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排序方式: 共有1093条查询结果,搜索用时 15 毫秒
81.
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Evidence for reduced recombination on the nondisjoined chromosomes 21 in Down syndrome 总被引:28,自引:0,他引:28
A C Warren A Chakravarti C Wong S A Slaugenhaupt S L Halloran P C Watkins C Metaxotou S E Antonarakis 《Science (New York, N.Y.)》1987,237(4815):652-654
Trisomy 21 usually results from nondisjunction during meiosis I. In order to determine whether nondisjunction results from failure of normal chromosome pairing or premature unpairing, recombination frequencies were estimated between DNA polymorphic markers on the long arm of chromosome 21 in families containing one individual with trisomy 21. The recombination frequencies on chromosomes 21 that had undergone nondisjunction were then compared to those on chromosomes 21 that had disjoined normally. The data indicate that recombination is reduced between DNA markers on nondisjoined chromosomes 21. These results are consistent with the hypothesis that reduced chiasma formation predisposes to nondisjunction, resulting in trisomy 21 in humans. 相似文献
83.
The construction of multicellular organisms depends on stem cells-cells that can both regenerate and produce daughter cells that undergo differentiation. Here, we show that the gaseous messenger ethylene modulates cell division in the cells of the quiescent center, which act as a source of stem cells in the seedling root. The cells formed through these ethylene-induced divisions express quiescent center-specific genes and can repress differentiation of surrounding initial cells, showing that quiescence is not required for these cells to signal to adjacent stem cells. We propose that ethylene is part of a signaling pathway that modulates cell division in the quiescent center in the stem cell niche during the postembryonic development of the root system. 相似文献
84.
Functional role of aspartic acid-27 in dihydrofolate reductase revealed by mutagenesis 总被引:12,自引:0,他引:12
E E Howell J E Villafranca M S Warren S J Oatley J Kraut 《Science (New York, N.Y.)》1986,231(4742):1123-1128
The crystal structures and enzymic properties of two mutant dihydrofolate reductases (Escherichia coli) were studied in order to clarify the functional role of an invariant carboxylic acid (aspartic acid at position 27) at the substrate binding site. One mutation, constructed by oligonucleotide-directed mutagenesis, replaces Asp27 with asparagine; the other is a primary-site revertant to Ser27. The only structural perturbations involve two internally bound water molecules. Both mutants have low but readily measurable activity, which increases rapidly with decreasing pH. The mutant enzymes were also characterized with respect to relative folate: dihydrofolate activities and kinetic deuterium isotope effects. It is concluded that Asp27 participates in protonation of the substrate but not in electrostatic stabilization of a positively charged, protonated transition state. 相似文献
85.
The herbicides N-(3-chloro-4-methyphenyl)-2-methylpentamamide (solan) and 1, 1'-dimethyl-4, 4'dipyridylium dichloride (paraquat) were less toxic to tomato plants when applied together than either one alone. However, the combination was more toxic to crabgrass and eggplant. This phenomenon may result from the interaction of the herbicides with photosynthesis. 相似文献
86.
The US food retailing industry continues to concentrate and consolidate. Power in the agriculture, food, and nutrition system
has shifted from producers to processors, and is now shifting to retailers. Currently, only eight food-retailing corporations
control the majority of food sales in the United States. Expanding on previous research by Lyson and Raymer (2000, Agriculture and Human Values 17: 199–208), this paper examines the characteristics of the boards of directors of the leading food retailing corporations
and the indirect interlocks that bind the food retailers into a corporate community.
Rachel
Schwartz
is a PhD Candidate in the Department of Development Sociology at Cornell University. Her research interests are focused on
the relationship between food retailers and food consumers in the United States, especially in regards to the construction
of the concept of “choice.”
Thomas Lyson
was Liberty Hyde Bailey Professor in the Department of Development Sociology at Cornell University. His interests included
the relationship of agriculture and food systems on community economic development and population health. His most recent
book, Civic Agriculture, developed a problem-solving model for food and agriculture issues. Dr. Lyson passed away in December
2006. 相似文献
87.
Morrow EM Yoo SY Flavell SW Kim TK Lin Y Hill RS Mukaddes NM Balkhy S Gascon G Hashmi A Al-Saad S Ware J Joseph RM Greenblatt R Gleason D Ertelt JA Apse KA Bodell A Partlow JN Barry B Yao H Markianos K Ferland RJ Greenberg ME Walsh CA 《Science (New York, N.Y.)》2008,321(5886):218-223
To find inherited causes of autism-spectrum disorders, we studied families in which parents share ancestors, enhancing the role of inherited factors. We mapped several loci, some containing large, inherited, homozygous deletions that are likely mutations. The largest deletions implicated genes, including PCDH10 (protocadherin 10) and DIA1 (deleted in autism1, or c3orf58), whose level of expression changes in response to neuronal activity, a marker of genes involved in synaptic changes that underlie learning. A subset of genes, including NHE9 (Na+/H+ exchanger 9), showed additional potential mutations in patients with unrelated parents. Our findings highlight the utility of "homozygosity mapping" in heterogeneous disorders like autism but also suggest that defective regulation of gene expression after neural activity may be a mechanism common to seemingly diverse autism mutations. 相似文献
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