Chromosome condensation in mitosis and meiosis of rye (Secale cereale L.)

Structural investigation and morphometry of meiotic chromosomes by scanning electron microscopy (in comparison to light microscopy) of all stages of condensation of meiosis I + II show remarkable differences during chromosome condensation in mitosis and meiosis I of rye (Secale cereale) with respect to initiation, mode and degree of condensation. Mitotic chromosomes condense in a linear fashion, shorten in length and increase moderately in diameter. In contrast, in meiosis I, condensation of chromosomes in length and diameter is a sigmoidal process with a retardation in zygotene and pachytene and an acceleration from diplotene to diakinesis. The basic structural components of mitotic chromosomes of rye are ``parallel fibers{''} and ``chromomeres{''} which become highly compacted in metaphase. Although chromosome architecture in early prophase of meiosis seems similar to mitosis in principle, there is no equivalent stage during transition to metaphase I when chromosomes condense to a much higher degree and show a characteristic ``smooth{''} surface. No indication was found for helical winding of chromosomes either in mitosis or in meiosis. Based on measurements, we propose a mechanism for chromosome dynamics in mitosis and meiosis, which involves three individual processes: (i) aggregation of chromatin subdomains into a chromosome filament, (ii) condensation in length, which involves a progressive increase in diameter and (iii) separation of chromatids. Copyright (C) 2003 S. Karger AG, Basel

Ultrastructural analysis of chromatin in meiosis I plus II of rye (Secale cereale L.)

Scanning electron microscopy (SEM) proves to be an appropriate technique for imaging chromatin organization in meiosis I and II of rye (Secale cereale) down to a resolution of a few nanometers. It could be shown for the first time that organization of basic structural elements (coiled and parallel fibers, chromomeres) changes dramatically during the progression to metaphase I and II. Controlled loosening with proteinase K (after fixation with glutaraldehyde) provides an enhanced insight into chromosome architecture even of highly condensed stages of meiosis. By selective staining with platinum blue, DNA content and distribution can be visualized within compact chromosomes as well as in a complex arrangement of fibers. Chromatin interconnecting threads, which are typically observed in prophase I between homologous and non-homologous chromosomes, stain clearly for DNA. In zygotene transversion of chromatid strands to their homologous counterparts becomes evident. In pachytene segments of synapsed and non-synapsed homologs alternate. At synapsed regions pairing is so intimate that homologous chromosomes form one filament of structural entity. Chiasmata are characterized by chromatid strands which traverse from one homolog to its counterpart. Bivalents are characteristically fused at their telomeric regions. In metaphase I and II there is no structural evidence for primary and secondary constrictions. Copyright (C) 2003 S. Karger AG, Basel

Airborne Measurements of Gravity Wave Breaking at the Tropopause

2000 FLORIDA AVE NW, WASHINGTON, DC, 2000

Excitation of weakly bound Rydberg electrons by half-cycle pulses

The interaction of a weakly bound Rydberg electron with an electromagnetic half-cycle pulse (HCP) is described with the help of a multidimensional semiclassical treatment. This approach relates the quantum evolution of the electron to its underlying classical dynamics. The method is nonperturbative and is valid for arbitrary spatial and temporal shapes of the applied HCP. On the basis of this approach angle- and energy-resolved spectra resulting from the ionization of Rydberg atoms by HCPs are analyzed. The different types of spectra obtainable in the sudden-impact approximation are characterized in terms of the appearing semiclassical scattering phenomena. Typical modifications of the spectra originating from finite pulse effects are discussed.Comment: Submitted to Phys. Rev.

Electroneutrality and the Friedel sum rule in a Luttinger liquid

Screening in one-dimensional metals is studied for arbitrary electron-electron interactions. It is shown that for finite-range interactions (Luttinger liquid) electroneutrality is violated. This apparent inconsistency can be traced to the presence of external screening gates responsible for the effectively short-ranged Coulomb interactions. We also draw attention to the breakdown of linear screening for wavevectors close to 2 K_f.Comment: 4 pages REVTeX, incl one figure, to appear in Phys.Rev.Let

Charge Exchange Processes between Excited Helium and Fully Stripped Ions

We made a classical trajectory Monte Carlo (CTMC) calculation of state selective cross sections for processes between some light ions and excited helium. The results, useful for analysis of spectroscopic data of fusion devices, are in good agreement with theoretical predictions of scaling laws.Comment: LaTex, 8 pages, 4 figures (available on request to the authors), DFPD/94/TH/57, to be published in Phys. Rev.

Syntbesis and Properries of the Selective Antimuscarinic Agent Cyclohexylphenyl(3-piperidinopropyl)silanol

Die Synthese des selektiven Antimuskarinikums Cyclohexylpheny\{3-piperidinopropyl)sila· nol (1 b) wird beschrieben. 1 b wurde - ausgehend von (3·Chlorpropyl)trimethoxysilan - durch eine vierstufige Reaktionsfolge erhalten und als Hydrochlorid 2b mit einer Gesamtausbeute von etwa 45°/o isoliert. - 1 b ist aufgrund seiner großen pharmakologischen Se· lektivität zu einer Standardsubstanz in der experimentellen Pharmakologie bei der Differenzierung von Muskarinrezeptoren geworden.The synthesis of thc selective antimuscarinic agent cyclohexylphenyl(3-piperidinopropyl)silanol (1 b) is described. Starting with (3-chloropropyl)trimethoxysilane, I b was obtained by four reaction steps and isolated as hydrochloride 2b with a total yield of about 45°/o. - Because of its high pharmacological selectivity 1 b has become a reference drug in experimental pharmacology for the differentiation of muscarinic rcceptors

Ion Collisions in Very Strong Electric Fields

A Classical Trajectory Monte Carlo (CTMC) simulation has been made of processes of charge exchange and ionization between an hydrogen atom and fully stripped ions embedded in very strong static electric fields ($O(10^{10}$ V/m$)$), which are thought to exist in cosmic and laser--produced plasmas. Calculations show that the presence of the field affects absolute values of the cross sections, enhancing ionization and reducing charge exchange. Moreover, the overall effect depends upon the relative orientation between the field and the nuclear motion. Other features of a null-field situation, such as scaling laws, are revisited.Comment: Latex, 13 pages, 11 figures (available upon request), to be published in Journal of Physics

Marker tolerant, immunocompetent animals as a new tool for regenerative medicine and long-term cell tracking

<p>Abstract</p> <p>Background</p> <p>Immune-mediated rejection of labeled cells is a general problem in transplantation studies using cells labeled with any immunogenic marker, and also in gene therapy protocols. The aim of this study was to establish a syngeneic model for long-term histological cell tracking in the absence of immune-mediated rejection of labeled cells in immunocompetent animals. We used inbred transgenic Fischer 344 rats expressing human placental alkaline phosphatase (hPLAP) under the control of the ubiquitous R26 promoter for this study. hPLAP is an excellent marker enzyme, providing superb histological detection quality in paraffin and plastic sections.</p> <p>Results</p> <p>Transplantation of cells from hPLAP transgenic (hPLAP-tg) F344 rats into wild-type (WT) F344 recipients failed because of immune-mediated rejection. Here we show that this problem can be overcome by inducing tolerance to the marker gene by transplantation of bone marrow from hPLAP-tg F344 rats into WT F344 hosts after lethal irradiation, or by neonatal exposure of WT F344 rats to hPLAP-tg F344 cells. As proof-of-principle, we injected bone marrow cells (BMC) from hPLAP-tg rats into the knee joint of marker tolerant, bone marrow-transplanted WT rats, and found successful engraftment and differentiation of donor cells. In addition, hPLAP-tg BMC injected intravenously in neonatally tolerized WT F344 hosts could be traced in lymph nodes, 2 months post-injection.</p> <p>Conclusion</p> <p>In combination with the excellent marker hPLAP, marker tolerant animals may open up new perspectives for all experiments requiring long-term histological tracking of genetically labeled cells.</p

From Luttinger liquid to Altshuler-Aronov anomaly in multi-channel quantum wires

A crossover theory connecting Altshuler-Aronov electron-electron interaction corrections and Luttinger liquid behavior in quasi-1D disordered conductors has been formulated. Based on an interacting non-linear sigma model, we compute the tunneling density of states and the interaction correction to the conductivity, covering the full crossover.Comment: 15 pages, 3 figures, revised version, accepted by PR