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481.
482.
Zusammenfassung Als Antifertilitätsmittel haben dasmeso-Isomer und die (±)-Isomere von Dimethylmyleran eine synergistische Wirkung auf die Spermatogenese von Ratten. Bei Mäusen und Wachteln hat ein 50% iges Gemisch der Isomere eine niedrigere Toxizität als die einzelnen Isomere, was bei höherer Dosis des Gemisches zu längerdauernder Sterilität führt. 相似文献
483.
Observations of the varying orientations of comet tails led to the suggestion of the existence of the solar wind--a continuous outflow of ionized material from the Sun. It is now well established that gas from comets is ionized by several processes and joins the solar wind, forming an ion (plasma) tail that points away from the Sun. The plasma environments of three comets have been measured in situ, but only in the upstream direction or less than 8,000 km downstream of the nucleus. Here we report a fortuitous crossing by a spacecraft of the plasma tail of comet Hyakutake (C/1996 B2), at a distance of more than 3.8 astronomical units (550 million kilometres) from its nucleus. This surpasses the tail length of 2 AU determined for the Great March Comet of 1843 (C/1843 D1). Our measurements reveal that, at this distance, the tail of comet Hyakutake was a structured entity at least 7 million kilometres in diameter. 相似文献
484.
Non-thermal heat-shock response to microwaves 总被引:18,自引:0,他引:18
de Pomerai D Daniells C David H Allan J Duce I Mutwakil M Thomas D Sewell P Tattersall J Jones D Candido P 《Nature》2000,405(6785):417-418
485.
Irradiation of dry or fully hydrated frozen DNA systems (conditions of direct damage) has been shown by electron-spin resonance spectroscopy to give rise to electron-gain centres localized on thymine (T.-) and electron loss centres ('holes') localized on guanine (G.+) with approximately equal yields. Our parallel studies on the development of both single- and double-strand breaks under comparable conditions provide good evidence that these radical centres are the precursors to such damage, and we and others have argued that this may be of relevance to the damage pathways in vivo. We now report evidence that when DNA is complexed to proteins as it is in the nuclei of eukaryotes, electron transfer from the histone to DNA is facile, leading to a significant increase in the yield of electron-gain centres in DNA as judged from their electron-spin resonance spectra. In contrast 'holes' generated in the protein are trapped and do not lead to any detectable increase in the yields of G.+. 相似文献
486.
487.
The dynamics of chromosome evolution in birds and mammals 总被引:20,自引:0,他引:20
Burt DW Bruley C Dunn IC Jones CT Ramage A Law AS Morrice DR Paton IR Smith J Windsor D Sazanov A Fries R Waddington D 《Nature》1999,402(6760):411-413
Comparative mapping, which compares the location of homologous genes in different species, is a powerful tool for studying genome evolution. Comparative maps suggest that rates of chromosomal change in mammals can vary from one to ten rearrangements per million years. On the basis of these rates we would expect 84 to 600 conserved segments in a chicken comparison with human or mouse. Here we build comparative maps between these species and estimate that numbers of conserved segments are in the lower part of this range. We conclude that the organization of the human genome is closer to that of the chicken than the mouse and by adding comparative mapping results from a range of vertebrates, we identify three possible phases of chromosome evolution. The relative stability of genomes such as those of the chicken and human will enable the reconstruction of maps of ancestral vertebrates. 相似文献
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489.
490.
The delayed rise of present-day mammals 总被引:1,自引:0,他引:1
Bininda-Emonds OR Cardillo M Jones KE MacPhee RD Beck RM Grenyer R Price SA Vos RA Gittleman JL Purvis A 《Nature》2007,446(7135):507-512
Did the end-Cretaceous mass extinction event, by eliminating non-avian dinosaurs and most of the existing fauna, trigger the evolutionary radiation of present-day mammals? Here we construct, date and analyse a species-level phylogeny of nearly all extant Mammalia to bring a new perspective to this question. Our analyses of how extant lineages accumulated through time show that net per-lineage diversification rates barely changed across the Cretaceous/Tertiary boundary. Instead, these rates spiked significantly with the origins of the currently recognized placental superorders and orders approximately 93 million years ago, before falling and remaining low until accelerating again throughout the Eocene and Oligocene epochs. Our results show that the phylogenetic 'fuses' leading to the explosion of extant placental orders are not only very much longer than suspected previously, but also challenge the hypothesis that the end-Cretaceous mass extinction event had a major, direct influence on the diversification of today's mammals. 相似文献