全文获取类型
收费全文 | 125篇 |
免费 | 0篇 |
专业分类
系统科学 | 3篇 |
教育与普及 | 1篇 |
现状及发展 | 26篇 |
研究方法 | 5篇 |
综合类 | 89篇 |
自然研究 | 1篇 |
出版年
2018年 | 1篇 |
2017年 | 2篇 |
2016年 | 1篇 |
2013年 | 1篇 |
2012年 | 8篇 |
2011年 | 3篇 |
2010年 | 1篇 |
2008年 | 4篇 |
2007年 | 7篇 |
2006年 | 8篇 |
2005年 | 4篇 |
2004年 | 4篇 |
2003年 | 6篇 |
2002年 | 5篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1980年 | 1篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1974年 | 8篇 |
1973年 | 1篇 |
1971年 | 4篇 |
1970年 | 6篇 |
1968年 | 1篇 |
1966年 | 4篇 |
1965年 | 2篇 |
1963年 | 1篇 |
1960年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有125条查询结果,搜索用时 0 毫秒
91.
Induction of angiogenesis during the transition from hyperplasia to neoplasia 总被引:147,自引:0,他引:147
It is now well established that unrestricted growth of tumours is dependent upon angiogenesis. Previous studies on tumour growth, however, have not revealed when or how the transition to an angiogenic state occurs during early tumour development. The advent of transgenic mice carrying oncogenes that reproducibly elicit tumours of specific cell types is providing a new format for studying multi-step tumorigenesis. In one of these models, transgenic mice expressing an oncogene in the beta-cells of the pancreatic islets heritably recapitulate a progression from normality to hyperplasia to neoplasia. We report here that angiogenic activity first appears in a subset of hyperplastic islets before the onset of tumour formation. A novel in vitro assay confirms that hyperplasia per se does not obligate angiogenesis. Rather, a few hyperplastic islets become angiogenic in vitro at a time when such islets are neovascularized in vivo and at a frequency that correlates closely with subsequent tumour incidence. These findings suggest that induction of angiogenesis is an important step in carcinogenesis. 相似文献
92.
Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2 总被引:18,自引:0,他引:18
Photosynthesis by marine phytoplankton in the Southern Ocean, and the associated uptake of carbon, is thought to be currently limited by the availability of iron. One implication of this limitation is that a larger iron supply to the region in glacial times could have stimulated algal photosynthesis, leading to lower concentrations of atmospheric CO2. Similarly, it has been proposed that artificial iron fertilization of the oceans might increase future carbon sequestration. Here we report data from a whole-ecosystem test of the iron-limitation hypothesis in the Southern Ocean, which show that surface uptake of atmospheric CO2 and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbon and ocean nutrients, forced with atmospheric iron fluxes to the region derived from the Vostok ice-core dust record. During glacial periods, predicted magnitudes and timings of atmospheric CO2 changes match ice-core records well. At glacial terminations, the model suggests that forcing of Southern Ocean biota by iron caused the initial approximately 40 p.p.m. of glacial-interglacial CO2 change, but other mechanisms must have accounted for the remaining 40 p.p.m. increase. The experiment also confirms that modest sequestration of atmospheric CO2 by artificial additions of iron to the Southern Ocean is in principle possible, although the period and geographical extent over which sequestration would be effective remain poorly known. 相似文献
93.
Sandilands A Terron-Kwiatkowski A Hull PR O'Regan GM Clayton TH Watson RM Carrick T Evans AT Liao H Zhao Y Campbell LE Schmuth M Gruber R Janecke AR Elias PM van Steensel MA Nagtzaam I van Geel M Steijlen PM Munro CS Bradley DG Palmer CN Smith FJ McLean WH Irvine AD 《Nature genetics》2007,39(5):650-654
We recently reported two common filaggrin (FLG) null mutations that cause ichthyosis vulgaris and predispose to eczema and secondary allergic diseases. We show here that these common European mutations are ancestral variants carried on conserved haplotypes. To facilitate comprehensive analysis of other populations, we report a strategy for full sequencing of this large, highly repetitive gene, and we describe 15 variants, including seven that are prevalent. All the variants are either nonsense or frameshift mutations that, in representative cases, resulted in loss of filaggrin production in the epidermis. In an Irish case-control study, the five most common European mutations showed a strong association with moderate-to-severe childhood eczema (chi2 test: P = 2.12 x 10(-51); Fisher's exact test: heterozygote odds ratio (OR) = 7.44 (95% confidence interval (c.i.) = 4.9-11.3), and homozygote OR = 151 (95% c.i. = 20-1,136)). We found three additional rare null mutations in this case series, suggesting that the genetic architecture of filaggrin-related atopic dermatitis consists of both prevalent and rare risk alleles. 相似文献
94.
今天的基因生物学充满了活力。它在有关细胞、酵母菌等单细胞生物以至复杂的人脑的生命科学中所占的份量与年俱增。这种奇妙的生物学热,在我最初踏人遗传学界时是完全无法想象的。1948年的生物学全是描述性的,处于科学标杆的最低部位,标杆的顶端是物理学。那时,爱因斯坦关于质.能互变的划时代理论已转化成为原子的巨大威力。如果不加约束的话,用它制成的武器完全可以摧毁人类文明的全部建筑物。因此,叙年代后期的物理学家们在因其使原子变成对社会有用之物而受人尊敬的同时,又担心他们发明的玩意儿一旦落入邪恶之手时,会干出何等蠢… 相似文献
95.
96.
Ding L Ley TJ Larson DE Miller CA Koboldt DC Welch JS Ritchey JK Young MA Lamprecht T McLellan MD McMichael JF Wallis JW Lu C Shen D Harris CC Dooling DJ Fulton RS Fulton LL Chen K Schmidt H Kalicki-Veizer J Magrini VJ Cook L McGrath SD Vickery TL Wendl MC Heath S Watson MA Link DC Tomasson MH Shannon WD Payton JE Kulkarni S Westervelt P Walter MJ Graubert TA Mardis ER Wilson RK DiPersio JF 《Nature》2012,481(7382):506-510
Most patients with acute myeloid leukaemia (AML) die from progressive disease after relapse, which is associated with clonal evolution at the cytogenetic level. To determine the mutational spectrum associated with relapse, we sequenced the primary tumour and relapse genomes from eight AML patients, and validated hundreds of somatic mutations using deep sequencing; this allowed us to define clonality and clonal evolution patterns precisely at relapse. In addition to discovering novel, recurrently mutated genes (for example, WAC, SMC3, DIS3, DDX41 and DAXX) in AML, we also found two major clonal evolution patterns during AML relapse: (1) the founding clone in the primary tumour gained mutations and evolved into the relapse clone, or (2) a subclone of the founding clone survived initial therapy, gained additional mutations and expanded at relapse. In all cases, chemotherapy failed to eradicate the founding clone. The comparison of relapse-specific versus primary tumour mutations in all eight cases revealed an increase in transversions, probably due to DNA damage caused by cytotoxic chemotherapy. These data demonstrate that AML relapse is associated with the addition of new mutations and clonal evolution, which is shaped, in part, by the chemotherapy that the patients receive to establish and maintain remissions. 相似文献
97.
Maune BM Borselli MG Huang B Ladd TD Deelman PW Holabird KS Kiselev AA Alvarado-Rodriguez I Ross RS Schmitz AE Sokolich M Watson CA Gyure MF Hunter AT 《Nature》2012,481(7381):344-347
Silicon is more than the dominant material in the conventional microelectronics industry: it also has potential as a host material for emerging quantum information technologies. Standard fabrication techniques already allow the isolation of single electron spins in silicon transistor-like devices. Although this is also possible in other materials, silicon-based systems have the advantage of interacting more weakly with nuclear spins. Reducing such interactions is important for the control of spin quantum bits because nuclear fluctuations limit quantum phase coherence, as seen in recent experiments in GaAs-based quantum dots. Advances in reducing nuclear decoherence effects by means of complex control still result in coherence times much shorter than those seen in experiments on large ensembles of impurity-bound electrons in bulk silicon crystals. Here we report coherent control of electron spins in two coupled quantum dots in an undoped Si/SiGe heterostructure and show that this system has a nuclei-induced dephasing time of 360 nanoseconds, which is an increase by nearly two orders of magnitude over similar measurements in GaAs-based quantum dots. The degree of phase coherence observed, combined with fast, gated electrical initialization, read-out and control, should motivate future development of silicon-based quantum information processors. 相似文献
98.
Micro-CT imaging elucidates jaw and pharynx structure in 14 nominal taxa belonging to Chrysopetalinae, Dysponetinae and Calamyzinae (Chrysopetalidae: Annelida). Systematic evaluation of chrysopetalid jaw form in each taxon is used to compare inter-generic relationships within and between each subfamily and with other polychaete families possessing lateral pairs of jaws. Jaw morphology proves diagnostic at all levels including agreement with current molecular phylogenetic analyses of the Chrysopetalidae. The greatest diversity of pharynx and jaw form, associated with sensory body characters, is found amongst taxa of the epibenthic Chrysopetalinae. It is hypothesized that the paired stylet jaws interlock to pierce prey tissue, and, in concert with a highly muscular pharynx, suck out prey fluids that pass down the internal groove of the stylets. Three generic groups are identified based on congruence of jaw form, and novel buccal structures are revealed in some taxa: a pharyngeal calcareous ‘ring’ and extensive pharyngeal glands. A continuum across free-living and facultative symbiont lifestyles is represented across all Chrysopetalinae. Dysponetinae comprises free-living, very small individuals that exhibit a meiofaunal lifestyle. All species possess one type of simple, tanned rod-like stylet jaws and certain simplified body structures. Calamyzinae species possess polyphyletic jaw forms and grades of radically simplified sensory morphologies. Free-living, bacteriovore calamyzins inhabit extreme chemosynthetic habitats and lack jaws, or possess modified jaws composed of an anterior platelet and posterior grooved jaw, present in the larvae and lost in the adult. New observations of an ectoparasitic Calamyzinae include details of a specialized mouth opening and stylet jaw present in a species that exhibits cryptic coloration, while grooved jaws are revealed for the first time in an obligate symbiont calamyzin. The grooved jaw form is considered a non-homoplasious synapomorphy that supports the monophyly of the family Chrysopetalidae. 相似文献
99.
This paper uses forecast combination methods to forecast output growth in a seven‐country quarterly economic data set covering 1959–1999, with up to 73 predictors per country. Although the forecasts based on individual predictors are unstable over time and across countries, and on average perform worse than an autoregressive benchmark, the combination forecasts often improve upon autoregressive forecasts. Despite the unstable performance of the constituent forecasts, the most successful combination forecasts, like the mean, are the least sensitive to the recent performance of the individual forecasts. While consistent with other evidence on the success of simple combination forecasts, this finding is difficult to explain using the theory of combination forecasting in a stationary environment. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
100.
Blaydon DC Ishii Y O'Toole EA Unsworth HC Teh MT Rüschendorf F Sinclair C Hopsu-Havu VK Tidman N Moss C Watson R de Berker D Wajid M Christiano AM Kelsell DP 《Nature genetics》2006,38(11):1245-1247
Anonychia and hyponychia congenita (OMIM 206800) are rare autosomal recessive conditions in which the only presenting phenotype is the absence or severe hypoplasia of all fingernails and toenails. After determining linkage to chromosome 20p13, we identified homozygous or compound heterozygous mutations in the gene encoding R-spondin 4 (RSPO4), a secreted protein implicated in Wnt signaling, in eight affected families. Rspo4 expression was specifically localized to developing mouse nail mesenchyme at embryonic day 15.5, suggesting a crucial role in nail morphogenesis. 相似文献