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排序方式: 共有124条查询结果,搜索用时 31 毫秒
101.
Wolterbeek, H. Th. 1987. Relationships between adsorption, chemicalstate and fluxes of cadmium applied as Cd(NO3)2 in isolatedxylem cell walls of tomato.J. exp. Bot. 38: 419432. Isolated xylem cell wall pieces were applied as membranes inion diffusion experiments. The cell walls were isolated fromtomato internodes (Lycopersicon esculentum Mill, cv. Tiny Tim)and sealed in a two-compartment diffusion system. In flux andadsorption calculations, the cell wall was regarded as a leakymembrane with parallel fluxes through Donnan Free Space (DFS)and Water Free Space (WFS). During the experiments absorptioninto and diffusion across the walls was determined of Cd2 +, applied as 115Cd(NO3)2. Flux experiments with 82Brindicated that excluded volume effects and path tortuosity resultedin apparent WFS diffusion coefficients in the walls which were0·012 times as high as in water. The free proton concentration in the DFS was shown to be relatedto a complex formation between fixed charges and Cd2 +. Thecell wall permeability for Cd2 + and NO3 varied withapplied and absorbed concentrations, and the Cd2 + flux curveshowed an inflexion point coinciding with a buffered degreeof dissociation of fixed charges in the DFS. The necessary couplingof fluxes of opposite charges resulted in relatively high NO3and small Cd2 + permeability of the DFS for strongly dilutedsolutions (P = 104 m s1 and 1011 m s1for NO3 and Cd2 + respectively). The results demonstratethe possible regulatory effects of the cell wall in processesof ion transfer from xylem vessels, or ion uptake in plant tissues. Key words: Cadmium, chemical state, DFS, WFS, ion flux, permeability, xylem cell walls, tomato, bromium, nitrate 相似文献
102.
Cloning and chromosomal localization of human genes encoding the three chains of type VI collagen. 总被引:22,自引:5,他引:17
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D Weil M G Mattei E Passage V C N''''Guyen D Pribula-Conway K Mann R Deutzmann R Timpl M L Chu 《American journal of human genetics》1988,42(3):435-445
Type VI collagen is a heterotrimer composed of three polypeptide chains, alpha 1(VI), alpha 2(VI), and alpha 3(VI). By immunological screening of an expression cDNA library, human cDNAs specific for each chain were isolated and characterized. Major mRNA species encoding these chains have a size of 4.2 kb (alpha 1), 3.5 kb (alpha 2), and 8.5 kb (alpha 3). The cDNA clones were also used to map the genes on human chromosomes by somatic cell hybrid analysis and in situ hybridization. The alpha 1 (VI) and alpha 2(VI) collagen genes were both located on chromosome 21, in band q223. This represents a third example of a possible physical proximity of two collagen loci. The alpha 3(VI) collagen gene was localized to chromosome 2, in the region 2q37. The alpha 3(VI) collagen gene is the fifth extracellular matrix gene to be localized to 2q, as four other extracellular matrix genes--i.e., the alpha 1(III) and alpha 2(V) collagen genes, the elastin gene, and the fibronectin gene--have been previously mapped to the distal region of the long arm of chromosome 2. 相似文献
103.
Lathe WC rd; Burke WD; Eickbush DG; Eickbush TH 《Molecular biology and evolution》1995,12(6):1094-1105
R1 is a non-long terminal repeat (non-LTR) retrotransposable element that
inserts into a specific sequence of insect 28S ribosomal RNA genes. We have
previously shown that this element has been maintained through vertical
transmission in the melanogaster species subgroup of Drosophila. To address
whether R1 elements have been vertically transmitted for longer periods of
evolutionary time, the analysis has been extended to 11 other species from
four species groups of the genus Drosophila (melanogaster, obscura,
testecea, and repleta). All sequenced elements appeared functional on the
basis of the preservation of their open-reading frames and consistently
higher rate of substitution at synonymous sites relative to replacement
sites. The phylogenetic relationships of the R1 elements from all species
analyzed were congruent with the species phylogenies, suggesting that the
R1 elements have been vertically transmitted since the inception of the
Drosophila genus, an estimated 50-70 Mya. The stable maintenance of R1
through the germ line appears to be the major mechanism for the widespread
distribution of these elements in Drosophila. In two species, D.
neotestecea of the testecea group and D. takahashii of the melanogaster
group, a second family of R1 elements was also present that differed in
sequence by 46% and 31%, respectively, from the family that was congruent
with the species phylogeny. These second families may represent occasional
horizontal transfers or, alternatively, they could reflect the ability of
R1 elements to diverge into new families within a species and evolve
independently.
相似文献
104.
In Papanicolaou-stained smears certain structures such as nucleoli, Pneumocystis carinii , Charcot-Leyden crystals, bacteria and fungi show a brilliant fluorescence. the morphological characteristics of microorganisms which can be detected by this system, especially mycobacteria, are described. This screening method offers the possibility of providing the clinician with a provisional diagnosis within hours. Proof of the nature of the organisms should be obtained by culture. 相似文献
105.
106.
N. TOUZET M. SENSENBRENNER TH. LENDER P. MANDEL 《Differentiation; research in biological diversity》1975,4(1-3):183-187
Dissociated cells from cerebral hemispheres of chick embryo at stages 17–18, 12–13 and 9–10, were cultivated for seven days. The cells were cultivated either completely covered with the nutrient medium in an atmosphere containing 5 percent CO2 or they were covered by only a thin film of nutrient medium in contact with air.
For the embryos at stages 17–18 or 12–13, under both culture conditions neurons differentiated after 3 or 4 days in culture, while for the embryos at stage 9–10, no neuronal differentiation occurred under either condition. The cells remained morphologically undifferentiated and formed aggregates of about 50 cells. Some fibroblasts were found to grow on the collagen matrix.
It is concluded that dissociated cells from embryos at stage 9–10 are incapable of auto differentiation under the present culture conditions. The possible causes of this inability to differentiate are discussed. 相似文献
For the embryos at stages 17–18 or 12–13, under both culture conditions neurons differentiated after 3 or 4 days in culture, while for the embryos at stage 9–10, no neuronal differentiation occurred under either condition. The cells remained morphologically undifferentiated and formed aggregates of about 50 cells. Some fibroblasts were found to grow on the collagen matrix.
It is concluded that dissociated cells from embryos at stage 9–10 are incapable of auto differentiation under the present culture conditions. The possible causes of this inability to differentiate are discussed. 相似文献
107.
Analysis of a genetic hitchhiking model, and its application to DNA polymorphism data from Drosophila melanogaster 总被引:6,自引:0,他引:6
Begun and Aquadro have demonstrated that levels of nucleotide variation
correlate with recombination rate among 20 gene regions from across the
genome of Drosophila melanogaster. It has been suggested that this
correlation results from genetic hitchhiking associated with the fixation
of strongly selected mutants. The hitchhiking process can be described as a
series of two-step events. The first step consists of a strongly selected
substitution wiping out linked variation in a population; this is followed
by a recovery period in which polymorphism can build up via neutral
mutations and random genetic drift. Genetic hitchhiking has previously been
modeled as a steady-state process driven by recurring selected
substitutions. We show here that the characteristic parameter of this
steady-state model is alpha v, the product of selection intensity (alpha =
2Ns) and the frequency of beneficial mutations v (where N is population
size and s is the selective advantage of the favored allele). We also
demonstrate that the steady-state model describes the hitchhiking process
adequately, unless the recombination rate is very low. To estimate alpha v,
we use the data of DNA sequence variation from 17 D. melanogaster loci from
regions of intermediate to high recombination rates. We find that alpha v
is likely to be > 1.3 x 10(-8). Additional data are needed to estimate
this parameter more precisely. The estimation of alpha v is important, as
this parameter determines the shape of the frequency distribution of
strongly selected substitutions.
相似文献
108.
ERVIN ÁRNYAS JUDIT BERECZKI ANDREA TÓTH KATALIN VARGA KATALIN PECSENYE ANDRÁS TARTALLY GYÖRGY KÖVICS DÓRA KARSA ZOLTÁN VARGA 《Ecological Entomology》2009,34(1):90-97
Abstract 1. The influence of infestation of the larval host plant Gentiana cruciata on the egg‐laying preferences of the xerophilous ecotype of Alcon Blue butterfly (Maculinea alcon) was studied in a semi‐dry grassland area (Aggtelek Karst Region, Northern Hungary). 2. We examined whether oviposition patterns of females differed when G. cruciata stems were uninfested compared with when they were infested by an aphid (Aphis gentianae) or a rust (Puccinia gentianae) species. 3. Females laid more than 90% of their eggs on fertile, uninfested G. cruciata stems, although these stems comprised only ~ 50% of the total stems available. Stems infested by aphids were similar to uninfested ones in properties that had a strong correlation with egg numbers, and yet there were significantly fewer eggs on infested stems than on intact ones. 4. Females never laid eggs on parts of Gentiana stems infested by aphids, and the presence of Lasius paralienus ants, which have a mutualistic interaction with Aphis gentianae, did not increase the repulsive effect of aphids. Infection of Gentiana by Puccinia did not influence the egg‐laying behaviour of females, even though the flowers and buds of infested stems exhibited a delayed development. 5. Aphid infestation can influence butterfly oviposition patterns through both direct and indirect effects. The presence of aphids directly excluded oviposition, but our data also indicated the possibility of an indirect effect of aphid infestation. Stems that had no aphids at the last egg counting, but were infested prior to it, had significantly fewer eggs than those that were never infested. 相似文献
109.
Henk C. den Bakker G. C. Zuccarello TH. W. Kuyper M. E. Noordeloos 《The New phytologist》2004,163(1):201-215
110.
Physiological characteristics of wild Atlantic salmon post-smolts during estuarine and coastal migration 总被引:2,自引:0,他引:2
S. O. Stefansson† B. TH. Björnsson‡ K. Sundell‡ G. Nyhammer S. D. McCormick§ 《Journal of fish biology》2003,63(4):942-955
Changes were measured in some of the major physiological variables associated with seawater adaptability, growth and energetics in wild Atlantic salmon Salmo salar smolts and post-smolts migrating from the river and through the estuary, fjord and coastal areas in the River Orkla and the Trondheimsfjord, Norway during late May to early June. Gill Na+ ,K+ -ATPase activity increased to levels of 12–16 µmol ADP mg protein−1 h−1 in post-smolts caught in higher salinity zones, probably representing long-term levels of Atlantic salmon post-smolts in oceanic conditions. Muscle moisture was regulated within narrow limits (77·7–78·7%) in fish from all zones during both years, suggesting that post-smolts adapt to marine conditions without any long-term disturbance of hydro-mineral balance. Lipid and glycogen content showed a general trend towards depletion from the river, through the fjord and into the ocean. There was, however, no significant change in protein content. The present results confirm that smolts are naturally 'energy deficient' during downstream migration, and suggest that post-smolts also mobilize energy reserves during their early marine phase, while protein is allocated for somatic growth. Plasma growth hormone (GH) levels increased transiently during passage through the estuary and fjord, with lower levels observed in post-smolts caught off-shore, i.e. in fish which were feeding on marine prey and had adapted to the marine environment. These physiological changes may confer substantial selective advantages during the critical early marine phase of anadromous salmonids, and hence are adaptive for long-term survival in sea water. 相似文献