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   223篇
  免费   16篇
  2022年   2篇
  2021年   4篇
  2020年   5篇
  2019年   4篇
  2018年   5篇
  2017年   7篇
  2016年   7篇
  2015年   10篇
  2014年   6篇
  2013年   21篇
  2012年   12篇
  2011年   13篇
  2010年   12篇
  2009年   8篇
  2008年   13篇
  2007年   8篇
  2006年   13篇
  2005年   11篇
  2004年   10篇
  2003年   8篇
  2002年   8篇
  2001年   3篇
  2000年   3篇
  1999年   4篇
  1998年   2篇
  1997年   5篇
  1996年   4篇
  1995年   1篇
  1994年   1篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1984年   3篇
  1981年   2篇
  1978年   3篇
  1976年   2篇
  1974年   2篇
  1973年   1篇
  1972年   1篇
  1971年   2篇
  1969年   2篇
  1968年   1篇
  1966年   2篇
排序方式: 共有239条查询结果,搜索用时 218 毫秒
11.
The transition from the Lower to the Middle Paleolithic in the Levant is a crucial event in human evolution, since it may involve the arrival of a new human population. In the current study, we present thermoluminescence (TL) dates obtained from 32 burnt flints retrieved from the late Lower Paleolithic (Acheulo-Yabrudian) and Early Middle Paleolithic (Mousterian) layers of Misliya Cave, Mount Carmel, Israel. Early Middle Paleolithic industries rich in Levallois and laminar products were assigned mean ages ranging from ∼250 to ∼160 ka (thousands of years ago), suggesting a production of this industry during MIS 7 and the early part of MIS 6. The mean ages obtained for the samples associated with the Acheulo-Yabrudian (strengthened by an isochron analysis) indicate a production of this cultural complex ∼250 ka ago, at the end of MIS 8. According to the Misliya TL dates, the transition from the Lower to the Middle Paleolithic in the site took place at the limit MIS 8/7 or during the early part of MIS 7. The dates, together with the pronounced differences in lithic technology strongly suggest the arrival of a new population during this period.  相似文献   
12.
Colonies of the hydrocoral Millepora dichotoma along the Gulf of Eilat are exhibiting unusual tissue lesions in the form of white spots. The emergence and rapid establishment of these multifocal tissue lesions was the first of its kind reported in this region. A characterization of this morphological anomaly revealed bleached tissues with a significant presence of bacteria in the tissue lesion area. To ascertain possible differences in microbial biota between the lesion area and non-affected tissues, we characterized the bacterial diversity in the two areas of these hydrocorals. Both culture-independent (molecular) and culture-dependent assays showed a shift in bacterial community structure between the healthy and affected tissues. Several 16S rRNA gene sequences retrieved from the affected tissues matched sequences of bacterial clones belonging to Alphaproteobacteria and Bacteroidetes members previously associated with various diseases in scleractinian corals.  相似文献   
13.
Odontoid process is an atypical and very rare localization of osteomyelitis. We reported the case of a 72-year-old hemodialysed man with methicillin-sensitive Staphylococcus aureus osteomyelitis of the odontoid process. Osteomyelitis was diagnosed at MRI and 18F-FDG PET/CT. While clinical examination and conventional radiographs were non contributive, 18F-FDG PET/CT also allowed the diagnosis of right foot osteomylitis and multiple vertebral septic localizations. 18F-FDG PET/CT done at month 3 demonstrated a regression of the odontoid and foot hypermetabolic activity. This case illustrates the atypical presentation of this septic localization and the usefulness of 18F-FDG PET/CT to perform whole body screening and detect septic metastasis.  相似文献   
14.
The aerosphere is utilized by billions of birds, moving for different reasons and from short to great distances spanning tens of thousands of kilometres. The aerosphere, however, is also utilized by aviation which leads to increasing conflicts in and around airfields as well as en‐route. Collisions between birds and aircraft cost billions of euros annually and, in some cases, result in the loss of human lives. Simultaneously, aviation has diverse negative impacts on wildlife. During avian migration, due to the sheer numbers of birds in the air, the risk of bird strikes becomes particularly acute for low‐flying aircraft, especially during military training flights. Over the last few decades, air forces across Europe and the Middle East have been developing solutions that integrate ecological research and aviation policy to reduce mutual negative interactions between birds and aircraft. In this paper we 1) provide a brief overview of the systems currently used in military aviation to monitor bird migration movements in the aerosphere, 2) provide a brief overview of the impact of bird strikes on military low‐level operations, and 3) estimate the effectiveness of migration monitoring systems in bird strike avoidance. We compare systems from the Netherlands, Belgium, Germany, Poland and Israel, which are all areas that Palearctic migrants cross twice a year in huge numbers. We show that the en‐route bird strikes have decreased considerably in countries where avoidance systems have been implemented, and that consequently bird strikes are on average 45% less frequent in countries with implemented avoidance systems in place. We conclude by showing the roles of operational weather radar networks, forecast models and international and interdisciplinary collaboration to create safer skies for aviation and birds.  相似文献   
15.
BACKGROUND: Traditionally, the bone maturity at birth has been estimated from the radiological presence and size of the ossified distal femoral epiphysis. This study was conducted in a search for a sonographic tool for the evaluation of neonatal bone maturity. METHODS: We examined sonographically 256 neonates within 24 h of birth. Gestational ages ranged from 36 to 42 weeks (mean: 39.4; median: 40). Birth weights ranged from 1,945 to 5,000 g (mean: 3,175; median: 3,180). The distal femoral epiphysis was imaged on the coronal plane sonogram of the distal femur with the knee at 90 degrees flexion and the distal femoral epiphysis maximal height was recorded. The acetabulum was imaged using Graf's method in the coronal plane image and the acetabular diameter recorded. RESULTS: It was found that plotting the distal femoral epiphysis against neonatal birth weight and gestational age provided a simple method for assessing the bone maturity. According to our study, a neonate can be regarded as bone maturity percentile X when plotting distal femoral epiphysis height or acetabulum diameter against birth weight and gestational age or when averaging the four readings. CONCLUSIONS: We suggest performing sonography of the distal femoral epiphysis as a bedside tool for the assessment of skeletal maturity in newborns.  相似文献   
16.
17.
18.
19.
Predatory bacteria seek and consume other live bacteria. Although belonging to taxonomically diverse groups, relatively few bacterial predator species are known. Consequently, it is difficult to assess the impact of predation within the bacterial realm. As no genetic signatures distinguishing them from non-predatory bacteria are known, genomic resources cannot be exploited to uncover novel predators. In order to identify genes specific to predatory bacteria, we developed a bioinformatic tool called DiffGene. This tool automatically identifies marker genes that are specific to phenotypic or taxonomic groups, by mapping the complete gene content of all available fully-sequenced genomes for the presence/absence of each gene in each genome. A putative ‘predator region’ of ~60 amino acids in the tryptophan 2,3-dioxygenase (TDO) protein was found to probably be a predator-specific marker. This region is found in all known obligate predator and a few facultative predator genomes, and is absent from most facultative predators and all non-predatory bacteria. We designed PCR primers that uniquely amplify a ~180bp-long sequence within the predators’ TDO gene, and validated them in monocultures as well as in metagenetic analysis of environmental wastewater samples. This marker, in addition to its usage in predator identification and phylogenetics, may finally permit reliable enumeration and cataloguing of predatory bacteria from environmental samples, as well as uncovering novel predators.  相似文献   
20.
It is well established that the vast majority of proteins of all taxonomical groups and species are initiated by an AUG codon, translated into the amino acid methionine (Met). Many attempts were made to evaluate the importance of the sequences surrounding the initiation codon, mostly focusing on the RNA sequence. However, the role and importance of the amino acids following the initiating Met residue were rarely investigated, mostly in bacteria and fungi. Herein, we computationally examined the protein sequences of all major taxonomical groups represented in the Swiss-Prot database, and evaluated the preference of each group to specific amino acids at the positions directly following the initial Met. The results indicate that there is a species-specific preference for the second amino acid of the majority of protein sequences. Interestingly, the preference for a certain amino acid at the second position changes throughout evolution from lysine in prokaryotes, through serine in lower eukaryotes, to alanine in higher plants and animals.  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