全文获取类型
收费全文 | 386篇 |
免费 | 11篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 7篇 |
教育与普及 | 2篇 |
理论与方法论 | 4篇 |
现状及发展 | 79篇 |
研究方法 | 90篇 |
综合类 | 212篇 |
自然研究 | 5篇 |
出版年
2022年 | 1篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 10篇 |
2017年 | 8篇 |
2016年 | 7篇 |
2015年 | 4篇 |
2014年 | 11篇 |
2013年 | 5篇 |
2012年 | 50篇 |
2011年 | 54篇 |
2010年 | 23篇 |
2009年 | 6篇 |
2008年 | 35篇 |
2007年 | 32篇 |
2006年 | 34篇 |
2005年 | 26篇 |
2004年 | 26篇 |
2003年 | 17篇 |
2002年 | 26篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1996年 | 1篇 |
1994年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1975年 | 3篇 |
1973年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有399条查询结果,搜索用时 15 毫秒
71.
Global variation in copy number in the human genome 总被引:3,自引:0,他引:3
Redon R Ishikawa S Fitch KR Feuk L Perry GH Andrews TD Fiegler H Shapero MH Carson AR Chen W Cho EK Dallaire S Freeman JL González JR Gratacòs M Huang J Kalaitzopoulos D Komura D MacDonald JR Marshall CR Mei R Montgomery L Nishimura K Okamura K Shen F Somerville MJ Tchinda J Valsesia A Woodwark C Yang F Zhang J Zerjal T Zhang J Armengol L Conrad DF Estivill X Tyler-Smith C Carter NP Aburatani H Lee C Jones KW Scherer SW Hurles ME 《Nature》2006,444(7118):444-454
Copy number variation (CNV) of DNA sequences is functionally significant but has yet to be fully ascertained. We have constructed a first-generation CNV map of the human genome through the study of 270 individuals from four populations with ancestry in Europe, Africa or Asia (the HapMap collection). DNA from these individuals was screened for CNV using two complementary technologies: single-nucleotide polymorphism (SNP) genotyping arrays, and clone-based comparative genomic hybridization. A total of 1,447 copy number variable regions (CNVRs), which can encompass overlapping or adjacent gains or losses, covering 360 megabases (12% of the genome) were identified in these populations. These CNVRs contained hundreds of genes, disease loci, functional elements and segmental duplications. Notably, the CNVRs encompassed more nucleotide content per genome than SNPs, underscoring the importance of CNV in genetic diversity and evolution. The data obtained delineate linkage disequilibrium patterns for many CNVs, and reveal marked variation in copy number among populations. We also demonstrate the utility of this resource for genetic disease studies. 相似文献
72.
The separation of the effects of environmental variability from the impacts of fishing has been elusive, but is essential for sound fisheries management. We distinguish environmental effects from fishing effects by comparing the temporal variability of exploited versus unexploited fish stocks living in the same environments. Using the unique suite of 50-year-long larval fish surveys from the California Cooperative Oceanic Fisheries Investigations we analyse fishing as a treatment effect in a long-term ecological experiment. Here we present evidence from the marine environment that exploited species exhibit higher temporal variability in abundance than unexploited species. This remains true after accounting for life-history effects, abundance, ecological traits and phylogeny. The increased variability of exploited populations is probably caused by fishery-induced truncation of the age structure, which reduces the capacity of populations to buffer environmental events. Therefore, to avoid collapse, fisheries must be managed not only to sustain the total viable biomass but also to prevent the significant truncation of age structure. The double jeopardy of fishing to potentially deplete stock sizes and, more immediately, to amplify the peaks and valleys of population variability, calls for a precautionary management approach. 相似文献
73.
74.
A geometric measure of dark energy with pairs of galaxies 总被引:1,自引:0,他引:1
Observations indicate that the expansion of the Universe is accelerating, which is attributed to a ‘dark energy’ component that opposes gravity. There is a purely geometric test of the expansion of the Universe (the Alcock–Paczynski test), which would provide an independent way of investigating the abundance (Ω(X)) and equation of state (W(X)) of dark energy. It is based on an analysis of the geometrical distortions expected from comparing the real-space and redshift-space shape of distant cosmic structures, but it has proved difficult to implement. Here we report an analysis of the symmetry properties of distant pairs of galaxies from archival data. This allows us to determine that the Universe is flat. By alternately fixing its spatial geometry at Ω(k)≡0 and the dark energy equation-of-state parameter at W(X)≡-1, and using the results of baryon acoustic oscillations, we can establish at the 68.3% confidence level that and -0.85>W(X)>-1.12 and 0.60<Ω(X)<0.80. 相似文献
75.
Christian P. Kubicek Monika Komon-Zelazowska Irina S. Druzhinina 《浙江大学学报(自然科学英文版)》2008,9(10):753-763
Hypocrea/Trichoderma is a genus of soil-borne or wood-decaying fungi containing members important to mankind as producers of industrial enzymes and biocontrol agents against plant pathogens, but also as opportunistic pathogens of immunocompromised humans and animals, while others can cause damage to cultivated mushroom. With the recent advent of a reliable, BarCode-aided identification system for all known taxa of Trichoderma and Hypocrea, it became now possible to study some of the biological fundamentals of the diversity in this fungal genus in more detail. In this article, we will therefore review recent progress in (1) the understanding of the geographic distribution of individual taxa; (2) mechanisms of speciation leading to development of mushroom diseases and facultative human mycoses; and (3) the possible correlation of specific traits of secondary metabolism and molecular phylogeny. 相似文献
76.
Greenberg JI Shields DJ Barillas SG Acevedo LM Murphy E Huang J Scheppke L Stockmann C Johnson RS Angle N Cheresh DA 《Nature》2008,456(7223):809-813
Angiogenesis does not only depend on endothelial cell invasion and proliferation: it also requires pericyte coverage of vascular sprouts for vessel stabilization. These processes are coordinated by vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) through their cognate receptors on endothelial cells and vascular smooth muscle cells (VSMCs), respectively. PDGF induces neovascularization by priming VSMCs/pericytes to release pro-angiogenic mediators. Although VEGF directly stimulates endothelial cell proliferation and migration, its role in pericyte biology is less clear. Here we define a role for VEGF as an inhibitor of neovascularization on the basis of its capacity to disrupt VSMC function. Specifically, under conditions of PDGF-mediated angiogenesis, VEGF ablates pericyte coverage of nascent vascular sprouts, leading to vessel destabilization. At the molecular level, VEGF-mediated activation of VEGF-R2 suppresses PDGF-Rbeta signalling in VSMCs through the assembly of a previously undescribed receptor complex consisting of PDGF-Rbeta and VEGF-R2. Inhibition of VEGF-R2 not only prevents assembly of this receptor complex but also restores angiogenesis in tissues exposed to both VEGF and PDGF. Finally, genetic deletion of tumour cell VEGF disrupts PDGF-Rbeta/VEGF-R2 complex formation and increases tumour vessel maturation. These findings underscore the importance of VSMCs/pericytes in neovascularization and reveal a dichotomous role for VEGF and VEGF-R2 signalling as both a promoter of endothelial cell function and a negative regulator of VSMCs and vessel maturation. 相似文献
77.
Why fishing magnifies fluctuations in fish abundance 总被引:1,自引:0,他引:1
Anderson CN Hsieh CH Sandin SA Hewitt R Hollowed A Beddington J May RM Sugihara G 《Nature》2008,452(7189):835-839
It is now clear that fished populations can fluctuate more than unharvested stocks. However, it is not clear why. Here we distinguish among three major competing mechanisms for this phenomenon, by using the 50-year California Cooperative Oceanic Fisheries Investigations (CalCOFI) larval fish record. First, variable fishing pressure directly increases variability in exploited populations. Second, commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly. Third, age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates. We find no evidence for the first hypothesis, limited evidence for the second and strong evidence for the third. Therefore, in California Current fisheries, increased temporal variability in the population does not arise from variable exploitation, nor does it reflect direct environmental tracking. More fundamentally, it arises from increased instability in dynamics. This finding has implications for resource management as an empirical example of how selective harvesting can alter the basic dynamics of exploited populations, and lead to unstable booms and busts that can precede systematic declines in stock levels. 相似文献
78.
Shotgun bisulphite sequencing of the Arabidopsis genome reveals DNA methylation patterning 总被引:10,自引:0,他引:10
Cokus SJ Feng S Zhang X Chen Z Merriman B Haudenschild CD Pradhan S Nelson SF Pellegrini M Jacobsen SE 《Nature》2008,452(7184):215-219
79.
从船侧倾斜拍摄图像中提取海冰密集度的方法 总被引:2,自引:0,他引:2
为消除提取海冰密集度时因摄影镜头倾斜造成图像变形可能引起的误差,基于摄影测量学原理,对倾斜拍摄引起的图像变形进行修正,建立从倾斜拍摄的图像中提取海冰密集度的精确方法和相应的简化算法.以2006年德国南极威德尔海冬季考察中获取的图像为例,讨论了现场观测时的系统标定方法和不同算法的计算结果.对比分析验证了图像变形修正的必要性,说明简化算法能够同时保证海冰密集度计算的精度和效率. 相似文献
80.
The superstatistics concept is a useful statistical method to describe inhomogeneous complex systems for which a system parameter βfluctuatesonalargespatio-temporal scale. Inthispaper weanalyze ameasuredtimeseriesofwindspeed fluctuationsandextract the superstatistical distribution function f(β) directly from the data. We construct suitable Langevin and Fokker-Planck models with a position dependent β-field and show that they reduce to standard type of superstatistics in the overdamped limit. 相似文献