全文获取类型
收费全文 | 114篇 |
免费 | 1篇 |
专业分类
系统科学 | 8篇 |
理论与方法论 | 4篇 |
现状及发展 | 30篇 |
研究方法 | 18篇 |
综合类 | 47篇 |
自然研究 | 8篇 |
出版年
2022年 | 1篇 |
2021年 | 1篇 |
2019年 | 5篇 |
2018年 | 11篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 4篇 |
2014年 | 3篇 |
2013年 | 6篇 |
2012年 | 9篇 |
2011年 | 10篇 |
2010年 | 2篇 |
2009年 | 5篇 |
2008年 | 5篇 |
2007年 | 5篇 |
2006年 | 5篇 |
2005年 | 6篇 |
2004年 | 12篇 |
2003年 | 5篇 |
2002年 | 8篇 |
2001年 | 1篇 |
1989年 | 1篇 |
1984年 | 2篇 |
1982年 | 1篇 |
排序方式: 共有115条查询结果,搜索用时 109 毫秒
11.
12.
Increasing dominance of large lianas in Amazonian forests 总被引:1,自引:0,他引:1
Phillips OL Vásquez Martínez R Arroyo L Baker TR Killeen T Lewis SL Malhi Y Monteagudo Mendoza A Neill D Núñez Vargas P Alexiades M Cerón C Di Fiore A Erwin T Jardim A Palacios W Saldias M Vinceti B 《Nature》2002,418(6899):770-774
Ecological orthodoxy suggests that old-growth forests should be close to dynamic equilibrium, but this view has been challenged by recent findings that neotropical forests are accumulating carbon and biomass, possibly in response to the increasing atmospheric concentrations of carbon dioxide. However, it is unclear whether the recent increase in tree biomass has been accompanied by a shift in community composition. Such changes could reduce or enhance the carbon storage potential of old-growth forests in the long term. Here we show that non-fragmented Amazon forests are experiencing a concerted increase in the density, basal area and mean size of woody climbing plants (lianas). Over the last two decades of the twentieth century the dominance of large lianas relative to trees has increased by 1.7-4.6% a year. Lianas enhance tree mortality and suppress tree growth, so their rapid increase implies that the tropical terrestrial carbon sink may shut down sooner than current models suggest. Predictions of future tropical carbon fluxes will need to account for the changing composition and dynamics of supposedly undisturbed forests. 相似文献
13.
Lincoln AJ Wickramasinghe D Stein P Schultz RM Palko ME De Miguel MP Tessarollo L Donovan PJ 《Nature genetics》2002,30(4):446-449
In a wide variety of animal species, oocyte maturation is arrested temporarily at prophase of meiosis I (ref. 1). Resumption of meiosis requires activation of cyclin-dependent kinase-1 (CDK1, p34cdc2), one component of maturation-promoting factor (MPF). The dual specificity phosphatases Cdc25a, Cdc25b and Cdc25c are activators of cyclin-dependent kinases; consequently, they are postulated to regulate cell-cycle progression in meiosis and mitosis as well as the DNA-damage response. We generated Cdc25b-deficient (Cdc25b-/-) mice and found that they are viable. As compared with wildtype cells, fibroblasts from Cdc25b-/- mice grew vigorously in culture and arrested normally in response to DNA damage. Female Cdc25b-/- mice were sterile, and Cdc25b-/- oocytes remained arrested at prophase with low MPF activity. Microinjection of wildtype Cdc25b mRNA into Cdc25b-/- oocytes caused activation of MPF and resumption of meiosis. Thus, Cdc25b-/- female mice are sterile because of permanent meiotic arrest resulting from the inability to activate MPF. Cdc25b is therefore essential for meiotic resumption in female mice. Mice lacking Cdc25b provide the first genetic model for studying the mechanisms regulating prophase arrest in vertebrates. 相似文献
14.
Peterson AT Ortega-Huerta MA Bartley J Sánchez-Cordero V Soberón J Buddemeier RH Stockwell DR 《Nature》2002,416(6881):626-629
Global climates are changing rapidly, with unexpected consequences. Because elements of biodiversity respond intimately to climate as an important driving force of distributional limitation, distributional shifts and biodiversity losses are expected. Nevertheless, in spite of modelling efforts focused on single species or entire ecosystems, a few preliminary surveys of fauna-wide effects, and evidence of climate change-mediated shifts in several species, the likely effects of climate change on species' distributions remain little known, and fauna-wide or community-level effects are almost completely unexplored. Here, using a genetic algorithm and museum specimen occurrence data, we develop ecological niche models for 1,870 species occurring in Mexico and project them onto two climate surfaces modelled for 2055. Although extinctions and drastic range reductions are predicted to be relatively few, species turnover in some local communities is predicted to be high (>40% of species), suggesting that severe ecological perturbations may result. 相似文献
15.
16.
17.
E San Miguel Salán 《Experientia》1984,40(8):861-862
Samples from 4 populations of D. melanogaster were treated with formaldehyde by the larval feeding method, and induced lethality was scored. The results showed relevant differences among the populations analyzed. 相似文献
18.
Epigenetic remodeling in colorectal cancer results in coordinate gene suppression across an entire chromosome band 总被引:8,自引:0,他引:8
We report a new mechanism in carcinogenesis involving coordinate long-range epigenetic gene silencing. Epigenetic silencing in cancer has always been envisaged as a local event silencing discrete genes. However, in this study of silencing in colorectal cancer, we found common repression of the entire 4-Mb band of chromosome 2q.14.2, associated with global methylation of histone H3 Lys9. DNA hypermethylation within the repressed genomic neighborhood was localized to three separate enriched CpG island 'suburbs', with the largest hypermethylated suburb spanning 1 Mb. These data change our understanding of epigenetic gene silencing in cancer cells: namely, epigenetic silencing can span large regions of the chromosome, and both DNA-methylated and neighboring unmethylated genes can be coordinately suppressed by global changes in histone modification. We propose that loss of gene expression can occur through long-range epigenetic silencing, with similar implications as loss of heterozygosity in cancer. 相似文献
19.
Generation of CRISPR/Cas9-mediated gene-targeted pigs via somatic cell nuclear transfer 总被引:1,自引:1,他引:0
20.