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31.
Summary The largest class of de novo chromosomal rearrangements in Down syndrome are rea(21q21q). Classically, these rearrangements have been termed Robertsonian translocations, implying an attachment of two different chromosome 21 homologues. Additionally, a Robertsonian translocation between two chromosomes 21 cannot be distinguished from an isochromosome composed of genetically identical arms by cytogenetic analyses. Therefore, we have used molecular techniques to differentiate between true Robertsonian translocations and isochromosomes. Samples were obtained from 12 probands, ascertained for de novo rearrangements between homologous chromosomes 21 [11 rea(21q21q) and 1 rea (21;21)(q22;q22)], their parents (n = 24) and available siblings (n = 7). The parental origins of the de novo rearrangements were assigned using molecular and cytogenetic analyses. Although not statistically significant, there was a two-fold increase in the number of paternally derived de novo rearrangements (n = 8) as compared with maternally derived rearrangements (n = 4). To distinguish between rob(21q21q) and i(21q), we used restriction fragment length polymorphisms (RFLPs) spanning the length of chromosome 21. Using all informative and partially informative RFLPs, we used the method of maximum likelihood to assign the most likely rearrangement definition (i or rob) and parental origin in each family. The maximum likelihood estimates indicated that all rearrangements tested (n = 8) were isochromosomes. C-banding revealed two centromeres in three cases indicating that a U-type exchange occurred between sister chromatids in these rearrangements. Our results suggest that the majority of de novo rea(21q21q) are isochromosomes derived from a single parental chromosome 21.  相似文献   
32.
The release of endogenous acetylcholine and dopamine and the appearance of their metabolites, choline and dihydroxyphenylacetic acid (DOPAC), from neostriatal slices prepared from Fischer 344 rats was examined under various experimental conditions. There was a dose-dependent increase in the amount of neurotransmitter or metabolite as the medium potassium concentration was increased from 5 to 50 mM. Over an eight minute period in Krebs Ringer bicarbonate buffer containing 25 mM potassium, the rate of release of acetylcholine was 6 to 13 times greater than that of dopamine. The dopamine endogenous to the slice preparation appeared to have little effect on the release of endogenous acetylcholine since manipulations that significantly altered dopamine release (depletion with 6-hydroxydopamine or uptake inhibition with nomifensine) had minimal effects on the cholinergic neurons. In contrast, increasing the endogenous acetylcholine in the preparation by inhibiting acetylcholinesterase resulted in a 1.2 to 12 fold increase in dopamine release depending upon the incubation time and the potassium concentration. These studies indicate that within the neostriatal slices there is minimal influence of the endogenous dopamine on the cholinergic neurons, whereas the extracellular acetylcholine can influence dopamine release when its concentration is increased by inhibition of acetylcholinesterase.  相似文献   
33.
Chemotaxis in the Marine Fungus Rhizophydium littoreum   总被引:1,自引:0,他引:1       下载免费PDF全文
Zoospores of the marine chytrid Rhizophydium littoreum are attracted to a variety of substances common to their environment. In general, carbohydrates and polysaccharides elicited strong concentration-dependent positive responses. There was no direct correlation between all substances used as foods and those stimulating positive responses. The chemotactic activities of this organism should, however, tend to bring it toward concentrated food sources.  相似文献   
34.
The human genetic material is packed hierarchically within the metaphase chromosome: the DNA moleculet together with histone proteins form 11 nm diameter nucleosomes, which are then ordered into the 30 nm thick chromatin fiber. Little is known about the packing of this fiber within the chromosome. We have developed a tracking algorithm with which we followed its path within a three-dimensional reconstruction of a human chromosome computed from a series of electron micrographie projections. Fiber segments were seen to form loops of 100–350 nm diameter. Our observations indicate that these loops — which themselves show no preferred orientation — are organised into regions of roughly 200 nm axial extent.  相似文献   
35.
36.
Stathmokinetic Analysis of Human Epidermal Cells in vitro   总被引:1,自引:0,他引:1  
Proliferation kinetics of cultured human epidermal cells is characterized in quantitative terms. Three distinct subpopulations of keratinocytes, two of which are cycling have been discriminated by two parameter DNA/RNA flow cytometry. Based on mathematical modelling, the cell cycle parameters of the cycling subpopulations have been assessed from stathmokinetic data collected at different time points after initiation of cultures (7–15 days). the first subpopulation is composed of low-RNA cells which resemble basal keratinocytes of epidermis and which show some characteristics of stem cells; these cells have a mean generation time of approximately 100 hr. the second subpopulation consists of high-RNA cells, resembling stratum spinosum cells of epidermis, which have an average generation time of approximately 40 hr. the third subpopulation consists of non-cycling cells with Go/G1 DNA content, with cytochemical features similar to those of cells in granular layer of epidermis. The results based on modelling can reproduce with acceptable accuracy the actual growth curve of the cultured cell population. Analysis of kinetics and differentiation of human keratinocytes is of interest in view of the recent application of cultured epidermal cell sheets for transplantation onto burn wounds. the results of this study also reveal the existence of regulatory mechanisms associated with proliferation and differentiation in the cultured epidermal cell population.  相似文献   
37.
In the leaf succulent Asclepiad Hoya carnosa (L.) R. Br., CAM photosynthesis occurred under well-watered conditions, as characterized by diurnal gas exchange and changes in titratable acidity. Following 10–12 days of severe water stress, the plants shifted from CAM to a modified CAM-idling mode of metabolism. CAM-idling was characterized by complete or almost complete stomatal closure accompanied by CAM-like diurnal changes in titratable acidity. H. carnosa plants maintained this CAM-idling mode of photosynthesis for at least 8 weeks. Upon reirrigation, the plants returned to the original CAM mode within 1 week. These results suggested that CAM-idling is a reversible, intermediate form of sustained metabolism which enables plant survival under conditions of extended drought.This work was supported in part by NSF Grant PCM 8200366 and in part by the Science and Education Administration of the United States Department of Agriculture under Competitive Grant 5901-0420-8-0018-0.  相似文献   
38.
The Rickettsia prowazekii ATP/ADP translocase (Tlc) gene (tlc), previously cloned in Escherichia coli was localized to a 1.6-kb chromosomal fragment. Nucleotide sequence analysis of this fragment revealed an open reading frame of 1494 bp that could encode a hydrophobic protein of 497 amino acids (aa) with an Mr of 56 668. Analysis of the deduced aa sequence revealed that it contained twelve potential membrane-spanning regions. Comparisons between the deduced aa sequence of the R. prowazekii ATP/ADP Tlc and the sequences of mitochondrial (mt) Tic revealed no detectable homologies between the rickettsial and mt sequences. The major protein synthesized in E. coli minicells containing the rickettsial gene exhibited an Mr of approx. 34000.  相似文献   
39.
Summary Resistance to streptomycin and lincomycin in plant cell culture is used as a color marker: resistant cells are green whereas sensitive cells are white on the selective medium. Streptomycin and lincomycin at appropriate concentrations do not kill sensitive Nicotiana cells. The selective value of plastid ribosomal DNA mutations, conferring resistance to streptomycin and lincomycin, was investigated by growing heteroplastidic cells on a selective medium. The heteroplastidic cells were obtained by protoplast fusion, and contained a mixed population of streptomycin resistant plastids from the N. tabacum line Nt-SR1-Kan2, and lincomycin resistant plastids from the N. plumbaginifolia line Np-LR400-Hyg1. Clones derived from protoplast fusion were selected by kanamycin and hygromycin resistance, transgenic nuclear markers. Somatic hybrids were then grown on a selective streptomycin or lincomycin medium, or in the absence of either drug to a 50 to 100 mg size callus. Southern analysis of a polymorphic region of plastid DNA (ptDNA) revealed that somatic hybrids grown on streptomycin contained almost exclusively ptDNA from the streptomycin resistant parent, somatic hybrids grown on lincomycin contained almost exclusively ptDNA from the lincomycin resistant parent whereas somatic hybrids grown in the absence of either drug contained mixed parental plastids. Sensitive ptDNA was below detection level in most clones on selective medium, but could be recovered upon subsequent culture in the presence of the appropriate drug. The drugs streptomycin and lincomycin provide a powerful selection pressure that should facilitate recovery of plastid transformants.  相似文献   
40.
The use of nitrogen-fixing trees such as black alder (Alnus glutinosa L. Gaertn.) as forest silvicultural tools has recently been recognized. The potential benefit of black alder in silvicultural practices may be reduced by nitrate fertilization. Fifteen-month-old, nodulated, black alder rooted cuttings were fertilized for 6 days with 0, 7.5 or 15 mM NO3 to determine the influence of nitrate on acetylene reduction, nodule respiration and net photosynthesis. Acetylene reduction, net photosynthesis and nodule respiration were measured on the second, fourth and sixth days of nitrate application. Nitrate treatment significantly reduced acetylene reduction and nodule respiration by day 4. Acetylene reduction was 75% lower and nodule respiration 36% lower for the 15 mM NO3 treatment when compared to that of the control treatment. By day 6, net photosynthesis and nodule respiration were significantly reduced by 29 and 59%, respectively, for seedlings treated with 15 mM NO3. This study suggests that nitrate fertilization has a profound influence on nitrogenase activity and that nitrogen-fixing tree species may respond to nitrate fertilization by shifting photosynthetic rates.  相似文献   
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