven mainly by a significant decrease in the operational costs compared to the costs of conventional nitrification and heterotrophic denitrification (11, 23). Oxygen-limited autotrophic nitrification and denitrification (OLAND) is one of the autotrophic processes used and is a one-stage procedure; i.e., partial nitritation and anammox occur in the same reactor (30). The “functional” autotrophic microorganisms in OLAND include aerobic ammonium-oxidizing bacteria (AerAOB) and anoxic ammonium-oxidizing bacteria (AnAOB). With oxygen, AerAOB oxidize ammonium to nitrite (nitritation), and with the nitrite AnAOB oxidize the residual ammonium to form dinitrogen gas and some nitrate (anammox). Additional aerobic nitrite oxidation to nitrate (nitratation) by nitrite-oxidizing bacteria (NOB) lowers the nitrogen removal efficiency, but it can, for instance, be prevented at low dissolved oxygen (DO) levels because the oxygen affinity of AerAOB is higher than that of NOB (16). Reactor configurations for the OLAND process can be based on suspended biomass growing in aggregates, like that in a sequencing batch reactor (SBR) (37) or a gas lift or upflow reactor (32). For suspended-growth systems there are two important challenges: biomass retention and equilibrated microbial activities.High biomass retention efficiency is a prerequisite in anammox technologies because of the slow growth of AnAOB (33). In suspended biomass systems, settling properties determine the retention of biomass and are related to the microbial aggregate morphology (floc or granule) and size. Granules can be defined as compact and dense aggregates with an approximately spherical external appearance that do not coagulate under decreased hydrodynamic shear conditions and settle significantly faster than flocs (18). Toh and coworkers calculated a lower sludge volume index for aerobic granules than for aerobic flocs and also showed that there was an increase in the settling velocity with increasing granule size (35). Hence, in terms of physical properties, large granules are preferable for suspended-growth applications.OLAND aggregate size not only influences settling properties but also affects the proportion of microbial nitrite production and consumption; lower AerAOB activity and higher AnAOB activity were observed with larger aggregates (25, 37). Theoretically, a microbial aggregate with equal nitrite production and nitrite consumption can remove ammonium autonomously, because of its independence from other aggregates for acquisition and conversion of nitrite. Hence, with an increasing aggregate size and thus with a decreasing ratio of nitrite production to nitrite consumption, three functional categories of aggregates can be distinguished: nitrite sources, autonomous nitrogen removers, and nitrite sinks. Because minimal nitrite accumulation is one of the prerequisites for high nitrogen removal efficiency in OLAND reactors, the presence of excess small aggregates is undesirable (9, 37).Although large granular aggregates are desirable for biomass retention and activity balance, so far no formation mechanisms have been proposed for OLAND granules, in contrast to the well-studied anaerobic (13) and aerobic (1) granules. In order to determine general and environment-specific determinants for aggregate size and architecture, three suspended-growth OLAND reactors with different inoculation and operation (mixing and aeration) parameters were selected, and these reactors were designated reactors A, B, and C (Table (Table1).1). The first objective of this study was to gain more insight into the relationship between OLAND aggregate size, AerAOB and AnAOB abundance, and the activity balance. The second objective was to propose pathways for aggregation and granulation by relating (dis)similarities in aggregate size distribution, morphology, and architecture to differences in reactor inoculation and operation.
Open in a separate windowaAggregates settling at a rate higher than the minimum settling velocity (MSV) were not washed out of the sequencing batch reactors (SBR). The MSV was calculated by dividing the vertical distance of the water volume decanted per cycle by the settling time.bSupernatant from a municipal sludge digestor.cEffluent from a potato-processing factory pretreated with anaerobic digestion and struvite precipitation.dObtained at the end of a reactor start-up study (37).eObtained at the end of a reactor start-up study (9). 相似文献
TABLE 1.
Overview of the three OLAND reactor systems from which suspended biomass samples were obtainedParameter | Reactor Aa | Reactor Ba | Reactor C |
---|---|---|---|
Reactor type | SBR | SBR | Upflow reactor |
Vol (m3) | 0.002 | 4.1 | 600 |
Reactor ht/diam ratio | 0.9 | 4 | 0.5-0.8 |
Inoculum | OLAND biofilm | Activated sludge | Anammox granules |
Wastewater | Synthetic | Domesticb | Industrialc |
Influent ammonium concn (mg N liter−1) | 230-330 | 800 | 250-350 |
Nitrogen removal rate (g N liter−1 day −1) | 0.45,d 1.1e | 0.65 | 1.3 |
Effluent nitrite concn (mg N liter−1) | 30-40d | 5-10 | 5-10 |
Influent COD/effluent COD (mg liter−1) | 0/0 | 240/220 | 200/150 |
pH | 7.4-7.8 | 7.4-7.6 | 8.0 |
Temp (°C) | 35 | 25 | 30-35 |
DO level (mg O2 liter−1) | 0.4-1.1 | 0.5-1.0 | 2.0-3.0 |
Mixing mechanism | Magnetic stirrer | Bladed impeller | Aeration |
Biomass retention mechanism | MSV, >0.73 m h−1 | MSV, >1.4 m h−1 | Three-phase separator |
Sampling time (months after start-up) | 2d | 8 | 30 |
90.
Tiziana Gigliobianco Bernard Lakaye Pierre Wins Benaïssa El Moualij Willy Zorzi Lucien Bettendorff 《BMC microbiology》2010,10(1):148