全文获取类型
收费全文 | 124篇 |
免费 | 0篇 |
专业分类
系统科学 | 1篇 |
教育与普及 | 1篇 |
现状及发展 | 28篇 |
研究方法 | 10篇 |
综合类 | 81篇 |
自然研究 | 3篇 |
出版年
2018年 | 1篇 |
2017年 | 2篇 |
2015年 | 2篇 |
2012年 | 5篇 |
2011年 | 7篇 |
2008年 | 8篇 |
2007年 | 5篇 |
2006年 | 6篇 |
2005年 | 10篇 |
2004年 | 11篇 |
2003年 | 3篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1996年 | 1篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1964年 | 2篇 |
1958年 | 1篇 |
1946年 | 1篇 |
排序方式: 共有124条查询结果,搜索用时 546 毫秒
31.
32.
33.
番木瓜环斑病毒复制酶基因的克隆和序列分析 总被引:6,自引:0,他引:6
用RT-PCR技术从PRV-AL中分离到复制酶(RP)基因,将基因克隆进载体pUC18,用双脱氧链终止法测定了基因序列,表明其全长为1602bp,与国内外报道的HA5-1、YK和Sm的RP基因相比,同源性分别达82.80%,95.07%和91.83%. 相似文献
34.
Meiotic gene conversion has an important role in allele diversification and in the homogenization of gene and other repeat DNA sequence families, sometimes with pathological consequences. But little is known about the dynamics of gene conversion in humans and its relationship to meiotic crossover. We therefore developed screening and selection methods to characterize sperm conversions in two meiotic crossover hot spots in the major histocompatibility complex (MHC) and one in the sex chromosomal pseudoautosomal pairing region PAR1 (ref. 9). All three hot spots are active in gene conversion and crossover. Conversion tracts are short and define a steep bidirectional gradient centered at the peak of crossover activity, consistent with crossovers and conversions being produced by the same recombination-initiating events. These initiations seem to be spread over a narrow zone, rather than occurring at a single site, and seem preferentially to yield conversions rather than crossovers. Crossover breakpoints are more broadly diffused than conversion breakpoints, suggesting either differences between conversion and crossover processing after initiation or the existence of a quality control checkpoint at which short interactions between homologous chromosomes are preferentially aborted as conversions. 相似文献
35.
36.
37.
38.
39.
Perry JN Firbank LG Champion GT Clark SJ Heard MS May MJ Hawes C Squire GR Rothery P Woiwod IP Pidgeon JD 《Nature》2004,428(6980):313-316
The UK Farm-Scale Evaluations (FSE) compared the effects on biodiversity of management of genetically modified herbicide-tolerant (GMHT) spring-sown crops with conventional crop management. The FSE reported larger weed abundance under GMHT management for fodder maize, one of three crops studied. Increased seed production may be important for the long-term persistence of these arable weeds and may benefit invertebrates, small mammals and seed-eating birds. In three-quarters of FSE maize fields, growers used atrazine on the conventionally managed half, reflecting contemporary commercial practice. Withdrawal of the triazine herbicides atrazine, simazine and cyanazine from approved lists of EU chemicals could therefore reduce or even reverse the reported benefits of GMHT maize. Here we analyse effects of applications of triazine herbicides in conventional maize regimes on key indicators, using FSE data. Weed abundances were decreased greatly relative to all other regimes whenever atrazine was applied before weeds emerged. Here, we forecast weed abundances in post-triazine herbicide regimes. We predict weed abundances under future conventional herbicide management to be considerably larger than that for atrazine used before weeds emerged, but still smaller than for the four FSE sites analysed that used only non-triazine herbicides. Our overall conclusion is that the comparative benefits for arable biodiversity of GMHT maize cropping would be reduced, but not eliminated, by the withdrawal of triazines from conventional maize cropping. 相似文献
40.
Modelling vaccination strategies against foot-and-mouth disease 总被引:1,自引:0,他引:1
Vaccination has proved a powerful defence against a range of infectious diseases of humans and animals. However, its potential to control major epidemics of foot-and-mouth disease (FMD) in livestock is contentious. Using an individual farm-based model, we consider either national prophylactic vaccination campaigns in advance of an outbreak, or combinations of reactive vaccination and culling strategies during an epidemic. Consistent with standard epidemiological theory, mass prophylactic vaccination could reduce greatly the potential for a major epidemic, while the targeting of high-risk farms increases efficiency. Given sufficient resources and preparation, a combination of reactive vaccination and culling might control ongoing epidemics. We also explore a reactive strategy, 'predictive' vaccination, which targets key spatial transmission loci and can reduce markedly the long tail that characterizes many FMD epidemics. These analyses have broader implications for the control of human and livestock infectious diseases in heterogeneous spatial landscapes. 相似文献