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21.
Condensed and dispersed chromatin fractions were isolated from human placental nuclei. The DNA of each fraction was purified and characterised by isopycnic centrifugation, thermal fractionation on hydroxylapatite (HAP) and sequence complexity studies. The DNAs had identical buoyant densities in neutral CsCl (1.698 g/cm3) and similar melting profiles on HAP. Analytical ultracentrifugation in Ag+-Cs2SO4, however, showed that satellite DNAs were present in the condensed fraction DNA (DNAC) but were not visible in the dispersed fraction DNA (DNAD). In addition, DNAC was found to be enriched in highly reiterated sequences (20% reassociated by C0t 10?3) which can be correlated with the presence of satellite DNAs, whereas DNAD contained only 3% of these fast reassociating sequences. In contrast DNAD contained 30% intermediate sequences (reassociating between C0t 10?3 and C0t 100) which represent only 10% of DNAC. The reassociated highly repeated sequences of DNAC showed the presence of two components in both CsCl density gradients and HAP thermal elution studies. This suggests that either there are sequence relationships resulting in partial mismatching between the different highly repeated DNA sequences in this fraction, or that highly repeated sequences are associated with less repetitious DNA. The results are discussed in terms of possible differences in genetic activity between the chromatin fractions. 相似文献
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Ian D. Hodkinson Jeremy M. Bird Jane K. Hill Robert Baxter . 《Ecological Entomology》2001,26(4):376-387
1. Salix lapponum host plants at an upper altitudinal site differed significantly in size, structural density, phenology, growth performance, and spatial isolation from those growing at a lower site. 2. Plant differences were paralleled by significant differences in psyllid population density and phenology parameters, with psyllid population density, percentage of catkins occupied, and phenological development relatively lower or retarded at the upper site. Population densities at the upper site, nevertheless, remained high. 3. Plant measurements were good predictors of insect density, often explaining up to 73% of the variance in abundance among plants at a given site. 4. Sets of four plant characters identified by best subsets regression were better predictors of psyllid density and development than single factors, although differences were often not great and the combinations of characters selected by multiple regression sometimes differed from the best single predictors. 5. Best single predictors of psyllid density on catkins were measurements of plant size, particularly height, length, and basal stem diameter. Shoot density and catkin phenology were occasionally important but plant isolation and prior growth performance were less important. 6. By contrast with density, age structure of the psyllid population was predicted best from plant phenological measurements, notably catkin phenology. 相似文献
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Jane Dawson 《BMJ (Clinical research ed.)》1987,295(6598):597-598
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Epithelial differentiation in Drosophila pupae 总被引:2,自引:0,他引:2
The construction of cell hairs on the wings in developing pupae of Drosophila provides a unique system for studies of the regulation of differentiation in the absence of cell division. Early steps in hair construction are the extrusion of cell hairs and the deposition of the external impervious layer called "cuticulin." Some properties of six of the most abundant proteins that are present during the early stages of hair construction are described. These proteins make up about 40% of the total protein of the preparation. 相似文献