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Finite-different frequency-domain analysis of a dielectric waveguide crossingCheng, Wei-chi 25 January 2010 (has links)
Multiple dielectric crossing waveguides are indispensable in building a complex optical integrated circuit. Since each input/output waveguide will have many crossings, it is important to design a low-loss waveguide crossing to ensure the overall radiation loss is kept at a minimum.
The beam propagation method (BPM) is usually the method of choice for modeling large but low-index-contrast waveguide devices. BPM assumes one-way propagation and adopts the paraxial approximation. It is neither able to consider reflection of electromagnetic (EM) fields nor to perform wide angle propagation of forward fields. Therefore, it can not be used to analyze perpendicular dielectric crossing waveguides. At a maximum 0.5 dB power loss per crossing, the difficulty of simulation a waveguide crossing is how to compute the complex coupling waves with high enough precision.
In this thesis, two-dimensional planar integrated optical waveguide crossing is studied in detail for the through and cross power coupling coefficients with the finite-difference frequency-domain (FD-FD) method. By exploiting the dual symmetries: the ¡§+¡¨ symmetry and the ¡§X¡¨ symmetry in the perpendicular crossing waveguide, we are able to compute the EM fields and their power coefficients without using artificial absorbing boundary conditions (ABC) nor using the perfectly matching layer (PML). We develop the layer-mode based transparent boundary condition (LM-TBC) [1] for launching the fundamental incident mode as well as transmitting the reflected and scattered wave fields off the crossing area. Numerical results including the field distribution, power coefficients are carefully verified and the convergent comparisons are also studied in the thesis.
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Recombination frequency, chiasma counts and the process of crossing-overNilsson, Nils-Otto. January 1994 (has links)
Thesis (doctoral)--Lund University, 1994. / Added t.p. with thesis statement inserted.
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Recombination frequency, chiasma counts and the process of crossing-overNilsson, Nils-Otto. January 1994 (has links)
Thesis (doctoral)--Lund University, 1994. / Added t.p. with thesis statement inserted.
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The photochemistry of unsaturated carbonyl compoundsWillasey-Wilsey, Sarah Louise January 1995 (has links)
No description available.
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Analysis of microwave crossing waveguide with analytic continuity mode-matching methodChan, Chia-Ta 02 July 2010 (has links)
We show that a 3-D microwave crossing waveguide can be solved by a 2-D scalar Helmholtz equation with combining boundary conditions for TE and TM modes. Furthermore the crossing waveguide possesses a symmetry along two diagonal axes passing through the origin. Computation of the EM wave fields is decomposed into four smaller tasks of computing reflection coefficient vector of a parallel plate waveguide terminated with a corner made of two perfectly electric or magnetic conducting walls (PECW/PMCW).
In this thesis, we propose a mixed Cartesian and polar coordinate mode-matching method to solve this 2-D corner cube microwave reflection problem. The solution is obtained by applying the continuity condition of both the tangential field and its normal derivative along a given curve inside the overlapped region of the two coordinate systems. We are able to compute up to the third decimal place of the reflection, through and cross transmission coefficients. All results pass the energy conservation test and are verified and compared with those computed by Integral equation method simulation.
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Induced mitotic recombination in Aspergillus strains differing in sensitivity to ultraviolet light.Shanfield, Bernice G. January 1968 (has links)
No description available.
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The effect of X chromosome inversions on crossing-over in the third chromosome of Drosophila melanogaster. --.Fraser, Frank Clarke. January 1941 (has links)
No description available.
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Induced mitotic recombination in Aspergillus strains differing in sensitivity to ultraviolet light.Shanfield, Bernice G. January 1968 (has links)
No description available.
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Caractérisation de variations naturelles de fréquence de crossovers chez le colza (Brassica napus) / Caracterization of natural variation of crossover rate in Oilseed rape (Brassica napus)Grandont, Laurie 09 March 2012 (has links)
La méiose est un processus fondamental qui conditionne la formation de gamètes et assure la stabilité des génomes tout en générant de la diversité par brassage génétique. La régularité méiotique nécessite la formation de crossing-overs (CO) exclusivement entre chromosomes homologues. Cette condition est plus difficile à remplir chez les espèces allopolyploïdes qui présentent plusieurs jeux de chromosomes toujours susceptibles de recombiner ensemble. Bien que la polyploïdie soit omniprésente chez les plantes, on connait peu de choses sur le déroulement de la méiose chez ces espèces. Au cours de ma thèse, je suis intéressée à l’effet de la polyploïdie sur la formation et sur la fréquence de CO en utilisant le colza (Brassica napus, AACC, 2n=38) comme modèle d’étude. J’ai notamment cherché à comprendre : (1) quel est l’effet du niveau de ploïdie sur la fréquence de crossovers, et (2) l’origine des variations de COs observées chez les plantes allohaploïdes (AC) produites à partir de différentes variétés de colza, en utilisant une palette d’approches cytologiques et cytogénétiques. Mes travaux ont permis de montrer que le niveau de ploïdie induit une augmentation de la fréquence de crossovers, et que cette augmentation est plus importante dans un contexte triploïde que tétraploïde. J’ai ainsi montré que la fréquence de CO augmente progressivement du diploïde (1,6 CO/bivalent) vers le tétraploïde (2 CO/bivalents) et quelle est maximale chez le triploïde (2,8 CO/bivalent). En ce qui concerne la deuxième question, j’ai montré que la différence entre les allohaploïdes de colzas apparaît tardivement au cours de la méiose. Elle semble être liée à une capacité différente à former des CO en fonction de la variété utilisée pour produire ces allohaploïdes et non pas à une différence dans la reconnaissance de l’homologie. Un de mes résultats original est que la protéine HEI10, impliquée dans la voie de formation des CO interférents, présente une dynamique différente entre les deux variétés, que ce soit à l’état euploïde (AACC) qu’allohaploïde (AC).Mes résultats conduisent à s’interroger sur la relation entre (i) la régulation du nombre de CO formés entre chromosomes homologues et (ii) la suppression des CO entre chromosomes non homologues chez les espèces allopolyploïdes. / Meiosis is a fundamental process required to produce gametes, ensure genome stability and generate diversity within species by creating new chromosome/allele combinations. For all these outcomes the exclusive formation of crossovers (CO) between homologous chromosomes is required. This condition is more difficult to fulfil in allopolyploid species that have more than two sets of chromosomes still able to recombine together. Although polyploidy has been particularly prevalent in plants, little is known about meiosis in polyploids. During my thesis I have analyzed the effect of polyploidy on CO formation and frequency, using oilseed rape (Brassica napus, AACC, 2n=38) as model. My work aimed to investigate (i) the effect of ploidy level on the rate of meiotic COs and (ii) the causes for the observed difference in CO rate between allohaploid plants (AC) produced from different B. napus varieties. To address these questions, I have combined a series of cytological, immunocytological and cytogenetical analyses.My work first indicates that polyploidization leads to increase CO frequency. I showed that the number of COs progressively increases from the diploid (1,6 CO/bivalent) to the tetraploid (2 CO/bivalent) and is maximal in the triploid (2,8 CO/bivalent). In the second part, I have shown that the difference of meiotic behaviors between B. napus allohaploids appears at a late stage of meiosis. This difference seems to be due to a difference in the propensity to form CO between the two varieties rather than a difference in the stringency of homology recognition. This difference could be related to the difference in the pattern and/or chronology of HEI10 (a key protein involved in the interfering CO pathway) signals along chromosomes during prophase I in both euploids (AACC) and allohaploids (AC).My results thus puts under the spotlight the link that may exist between (i) the regulation of CO rate between homologous chromosomes and (ii) the suppression of COs between non-homologous chromosomes in polyploid species.
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Role of DNA methylation in meiotic recombination in Arabidopsis thaliana / Rôle de la méthylation de l’ADN dans la recombinaison meiotique chez Arabidopsis thalianaLahouze, Benoit 03 July 2015 (has links)
Pendant la méiose, la division cellulaire qui forme les cellules haploïdes, les chromosomes homologues hérités de chacun des deux parents sont appariés et échangent des segments réciproques appelés crossing-overs (CO). Les CO ne sont pas distribués au hasard dans le génome et leur taux varie le long des chromosomes. Certains des mécanismes responsable ont été décrits chez les mammifères et la levure mais ne sont pas conservés chez les plantes. Les CO sont fortement inhibés dans l'hétérochromatine qui est riche en éléments répétés. Le degré élevé de méthylation d l'ADN qui caractérise les séquences répétées pourrait être un inhibiteur des CO. Cela a été clairement démontré chez le champignon Ascobolus immersus et des études récentes ont montré que la perte de méthylation modifiait la distribution des CO chez Arabidopsis thaliana. Le but de ma thèse a été de décrire plus précisément le rôle de la méthylation de l'ADN dans le contrôle des CO en l'absence de polymorphisme de séquence qui affecte aussi la recombinaison.Pour cela, j'ai mesuré la recombinaison dans différentes plantes dans lesquelles la méthylation de l'ADN a été partiellement ou totalement enlevée grâce à la mutation du gène ddm1. Pour tester l'effet opposé d'un gain de méthylation, j'ai aussi essayé de cibler la methylation de l'ADN à un point chaud de recombinaison connu. Mes résultats montrent que la parte de la méthylation de l'ADN entraîne une augmentation globale de la recombinaison. Paradoxalement, l'heterochromatine qui est normalement très méthylée est moins affectée par la perte de méthylation que le reste du chromosome, probablement car la méthylation de l'ADN a des effets à distance. L'augmentation de CO est accentuée dans les générations successives du mutant ddm1. Cependant, l'effet le plus important est observé dans les hétérozygotes où la moitié du génome seulement est hypométhylée, ce qui suggère un rôle complexe de la méthylation. Finalement, j'ai pu montrer que le polymorphisme affecte la recombinaison surtout dans l'hétérochromatine mais pas dans le sens attendu puisque les plantes homozygotes recombinent moins que les plantes hétérozygotes. / During meiosis, the cellular division that gives rise to haploid cells, homologous chromosomes inherited from each parent are paired and are subjected to reciprocal exchanges of chromosome segments called crossing-overs (COs). COs are not randomly distributed in the genome. Some of the involved mechanisms have recently been described in mammals and yeast bu they are not conserved in plants. Repeat-rich heterochromatin is suppressed for COs. The high level of DNA methylation associated with repeats could be an inhibitor of COs. This was clearly demonstrated in the fungus Ascobolus immersus and recent studies have shown that the loss of DNA methylation also affects COs in Arabidopsis thaliana. The aim of my thesis was to describe more precisely the role of DNA methylation in the control of CO distribution in the absence of any DNA sequence polymorphism which are known to affect recombination. For this purpose, I measured recombination in different plants where DNA methylation has been partially or completely removed thanks to the mutation of the DDM1 gene. To test the opposed effect of a gain of DNA methylation,.I also tried to target DNA methylation at a known recombination hotspot. My results show that the loss of DNA methylation induces a global increase of recombination. Paradoxically, the normally highly methylated heterochromatin is less affected by this loss than the rest of the chromosome, probably because DNA methylation has distal effects. The increased recombination is exacerbated in successive generations of the hypomethylated ddm1 mutants. However, the strongest effect is seen in the heterozygotes where only half of the genome is hypomethylated, suggesting a complex role in the control of CO distribution. Finally, I show that DNA sequence polymorphism affects mainly recombination in the heterochromatin but not in the expected sense, since homozygous plants recombine less than heterozygous.
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