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991.
Zimbabwe's communal farmers are now less food secure than they were two generations ago. The roots of this decline lie not only in the confinement of Africans to marginal land but also in the historic forced replacement of their sustainable, indigenous farming system with one whose productivity now relies on the use of large amounts of expensive chemical inputs. Environmentally-friendly, traditional farming practices such as pyro-culture, minimum tillage, mixed cropping, and bush fallowing were completely wiped out and replaced with a highly technical western or ganic farming system based on plough cultivation and continuous monocultures of commodity crops, that were supposed to be sustained by liberal amounts of green and animal manures. This gave rise to an effective agricultural hegemony that was due, mainly, to the zealous dedication of an American Mormon missionary whose motives were evangelical rather than scientific. 相似文献
992.
McPherson JP Lemmers B Chahwan R Pamidi A Migon E Matysiak-Zablocki E Moynahan ME Essers J Hanada K Poonepalli A Sanchez-Sweatman O Khokha R Kanaar R Jasin M Hande MP Hakem R 《Science (New York, N.Y.)》2004,304(5678):1822-1826
Mus81-Eme1 endonuclease has been implicated in the rescue of stalled replication forks and the resolution of meiotic recombination intermediates in yeast. We used gene targeting to study the physiological requirements of Mus81 in mammals. Mus81-/- mice are viable and fertile, which indicates that mammalian Mus81 is not essential for recombination processes associated with meiosis. Mus81-deficient mice and cells were hypersensitive to the DNA cross-linking agent mitomycin C but not to gamma-irradiation. Remarkably, both homozygous Mus81-/- and heterozygous Mus81+/- mice exhibited a similar susceptibility to spontaneous chromosomal damage and a profound and equivalent predisposition to lymphomas and other cancers. These studies demonstrate a critical role for the proper biallelic expression of the mammalian Mus81 in the maintenance of genomic integrity and tumor suppression. 相似文献
993.
Persistent increase in brain serotonin turnover after chronic administration of LSD in the rat 总被引:1,自引:0,他引:1
Lysergic acid diethylamide at doses of 20 micrograms per kilogram per day was administered orally to rats for I month. Eighteen hours after the final dose a 25 to 30 percent increase in the synthesis and turnover of serotonin was noted, as well as a moderate but significant increase in the concentration of tryptophan (18 percent) and serotonin (13 percent) in the brain. 相似文献
994.
In the experiments described the following observations have been made: (1) The toxic and diabetogenic action of alloxan increases in rats fed a low protein diet and even more so in the case of a high lard or ox fat diet. (2) The action of these fat diets was corrected by the addition of methionine, thiouracil, or coconut oil, but there was no modification either by addition of choline or sulfonamide or by increase in the protein proportion. (3) In rats fed other high fat diets (olive oil, butter) the actions of alloxan were not modified, but there was a slight diminution when high oleomargarine or corn oil diets were fed. However, complete protection was observed when a high coconut oil diet was administered. (4) The unfavorable effect of the high lard diet was observed also on the initiation and evolution of diabetes due to subtotal pancreatectomy. Contrarily, feeding a high protein diet and especially treatment with thiouracil had a favorable effect. (5) Diabetes appeared first in subtotal pancreatectomized rats which were overfed, then in those which ate ad libitum, and finally in those which were underfed. Diabetes appeared in rats fed a single meal before it did in those eating the same amount of food divided into three meals. 相似文献
995.
Middle Paleolithic assemblages from the Indian subcontinent before and after the Toba super-eruption
Petraglia M Korisettar R Boivin N Clarkson C Ditchfield P Jones S Koshy J Lahr MM Oppenheimer C Pyle D Roberts R Schwenninger JL Arnold L White K 《Science (New York, N.Y.)》2007,317(5834):114-116
The Youngest Toba Tuff (YTT) eruption, which occurred in Indonesia 74,000 years ago, is one of Earth's largest known volcanic events. The effect of the YTT eruption on existing populations of humans, and accordingly on the course of human evolution, is debated. Here we associate the YTT with archaeological assemblages at Jwalapuram, in the Jurreru River valley of southern India. Broad continuity of Middle Paleolithic technology across the YTT event suggests that hominins persisted regionally across this major eruptive event. 相似文献
996.
Terracciano A Abdel-Khalek AM Adám N Adamovová L Ahn CK Ahn HN Alansari BM Alcalay L Allik J Angleitner A Avia MD Ayearst LE Barbaranelli C Beer A Borg-Cunen MA Bratko D Brunner-Sciarra M Budzinski L Camart N Dahourou D De Fruyt F de Lima MP del Pilar GE Diener E Falzon R Fernando K Ficková E Fischer R Flores-Mendoza C Ghayur MA Gülgöz S Hagberg B Halberstadt J Halim MS Hrebícková M Humrichouse J Jensen HH Jocic DD Jónsson FH Khoury B Klinkosz W Knezević G Lauri MA Leibovich N Martin TA 《Science (New York, N.Y.)》2005,310(5745):96-100
Most people hold beliefs about personality characteristics typical of members of their own and others' cultures. These perceptions of national character may be generalizations from personal experience, stereotypes with a "kernel of truth," or inaccurate stereotypes. We obtained national character ratings of 3989 people from 49 cultures and compared them with the average personality scores of culture members assessed by observer ratings and self-reports. National character ratings were reliable but did not converge with assessed traits. Perceptions of national character thus appear to be unfounded stereotypes that may serve the function of maintaining a national identity. 相似文献
997.
Large RNA molecules, such as ribozymes, fold with well-defined tertiary structures that are important for their activity. There are many instances of ribozymes with identical function but differences in their secondary structures, suggesting alternative tertiary folds. Here, we report a crystal structure of the 161-nucleotide specificity domain of an A-type ribonuclease P that differs in secondary and tertiary structure from the specificity domain of a B-type molecule. Despite the differences, the cores of the domains have similar three-dimensional structure. Remarkably, the similar geometry of the cores is stabilized by a different set of interactions involving distinct auxiliary elements. 相似文献
998.
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
999.
1000.
Chiu DT Wilson CF Ryttsén F Strömberg A Farre C Karlsson A Nordholm S Gaggar A Modi BP Moscho A Garza-López RA Orwar O Zare RN 《Science (New York, N.Y.)》1999,283(5409):1892-1895
Individual phospholipid vesicles, 1 to 5 micrometers in diameter, containing a single reagent or a complete reaction system, were immobilized with an infrared laser optical trap or by adhesion to modified borosilicate glass surfaces. Chemical transformations were initiated either by electroporation or by electrofusion, in each case through application of a short (10-microsecond), intense (20 to 50 kilovolts per centimeter) electric pulse delivered across ultramicroelectrodes. Product formation was monitored by far-field laser fluorescence microscopy. The ultrasmall characteristic of this reaction volume led to rapid diffusional mixing that permits the study of fast chemical kinetics. This technique is also well suited for the study of reaction dynamics of biological molecules within lipid-enclosed nanoenvironments that mimic cell membranes. 相似文献