全文获取类型
收费全文 | 1062篇 |
免费 | 110篇 |
专业分类
1172篇 |
出版年
2021年 | 7篇 |
2020年 | 5篇 |
2019年 | 5篇 |
2018年 | 14篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 23篇 |
2014年 | 17篇 |
2013年 | 33篇 |
2012年 | 59篇 |
2011年 | 52篇 |
2010年 | 29篇 |
2009年 | 40篇 |
2008年 | 58篇 |
2007年 | 51篇 |
2006年 | 44篇 |
2005年 | 50篇 |
2004年 | 34篇 |
2003年 | 37篇 |
2002年 | 41篇 |
2001年 | 47篇 |
2000年 | 63篇 |
1999年 | 62篇 |
1998年 | 15篇 |
1997年 | 19篇 |
1996年 | 15篇 |
1995年 | 11篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 29篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 24篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 9篇 |
1985年 | 23篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 11篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1967年 | 5篇 |
1966年 | 8篇 |
排序方式: 共有1172条查询结果,搜索用时 0 毫秒
81.
Dobson R 《BMJ (Clinical research ed.)》2000,320(7240):958
82.
83.
Dobson R 《BMJ (Clinical research ed.)》2000,320(7228):138
84.
Dobson R 《BMJ (Clinical research ed.)》2000,321(7274):1431
85.
Evolutionary theory suggests that mating systems should have substantial effects on gene dynamics of local populations. In polygynous species, local 'breeding groups' may produce significant genetic structure, due to genetic differences among groups, and rate of loss of genetic variation from such populations may be considerably slowed. We examined possible influences of the variable mating system and family group structure on genetic properties of a population of plateau pikas (Ochotona curzoniae). Pika gene dynamics were examined via F-statistics and effective population sizes (N(e)), calculated from genetic correlations within and among individuals and families. Genetic correlations were estimated from mating patterns, population demography, and dispersal patterns. Substantial genetic structure within the population was indicated by a strongly positive F(LS). Genetic influence of natal dispersal out of pika families was indicated by a strongly negative inbreeding statistic (F(IL)=-0.34). Effective size of the population was not greatly different from the census population, whereas a traditional estimate of effective size of the population was much lower, indicating that the family structure of the pikas results in a slowed loss of genetic variation over time. Thus, even though mating patterns of plateau pikas were variable, family structure had a strong influence on pika gene dynamics. 相似文献
86.
The ecology and evolution of pollen odors 总被引:9,自引:0,他引:9
The literature is reviewed and new evidence presented that pollen produces odors, which serve multiple functions in pollination and defense. Pollen odor, which originates from pollenkitt, comprises volatiles that belong to the same chemical classes found in flower scents, that are in species-specific mixtures, and that contrast with odors of other floral parts. Pollen can also take up volatiles from surrounding floral odors, but this adsorption is selective and varies among species. Pollen odors are more pronounced in insect- than bird- or wind-pollinated plants, suggesting that volatile emission evolved in part under selection to attract pollinators. Pollen-feeding insects can perceive pollen odor and use it to discriminate between different pollen types and host plants. Pollen odor influences bee foraging, including the location of pollen sources, discrimination of flowers with different amounts of pollen, and hostplant recognition by pollen-specialist species. In the few wind-pollinated plants studied, odors of male flowers or pollen are comparatively high in -methyl alcohols and ketones; these volatiles may serve in pollen defense, with some known to repel insects. Pollen odor often includes chemicals with documented defense activity, which is probably aimed mainly at nonpollinator pollen-feeding insects and pathogens; an involvement in pollen allelopathy is also possible. Pollen volatiles comprise chemically diverse compounds that may play multiple roles, and their emission in pollen odor undoubtedly evolved under the principle, and often conflicting, selective pressures to both protect the male gametophyte and increase its dispersal by animals. 相似文献
87.
Characterization of the oligomeric states of insulin in self-assembly and amyloid fibril formation by mass spectrometry 总被引:8,自引:0,他引:8 下载免费PDF全文
Nettleton EJ Tito P Sunde M Bouchard M Dobson CM Robinson CV 《Biophysical journal》2000,79(2):1053-1065
The self-assembly and aggregation of insulin molecules has been investigated by means of nanoflow electrospray mass spectrometry. Hexamers of insulin containing predominantly two, but up to four, Zn(2+) ions were observed in the gas phase when solutions at pH 4.0 were examined. At pH 3.3, in the absence of Zn(2+), dimers and tetramers are observed. Spectra obtained from solutions of insulin at millimolar concentrations at pH 2.0, conditions under which insulin is known to aggregate in solution, showed signals from a range of higher oligomers. Clusters containing up to 12 molecules could be detected in the gas phase. Hydrogen exchange measurements show that in solution these higher oligomers are in rapid equilibrium with monomeric insulin. At elevated temperatures, under conditions where insulin rapidly forms amyloid fibrils, the concentration of soluble higher oligomers was found to decrease with time yielding insoluble high molecular weight aggregates and then fibrils. The fibrils formed were examined by electron microscopy and the results show that the amorphous aggregates formed initially are converted to twisted, unbranched fibrils containing several protofilaments. Fourier transform infrared spectroscopy shows that both the soluble form of insulin and the initial aggregates are predominantly helical, but that formation of beta-sheet structure occurs simultaneously with the appearance of well-defined fibrils. 相似文献
88.
Ultrastructural organization of amyloid fibrils by atomic force microscopy 总被引:6,自引:0,他引:6 下载免费PDF全文
Chamberlain AK MacPhee CE Zurdo J Morozova-Roche LA Hill HA Dobson CM Davis JJ 《Biophysical journal》2000,79(6):3282-3293
Atomic force microscopy has been employed to investigate the structural organization of amyloid fibrils produced in vitro from three very different polypeptide sequences. The systems investigated are a 10-residue peptide derived from the sequence of transthyretin, the 90-residue SH3 domain of bovine phosphatidylinositol-3'-kinase, and human wild-type lysozyme, a 130-residue protein containing four disulfide bridges. The results demonstrate distinct similarities between the structures formed by the different classes of fibrils despite the contrasting nature of the polypeptide species involved. SH3 and lysozyme fibrils consist typically of four protofilaments, exhibiting a left-handed twist along the fibril axis. The substructure of TTR(10-19) fibrils is not resolved by atomic force microscopy and their uniform appearance is suggestive of a regular self-association of very thin filaments. We propose that the exact number and orientation of protofilaments within amyloid fibrils is dictated by packing of the regions of the polypeptide chains that are not directly involved in formation of the cross-beta core of the fibrils. The results obtained for these proteins, none of which is directly associated with any human disease, are closely similar to those of disease-related amyloid fibrils, supporting the concept that amyloid is a generic structure of polypeptide chains. The detailed architecture of an individual fibril, however, depends on the manner in which the protofilaments assemble into the fibrillar structure, which in turn is dependent on the sequence of the polypeptide and the conditions under which the fibril is formed. 相似文献
89.
Hawks J Oh S Hunley K Dobson S Cabana G Dayalu P Wolpoff MH 《Journal of human evolution》2000,39(1):1-22
This analysis investigates the ancestry of a single modern human specimen from Australia, WLH-50 (Thorne et al., in preparation; Webb, 1989). Evaluating its ancestry is important to our understanding of modern human origins in Australasia because the prevailing models of human origins make different predictions for the ancestry of this specimen, and others like it. Some authors believe in the validity of a complete replacement theory and propose that modern humans in Australasia descended solely from earlier modern human populations found in Late Pleistocene Africa and the Levant. These ancestral modern populations are believed to have completely replaced other archaic human populations, including the Ngandong hominids of Indonesia. According to this recent African origin theory, the archaic humans from Indonesia are classified as Homo erectus, a different evolutionary species that could not have contributed to the ancestry of modern Australasians. Therefore this theory of complete replacement makes clear predictions concerning the ancestry of the specimen WLH-50. We tested these predictions using two methods: a discriminant analysis of metric data for three samples that are potential ancestors of WLH-50 (Ngandong, Late Pleistocene Africans, Levant hominids from Skhul and Qafzeh) and a pairwise difference analysis of nonmetric data for individuals within these samples. The results of these procedures provide an unambiguous refutation of a model of complete replacement within this region, and indicate that the Ngandong hominids or a population like them may have contributed significantly to the ancestry of WLH-50. We therefore contend that Ngandong hominids should be classified within the evolutionary species, Homo sapiens. The Multiregional model of human evolution has the expectation that Australasian ancestry is in all three of the potentially ancestral groups and best explains modern Australasian origins. 相似文献
90.
The Nova Scotia (type D) form of Niemann-Pick disease is caused by a G3097-->T transversion in NPC1.
W L Greer D C Riddell T L Gillan G S Girouard S M Sparrow D M Byers M J Dobson P E Neumann 《American journal of human genetics》1998,63(1):52
Niemann-Pick type D (NPD) disease is a progressive neurodegenerative disorder characterized by the accumulation of tissue cholesterol and sphingomyelin. This disorder is relatively common in southwestern Nova Scotia, because of a founder effect. Our previous studies, using classic linkage analysis of this large extended kindred, defined the critical gene region to a 13-cM chromosome segment between D18S40 and D18S66. A recently isolated gene from this region, NPC1, is mutated in the majority of patients with Niemann-Pick type C disease. We have identified a point mutation within this gene (G3097-->T; Gly992-->Trp) that shows complete linkage disequilibrium with NPD, confirming that NPD is an allelic variant of NPC1. 相似文献