全文获取类型
收费全文 | 334篇 |
免费 | 34篇 |
出版年
2021年 | 9篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 4篇 |
2015年 | 13篇 |
2014年 | 15篇 |
2013年 | 27篇 |
2012年 | 20篇 |
2011年 | 12篇 |
2010年 | 17篇 |
2009年 | 20篇 |
2008年 | 12篇 |
2007年 | 10篇 |
2006年 | 15篇 |
2005年 | 10篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 9篇 |
2001年 | 3篇 |
2000年 | 8篇 |
1999年 | 5篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1969年 | 5篇 |
1967年 | 3篇 |
1966年 | 3篇 |
1965年 | 2篇 |
1963年 | 2篇 |
1956年 | 2篇 |
1935年 | 1篇 |
1933年 | 1篇 |
1914年 | 1篇 |
排序方式: 共有368条查询结果,搜索用时 15 毫秒
341.
342.
343.
Phosphonic acids are the only phosphorus-containing organic compounds detected in the Murchison meteorite. We earlier described
the synthesis of methyl-, hydroxymethyl-, and 1-hydroxyethyl phosphonic acids using sodium phosphite as a source of phosphite
radicals. We now show that ultraviolet irradiation of dilute aqueous solutions of acetylene in the presence of sodium phosphite
leads to the synthesis of vinyl phosphonic acid. At neutral to basic pH, vinyl phosphonic acid reacts under photochemical
conditions to produce phosphonoacetaldehyde and 2-hydroxyethyl phosphonic acid as the major products, as well as smaller yields
of 1-hydroxyethyl phosphonic acid, phosphonoacetic acid, and ethyl phosphonic acid. Of these products, phosphonoacetaldehyde
is particularly interesting as a potential precursor of prebiotic carbohydrate derivatives.
Received: 22 July 1996 / Accepted: 13 September 1996 相似文献
344.
Abstract The sulfur cycle in a microbial mat was studied by determining viable counts of sulfate-reducing bacteria, chemolithoautotrophic sulfur bacteria and anoxygenic phototrophic bacteria. All three functional groups of sulfur bacteria revealed a maximum population density in the uppermost 5 mm of the mat: 1.1 × 108 cells of sulfate reducers cm−3 sediment, 2.0 × 109 cells of chemolithoautotrophs cm−3 sediment, and 4.0 × 107 cells of anoxygenic phototrophs cm−3 sediment. Bacterial dynamics were studied by sulfate reduction rate measurements, both under anoxic conditions (dark incubation) and oxic conditions (incubation in the light), and determination of the vertical distribution of the potential rate of thiosulfate consumption under oxic conditions. Sulfate reduction rates in the top 5 mm of the sediment were 566 nmol cm−3 d−1 in the absence of oxygen, and 123 nmol cm−3 d−1 in the presence of oxygen. In the latter case, the maximum rate was found in the 5–10-mm depth horizon (361 nmol cm−3 d−1 ). Biological consumption of amended thiosulfate was rapid and decreased with depth, while in the presence of molybdate, thiosulfate consumption decreased to 10–30% of the original rate. 相似文献
345.
The efficiency of marker-assisted introgression in backcross populations derived from inbred lines was investigated by simulation. Background genotypes were simulated assuming that a genetic model of many genes of small effects in coupling phase explains the observed breed difference and variance in backcross populations. Markers were efficient in introgression backcross programs for simultaneously introgressing an allele and selecting for the desired genomic background. Using a marker spacing of 10-20 cM gave an advantage of one to two backcross generations selection relative to random or phenotypic selection. When the position of the gene to be introgressed is uncertain, for example because its position was estimated from a trait gene mapping experiment, a chromosome segment should be introgressed that is likely to include the allele of interest. Even for relatively precisely mapped quantitative trait loci, flanking markers or marker haplotypes should cover ~10-20 cM around the estimated position of the gene, to ensure that the allele frequency does not decline in later backcross generations. 相似文献
346.
347.
Zhi Wei Wei Wang Jonathan Bradfield Jin Li Christopher Cardinale Edward Frackelton Cecilia Kim Frank Mentch Kristel Van?Steen Peter?M. Visscher Robert?N. Baldassano Hakon Hakonarson the International IBD Genetics Consortium 《American journal of human genetics》2013,92(6):1008-1012
We performed risk assessment for Crohn’s disease (CD) and ulcerative colitis (UC), the two common forms of inflammatory bowel disease (IBD), by using data from the International IBD Genetics Consortium’s Immunochip project. This data set contains ∼17,000 CD cases, ∼13,000 UC cases, and ∼22,000 controls from 15 European countries typed on the Immunochip. This custom chip provides a more comprehensive catalog of the most promising candidate variants by picking up the remaining common variants and certain rare variants that were missed in the first generation of GWAS. Given this unprecedented large sample size and wide variant spectrum, we employed the most recent machine-learning techniques to build optimal predictive models. Our final predictive models achieved areas under the curve (AUCs) of 0.86 and 0.83 for CD and UC, respectively, in an independent evaluation. To our knowledge, this is the best prediction performance ever reported for CD and UC to date. 相似文献
348.
The waggle dance of the honey bee is used to recruit nest mates to a resource, though direction indicated for a resource may
vary greatly within a single dance. Some authors suggest that this variation exits as an adaptation to distribute recruits
across a patch of flowers, and that, due to the variation’s inverse relationship with distance, the shape of the recruit distribution
will remain constant for resources at different distances. In this study, we test this hypothesis by examining how variation
in the indication of direction and distance changes with respect to distance. We find that imprecision in the communication
of direction does not diminish rapidly enough to accommodate an adaptive-error hypothesis, and we also find that variation
in the indication of distance has a positive relationship with the distance of a resource from the hive. 相似文献
349.
350.
P. M. Visscher C. S. Haley 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1996,93(5-6):691-702
Quantitative trait locus (QTL) mapping studies often employ segregating generations derived from a cross between genetically divergent inbred lines. In the analysis of such data it is customary to fit a single QTL and use a null hypothesis which assumes that the genomic region under study contributes no genetic variance. To explore the situation in which multiple linked genes contribute to the genetic variance, we simulated an F2-mapping experiment in which the genetic difference between the two original inbred strains was caused by a large number of loci, each having equal effect on the quantitative trait. QTLs were either in coupling, dispersion or repulsion phase in the base population of inbred lines, with the expected F2 genetic variance explained by the QTLs being equivalent in the three models. Where QTLs were in coupling phase, one inbred line was fixed for all plus alleles, and the other line was fixed for minus alleles. Where QTLs were in dispersion phase, they were assumed to be randomly fixed for one or other allele (as if the inbred lines had evolved from a common ancestor by random drift). Where QTLs were in repulsion phase alleles within an inbred line were alternating plus and minus at adjacent loci, and alternative alleles were fixed in the two inbred lines. In all these genetic models a standard interval mapping test statistic used to determine whether there is a QTL of large effect segregating in the population was inflated on average. Furthermore, the use of a threshold for QTL detection derived under the assumption that no QTLs were segregating would often lead to spurious conclusions regards the presence of genes of large effects (i.e. type I errors). The employment of an alternative model for the analysis, including linked markers as cofactors in the analysis of a single interval, reduced the problem of type I error rate, although test statistics were still inflated relative to the case of no QTLs. It is argued that in practice one should take into account the difference between the strains or the genetic variance in the F2 population when setting significance thresholds. In addition, tests designed to probe the adequacy of a single-QTL model or of an alternative infinitesimal coupling model are described. Such tests should be applied in QTL mapping studies to help dissect the true nature of genetic variation. 相似文献