首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   214篇
  免费   22篇
  2022年   2篇
  2021年   5篇
  2018年   6篇
  2017年   6篇
  2016年   5篇
  2015年   6篇
  2014年   7篇
  2013年   12篇
  2012年   13篇
  2011年   11篇
  2010年   10篇
  2009年   4篇
  2008年   9篇
  2007年   7篇
  2006年   12篇
  2005年   9篇
  2004年   8篇
  2003年   6篇
  2002年   7篇
  2001年   5篇
  2000年   9篇
  1999年   3篇
  1998年   3篇
  1997年   4篇
  1996年   3篇
  1995年   2篇
  1994年   3篇
  1993年   2篇
  1992年   3篇
  1991年   8篇
  1990年   2篇
  1989年   3篇
  1988年   3篇
  1987年   1篇
  1986年   3篇
  1985年   5篇
  1984年   3篇
  1983年   4篇
  1982年   2篇
  1981年   3篇
  1980年   2篇
  1978年   2篇
  1977年   1篇
  1976年   4篇
  1975年   1篇
  1974年   2篇
  1973年   1篇
  1970年   1篇
  1968年   1篇
  1967年   1篇
排序方式: 共有236条查询结果,搜索用时 15 毫秒
91.
Knowing the information dissemination mechanisms of different media and having an efficient information dissemination plan for disaster pre-warning plays a very important role in reducing losses and ensuring the safety of human beings. In this paper we established models of information dissemination for six typical information media, including short message service (SMS), microblogs, news portals, cell phones, television, and oral communication. Then, the information dissemination capability of each medium concerning individuals of different ages, genders, and residential areas was simulated, and the dissemination characteristics were studied. Finally, radar graphs were used to illustrate comprehensive assessments of the six media; these graphs show directly the information dissemination characteristics of all media. The models and the results are essential for improving the efficiency of information dissemination for the purpose of disaster pre-warning and for formulating emergency plans which help to reduce the possibility of injuries, deaths and other losses in a disaster.  相似文献   
92.
The regional distribution of the mRNA encoding the 5-HT1A serotonin receptor (whose selective agonists are potential anxiolytic and antidepressant drugs) was investigated in rat brain sections by in situ hybridization histochemistry using two sets of [32P]labelled nucleoprobes, a riboprobe of 156 bases and oligoprobes of 30 bases corresponding to highly selective portions within the third intracellular loop and the N terminus domain of the amino acid sequence. These probes allowed the visualization of the 5-HT1A mRNA mainly in the limbic regions: dentate gyrus and area CA1 of the hippocampus, amygdala, entorhinal cortex, lateral septum and the dorsal raphe nucleus. These structures were also those which could be labelled by the specific 5-HT1A radioligand [125I]BH-8-MeO-N-PAT and antibodies raised against a synthetic 26 amino acid peptide whose sequence was taken from the most selective portion of the rat 5-HT1A receptor protein. These data suggest that the 5-HT1A receptors are not transported to a long distance from their site of synthesis, as it has been already reported for the somato-dendritic 5-HT1A autoreceptors in the dorsal raphe nucleus. Combined autoradiographic quantification of the 5-HT1A binding sites (labelled by a selective radioligand such as [125I]BH-8-MeO-N-PAT, the 5-HT1A receptor binding subunit (by radioimmunohistochemistry) and the 5-HT1A mRNA on adjacent brain sections should be a relevant approach for assessing the molecular mechanisms responsible for the functional alterations of these receptors under various pathological and pharmacological conditions.  相似文献   
93.
Translation of the rpoC genes in Escherichia coli and Salmonella typhimurium is known to start from the GUG codon. Now, using toeprint analysis we have shown UUG to be the initiation codon of the Pseudomonas putida rpoC gene. IF3 does not seem to proofread initiation at the UUG codon. The rpoC genes of P. putida, E. coli, and S. typhimurium, which use rare start codons, have strong SD-domains AGGAGG (P. p.) and GGGAG (E. c., S. t.), optimal seven-nucleotide spacing between SD and start codons, and good second codon AAA. We suggest that rpoC presents an infrequent case of the regulation of translation initiation by selecting the start codon.  相似文献   
94.
Neurons and glia from the central nervous system of the adult teleost Carassius auratus have been grown as explant cultures of minced brain tissue and as trypsin dissociated cells. These cultures exhibit extensive neurite growth from two neuronal types, have organotypic ultrastructure, and contain electrically active cells. Autoradiographic data indicate that these neurons do not divide in culture, and histological evidence suggests that some mature neurons survive explantation and regenerate processes. However, explantation of brain fragments not containing undifferentiated cells, localized in the ventricular and subventricular zones in the brains of fish, resulted in mesenchymal and glial cell cultures only. Therefore, a contribution to the population of cells in culture by undifferentiated cells must be considered. The cultured neurons remained viable for at least 19 weeks and ultrastructural and electrophysiological data indicate synaptic interaction between cells in explant cultures.  相似文献   
95.

Background

The benefit of pelvic lymphadenectomy in patients with cancer of the urinary bladder remains controversial. Though the inclusion of lymph node dissection in conjunction with radical cystectomy for patients with clinically negative nodes is well accepted, however, the extent of the nodal dissection remains contentious, particularly in patients with gross disease and T1G3 cancer. The extent of the primary bladder tumor, number of lymph nodes removed and the lymph node tumor burden are important prognostic variables in patients undergoing cystectomy. We analyzed the impact of the extent of lymphadenectomy during radical cystectomy on survival in the contemporary literature.

Methods

A Pubmed search was carried out for the literature published over the last 15 years using bladder cancer, radical cystectomy, survival, lymphadenectomy and complications as the key words. We have discussed the extent of lymphadenectomy on survival and its anatomical basis to determine the optimal number of lymph nodes to be removed and the concept of node density.

Results

Evidence from contemporary literature indicate significantly increased survival rates after cystectomy in patients with bladder cancer diagnosed with stages III or IV disease who have had relatively more lymph nodes examined, suggesting that even some patients with higher stage disease may benefit from extended pelvic lymphadenectomy at the time of cystectomy. Studies also indicate that more extensive lymphadenectomy significantly improved the prognosis of patients with bladder cancer, not only by providing prognostic information but perhaps it is also due to its inherent therapeutic value.

Conclusion

Extended lymph node dissection improves local control and survival. However, in the absence of controlled randomized trial this remains a dubitable issue.  相似文献   
96.
A special Escherichia coli strain capable of producing a leaderless lacZ mRNA from the chromosomal lac promoter was constructed to study the mechanism of leaderless mRNA translation. The translation efficiency of this noncanonical mRNA is very low in comparison with the canonical cellular templates, but it increases by one order of magnitude in the presence of chromosomal mutations in the genes encoding the ribosomal S1 and S2 proteins. The new strain possesses obvious advantages over the commonly used plasmid constructs (first of all, due to the constant dosage of lacZ gene in the cell) and opens possibilities for investigation of the specific conditions for leaderless mRNA translation in vivo using molecular genetic approaches.  相似文献   
97.
The members of tribe Microlicieae in the flowering plant family Melastomataceae are nearly all endemic to the cerrado biome of Brazil. Traditional classifications of the Melastomataceae have attributed between 15 and 17 genera to the Microlicieae, but subsequent revisions have circumscribed the tribe more narrowly. The monophyly and intergeneric relationships of the Microlicieae were evaluated through phylogenetic analyses with molecular and morphological data sets. Incorporation of DNA sequences from the intron of the chloroplast gene rpl16 into a previously generated family-wide data set yielded a clade comprising Chaetostoma, Lavoisiera, Microlicia, Rhynchanthera, Stenodon, and Trembleya ("core Microlicieae"), with Rhynchanthera as the first-diverging lineage. The other four genera of Microlicieae sampled are placed in other clades: Eriocnema with Miconieae; Siphanthera with Aciotis, Nepsera, and Acisanthera of Melastomeae; Castratella as sister to Monochaetum of Melastomeae; and Cambessedesia as part of an unresolved polytomy in a large clade that includes most Melastomataceae. Analyses of the chloroplast genes rbcL and ndhF that included three core genera produced similar results, as did the combined analysis of all three data sets. Combined parsimony analyses of DNA sequences from rpl16 and the nuclear ribosomal intercistronic transcribed spacer (ITS) region of 22 species of core Microlicieae yielded generally low internal support values. Lavoisiera, recently redefined on the basis of several morphological characters, was strongly supported as monophyletic. A morphological phylogenetic analysis of the Microlicieae based on 10 parsimony-informative characters recovered a monophyletic core Microlicieae but provided no further resolution among genera. Penalized likelihood analysis with two calibration time windows produced an age estimate of 3.7 million years for the time of initial divergence of strictly Brazilian core Microlicieae. This date is in general agreement with the estimated age of the most active stage of development of cerrado vegetation and implies an adaptive shift from hydric to seasonally dry habitats during the early evolution of this group.  相似文献   
98.
Influenza in humans is characterised by strongly annual dynamics and antigenic evolution leading to partial escape from prior host immunity. The variability of new epidemic strains depends on the amount of virus currently circulating. In this paper, the amount of antigenic variation produced each year is dependent on the epidemic size. Our model reduces to a one-dimensional map and a full mathematical analysis is presented. This simple system suggests some basic principles which may be more generally applicable. In particular, for diseases with antigenic drift, vaccination may be doubly beneficial. Not only does it protect the population through classical herd immunity, but the overall case reduction reduces the chance of new variants being produced; hence, subsequent epidemics may be milder as a result of this positive feedback. Also, a disease with a high innate rate of antigenic variation will always be able to invade a susceptible population, whereas a disease with less potential for variation may require several introduction events to become endemic.  相似文献   
99.
Translation initiation of hepatitis C virus (HCV) RNA occurs through an internal ribosome entry site (IRES) located at its 5' end. As a positive-stranded virus, HCV uses the genomic RNA template for translation and replication, but the transition between these two processes remains poorly understood. HCV core protein (HCV-C) has been proposed as a good candidate to modulate such a regulation. However, current data are still the subject of controversy in attributing any potential role in HCV translation to the HCV core protein. Here we demonstrate that HCV-C displays binding activities toward both HCV IRES and the 40 S ribosomal subunit by using centrifugation on sucrose gradients. To gain further insight into these interactions, we investigated the effect of exogenous addition of purified HCV-C on HCV IRES activity by using an in vitro reporter assay. We found that HCV IRES-mediated translation was specifically modulated by HCV-C provided in trans, in a dose-dependent manner, with up to a 5-fold stimulation of the IRES efficiency upon addition of low amounts of HCV-C, followed by a decrease at high doses. Interestingly, mutations within some domains of the IRES as well as the presence of an upstream reporter gene both lead to changes in the expected effects, consistent with the high dependence of HCV IRES function on its overall structure. Collectively, these results indicate that the HCV core protein is involved in a tight modulation of HCV translation initiation, depending on its concentration, and they suggest an important biological role of this protein in viral gene expression.  相似文献   
100.
The diatom Achnanthes brevipes C.A. Ag. was cultured in the presence of limiting concentrations of nitrogen (N) or inorganic phosphate (Pi). Growth, in terms of final yield, was more affected by N limitation than Pi limitation; N limitation had a greater effect also on protein and chlorophyll content. Carbohydrate concentrations increased under both nutrient starvation treatments, but N or Pi limitation had different effects. Total (intracellular plus extracellular) sugar content increased when cells were exposed to both types of nutrient limitation, but the extracellular polysaccharide fraction increased only in the presence of Pi starvation. Analyses were performed to identify the metabolic changes occurring in cells exposed to low phosphate because this was the main condition that affected carbohydrate extrusion. Activities of several enzymes involved in carbohydrate metabolism showed that under Pi limitation there was no activation of alternative reactions that were found to result in Pi liberation, instead of its consumption, in some higher plants and in the green alga Selenastrum minutum Naeg. Collins. Results showed that activities of pyruvate kinase, phosphorylating NAD-dependent 3-phosphate-glyceraldehyde dehydrogenase, and 3-phospho-glycerate kinase were inhibited under Pi-limited conditions compared with control cells, indicating limited glucose catabolism. Activity of uridine diphosphate glucose pyrophosphorylase, a key enzyme for the biosynthesis of the storage compound crysolaminarin, was also partly inhibited in Pi-stressed cells. Our findings suggest that carbohydrate catabolism in A. brevipes is limited under Pi deficiency, whereas extracellular extrusion of carbohydrate is favored.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号