Résumé Dans la présente thèse, nous avons étudié la déformation anisotrope par bombardement ionique de nanoparticules d'or intégrées dans une matrice de silice amorphe ou d'arséniure d’aluminium cristallin. On s’est intéressé à la compréhension du mécanisme responsable de cette déformation pour lever toute ambigüité quant à l’explication de ce phénomène et pour avoir une interprétation consistante et unique. Un procédé hybride combinant la pulvérisation et le dépôt chimique en phase vapeur assisté par plasma a été utilisé pour la fabrication de couches nanocomposites Au/SiO2 sur des substrats de silice fondue. Des structures à couches simples et multiples ont été obtenues. Le chauffage pendant ou après le dépôt active l’agglomération des atomes d’Au et par conséquent favorise la croissance des nanoparticules. Les nanocomposites Au/AlAs ont été obtenus par implantation ionique de couches d’AlAs suivie de recuit thermique rapide. Les échantillons des deux nanocomposites refroidis avec de l’azote liquide ont été irradiés avec des faisceaux de Cu, de Si, d’Au ou d’In d’énergie allant de 2 à 40 MeV, aux fluences s'étendant de 1×1013 à 4×1015 ions/cm2, en utilisant le Tandem ou le Tandetron. Les propriétés structurales et morphologiques du nanocomposite Au/SiO2 sont extraites en utilisant des techniques optiques car la fréquence et la largeur de la résonance plasmon de surface dépendent de la forme et de la taille des nanoparticules, de leur concentration et de la distance qui les séparent ainsi que des propriétés diélectriques du matériau dans lequel les particules sont intégrées. La cristallinité de l’arséniure d’aluminium est étudiée par deux techniques: spectroscopie Raman et spectrométrie de rétrodiffusion Rutherford en mode canalisation (RBS/canalisation). La quantité d’Au dans les couches nanocomposites est déduite des résultats RBS. La distribution de taille et l’étude de la transformation de forme des nanoparticules métalliques dans les deux nanocomposites sont déterminées par microscopie électronique en transmission. Les résultats obtenus dans le cadre de ce travail ont fait l’objet de trois articles de revue. La première publication montre la possibilité de manipuler la position spectrale et la largeur de la bande d’absorption des nanoparticules d’or dans les nanocomposites Au/SiO2 en modifiant leur structure (forme, taille et distance entre particules). Les nanoparticules d’Au obtenues sont presque sphériques. La bande d’absorption plasmon de surface (PS) correspondante aux particules distantes est située à 520 nm. Lorsque la distance entre les particules est réduite, l’interaction dipolaire augmente ce qui élargit la bande de PS et la déplace vers le rouge (602 nm). Après irradiation ionique, les nanoparticules sphériques se transforment en ellipsoïdes alignés suivant la direction du faisceau. La bande d’absorption se divise en deux bandes : transversale et longitudinale. La bande correspondante au petit axe (transversale) est décalée vers le bleu et celle correspondante au grand axe (longitudinale) est décalée vers le rouge indiquant l’élongation des particules d’Au dans la direction du faisceau. Le deuxième article est consacré au rôle crucial de la déformation plastique de la matrice et à l’importance de la mobilité des atomes métalliques dans la déformation anisotrope des nanoparticules d’Au dans les nanocomposites Au/SiO2. Nos mesures montrent qu'une valeur seuil de 2 keV/nm (dans le pouvoir d'arrêt électronique) est nécessaire pour la déformation des nanoparticules d'or. Cette valeur est proche de celle requise pour la déformation de la silice. La mobilité des atomes d’Au lors du passage d’ions est confirmée par le calcul de la température dans les traces ioniques. Le troisième papier traite la tentative de formation et de déformation des nanoparticules d’Au dans une matrice d’arséniure d’aluminium cristallin connue pour sa haute résistance à l’amorphisation et à la déformation sous bombardement ionique. Le résultat principal de ce dernier article confirme le rôle essentiel de la matrice. Il s'avère que la déformation anisotrope du matériau environnant est indispensable pour la déformation des nanoparticules d’or. Les résultats expérimentaux mentionnés ci-haut et les calculs de températures dans les traces ioniques nous ont permis de proposer le scénario de déformation anisotrope des nanoparticules d’Au dans le nanocomposite Au/SiO2 suivant: - Chaque ion traversant la silice fait fondre brièvement un cylindre étroit autour de sa trajectoire formant ainsi une trace latente. Ceci a été confirmé par la valeur seuil du pouvoir d’arrêt électronique. - L’effet cumulatif des impacts de plusieurs ions conduit à la croissance anisotrope de la silice qui se contracte dans la direction du faisceau et s’allonge dans la direction perpendiculaire. Le modèle de chevauchement des traces ioniques (overlap en anglais) a été utilisé pour valider ce phénomène. - La déformation de la silice génère des contraintes qui agissent sur les nanoparticules dans les plans perpendiculaires à la trajectoire de l’ion. Afin d’accommoder ces contraintes les nanoparticules d’Au se déforment dans la direction du faisceau. - La déformation de l’or se produit lorsqu’il est traversé par un ion induisant la fusion d’un cylindre autour de sa trajectoire. La mobilité des atomes d’or a été confirmée par le calcul de la température équivalente à l’énergie déposée dans le matériau par les ions incidents. Le scénario ci-haut est compatible avec nos données expérimentales obtenues dans le cas du nanocomposite Au/SiO2. Il est appuyé par le fait que les nanoparticules d’Au ne se déforment pas lorsqu’elles sont intégrées dans l’AlAs résistant à la déformation. / Abstract In the present thesis, we study the anisotropic deformation of gold nanoparticles embedded in amorphous silica or crystalline aluminum arsenide, under ion bombardment. We try to comprehend the mechanism responsible for this deformation and to remove any ambiguity related to the explanation of this phenomenon. A hybrid process combining sputtering and plasma enhanced chemical vapour deposition was used to fabricate Au/SiO2 layers on fused silica substrates. Structures with single and multilayer were obtained. Heating during or after deposition activates the Au atom agglomeration and favours the growth of the nanoparticles. Also, a Au/AlAs nanocomposite was obtained by ion implantation of AlAs films, followed by rapid thermal annealing. The samples of the two nanocomposites, cooled with liquid nitrogen, were irradiated with 2 to 40 MeV Cu, Si, Au or In ion beams, at fluences ranging from 1×1013 to 4×1015 ions/cm2, using a Tandem or Tandetron accelerator. The structural and morphological properties of the Au/SiO2 nanocomposite were extracted by optical means; the frequency and the width of surface plasmon resonance band depend on the nanoparticle shape and size, their concentration, the inter-particle distance and the dielectric properties of material in which the particles are embedded. The aluminum arsenide crystallinity was studied by two techniques: Raman spectroscopy and Rutherford backscattering spectrometry in channelling configuration (RBS/ channelling). The Au concentration in the nanocomposite layers was deducted from RBS results. The size distribution and metallic nanoparticles shape transformation in both nanocomposites were observed by electronic transmission microscopy. The results obtained within the framework of this work are the subject of three journal papers. The first publication shows the possibility of manipulating the width and spectral position of the gold nanoparticle absorption band in Au/SiO2 nanocomposites by modifying their structure (form, size and inter-particle distance). The obtained Au nanoparticles are nearly spherical. The surface plasmon (PS) absorption band corresponding to the distant particles is located at 520 nm. After ion irradiation, the spherical nanoparticles transform into ellipsoids aligned along the ion beam. The absorption band splits into two bands: transversal and longitudinal. The band corresponding to the ellipsoids small axis (transversal) is blue-shifted and that corresponding to the long axis (longitudinal) is red-shifted indicating the elongation of particles in the beam direction. The second paper is consecrated to the crucial role of the plastic deformation of the matrix and to the importance of the metal atomic mobility in the anisotropic nanoparticles deformation in Au/SiO2 nanocomposites. Our measurements show that a threshold value of 2 keV/nm (electronic stopping power) is necessary for the deformation of Au nanoparticles. This value is close to that required for silica deformation. Mobility of the Au atoms at the time of the ion passage is confirmed by temperature calculation within the ionic track. The third paper treats the attempt of formation and deformation of Au nanoparticles in crystalline aluminum arsenide matrix known by its high resistance to amorphisation and deformation under ionic bombardment. The principal result of the last article confirms the essential role of the matrix. It proves that the anisotropic deformation of surrounding material is indispensable for gold nanoparticles deformation. The experimental results mentioned above and temperature calculations within ionic tracks allowed us to propose the following anisotropic deformation scenario of Au nanoparticles embedded in Au/SiO2 nanocomposite: - Each ion crossing the silica melts (very briefly) a narrow cylinder around its trajectory forming thus a latent track. This is consistent with the observed threshold value in the electronic stopping power. - The cumulative effect of many separate ion impacts leads to the anisotropic growth of the silica matrix which contracts in the direction of the beam and elongates in the perpendicular direction. The overlap model of the ionic tracks was used to validate this phenomenon. - The deformation of silica generates strains which act on the nanoparticles in the plane perpendicular to the ion trajectory. In order to accommodate these strains, the Au nanoparticles deform in the beam direction. - The deformation of nanoparticles occurs each time an ion traverses the gold particle and melts a cylinder around its trajectory. The mobility of the gold atoms was confirmed by a calculation of the equivalent temperature from the deposited energy in the material by incident ions. The scenario above is compatible with our experimental data obtained in the case of the Au/SiO2 nanocomposite. It is further supported by the fact that the Au nanoparticules do not deform when they are integrated in AlAs which is resistant to the deformation.

The reliability of modern gas turbine engines is largely due to careful damage tolerant design a method of structural design based on the assumption that flaws (cracks) exist in any structure and will continue to grow with usage. With proper monitoring, largely in the form of periodic inspections at conservative intervals reliability and safety is maintained. These methods while reliable can lead to the early retirement of some components and unforeseen failure if design assumptions fail to reflect reality. With improvements to sensor and computing technology there is a growing interest in a system that could continuously monitor the health of structural aircraft as well as forecast future damage accumulation in real-time. Through the use of two-dimensional and three-dimensional numerical modeling the initial goals and findings for this continued work include: (a) establishing measurable parameters directly linked to the health of the blade and (b) the feasibility of detecting accumulated damage to the structural material and thermal barrier coating as well as the onset of damage causing structural failure.

Digital image correlation techniques (DICT), a non-contact deformation measuring technique based on gray value digital images, have become increasingly used over the last years. By using the DIC technique during a tensile test, the deformation behavior of different engineering material under an applied load can be determined and analyzed. Digital images, acquired from a tensile test, can be correlated by using DICT software and from that the local or global mechanical properties can be calculated. The local or global mechanical properties determination of a flat test specimens are based on the displacements or changes in a previous stochastic sprayed or natural pattern. The used material for this purpose is cast silicon (Si) based aluminium (Al) component, designated as AlSi7Mg0.3 (Anticorodal-78 dv). The hypoeutectic Al- Si alloy is widely applicable for engine constructions, vehicle and aerospace constructions, shipbuilding, electrical engineering and constructions for food industry. There are many microstructural parameters in a binary system Al- Si alloys, which the mechanical properties can be depended on, for instance phase distribution, Secondary Dendrite Arm Spacing (SDAS), morphology of Si particles (Roundness) and microscopic defects or pores. All these parameters can contribute to enhance the proper mechanical performance (e.g. Strength and ductility) in the Al-Si cast components.

The objective of this research was to investigate the occurrence of mycoflora in rice plants and rice seeds in South Africa and their negative impact. A total of six species of Fusarium were isolated from diseased rice plants and rice seeds and identified as F. anthophilum, F. chlamydosporum, F. compactum, F. equiseti, F. fujikuroi and F. semitectum. In the translation elongation factor data set, Fusarium equiseti isolates grouped together within the F. incarnatum - equiseti Species Complex (FIESC). The isolates from rice clustered together in a single clade with the F. equiseti and F. incarnatum isolates forming two separate sub-clades.The isolates of F. equiseti present a new phylogenetically distinct species in FIESC. In the pathogenicity tests, isolates of both F. anthophilum and F. fujikuroi caused bakanae disease to rice plants. Fifty four rice cultivars and lines were tested by the standardized test tube inoculation method for resistance and susceptibility against bakanae isolate of F. anthophilum and the bakanae isolate of F. fujikuroi. None of the rice cultivars and lines was found to be resistant to bakanae isolates of Fusarium spp. The fungicide, benomyl was found to be most effective as a seed treatment for controlling bakanae disease of rice due to isolates of both F. anthophilum and F. fujikuroi. Thiram was found to be the least effective fungicide for controlling bakanae disease of rice caused by isolates of both the Fusarium spp. Apart from Fusarium species, other fungi that were also isolated from diseased rice plants and rice seeds were identified as Alternaria alternata, Alternaria longipes, Cochliobolus miyabeanus, Nigrospora sphaerica, Phoma eupyrena, Phoma jolyana, Phoma sorghina and Pithomyces sp. In mycotoxin tests, the isolates of both F. anthophilum and F. fujikuroi produced moniliformin. None of the isolates of F. anthophilum and F. fujikuroi produced fumonisins. This research is important as it identifies many fungal species in rice plants and seeds in South Africa for the first time. Currently, there is very little literature that makes reference to such findings under South African conditions. In addition, this investigation unravels previously unknown information on the resistance of rice to bakanese disease. Finally, information is provided on the effectiveness of commonly used fungicides (benomyl and thiram) to control rice diseases. This knowledge is crucial information that is useful to plant pathologists, the farming community and the scientists that are involved in strategies of fighting or reducing rice diseases so as to help contribute to food security. / Environmental Sciences / D. Phil. (Environmental Science)

The present work aims to determine the physiological and sanitary quality of Sweet sorghum (Sorghum bicolor (L.) Moench) seeds, as well as to identify pathogens associated with seed, their transmission to seedlings and the subsequent pathogenicity of isolates obtained, In addition, molecularly identify the fungal species pathogenic to this crop. The experiments were carried out in the Teaching and Seed Research Laboratory (TSRL), of the Plant Engineering Department; In the Elocy Minussi Phytopathology Laboratory, Department of Plant Protection, and at the Biological Institute of São Paulo. Sweet sorghum seeds were used, all without chemical treatment. Sanitary quality was evaluated by sanity test, and physiological characteristics by germination and vigor tests (seedling length, dry mass, emergence, rate of emergence and accelerated aging). It was performed the test of transmission of the pathogens associated to the seeds and the subsequent pathogenicity of the obtained isolates, culminating with the molecular characterization of the identified pathogens, in which were sequenced the Internal Transcribed Spacer (ITS) genomic regions and the Elongation Factor 1 - alpha (TEF1-α) gene. The design used was the completely randomized design, with four cultivars of Sweet sorghum (BRS 506, F19, BRS 511 and BRS 509); For the evaluation of pathogenicity, the factorial scheme is represented by four cultivars and three isolates of Fusarium spp., besides the witness. The seeds of the BRS 509 cultivar were considered to have lower physiological quality than the other cultivars. The DNA sequencing allowed identifying the Fusarium thapsinum species as a pathogenic agent in the sweet sorghum crop, and proven its transmission via seeds. / O presente trabalho teve como objetivo determinar a qualidade fisiológica e sanitária de sementes de cultivares de sorgo sacarino (Sorghum bicolor (L.) Moench), bem como identificar os patógenos associados à semente, sua transmissão às plântulas e a posterior patogenicidade de isolados obtidos, além disso, identificar molecularmente as espécies fúngicas patogênicas a esta cultura. Os experimentos foram realizados no Laboratório Didático e de Pesquisas em Sementes (LDPS), do Departamento de Fitotecnia; no Laboratório de Fitopatologia Elocy Minussi, do Departamento de Defesa Fitossanitária e, no Instituto Biológico de São Paulo. Foram utilizadas sementes de sorgo sacarino, todas sem tratamento químico. A qualidade sanitária foi avaliada pelo teste de sanidade, e as características fisiológicas por meio dos testes de germinação e de vigor (comprimento de plântulas, massa seca, emergência, índice de velocidade de emergência e envelhecimento acelerado). Foi realizado o teste de transmissão dos patógenos associados à semente e a posterior patogenicidade dos isolados fúngicos obtidos, culminando com a caracterização molecular dos patógenos identificados, na qual foram sequenciadas as regiões genômicas Internal Transcribed Spacer (ITS) e o gene do fator de elongação 1-α (TEF1α). O delineamento utilizado foi o inteiramente casualizado com quatro cultivares de sorgo sacarino (BRS 506, Fepagro 19, BRS 511 e BRS 509); já para a avaliação da patogenicidade, o esquema fatorial foi representado pelas quatro cultivares e três isolados de Fusarium sp., além da testemunha. As sementes da cultivar BRS 509 foram consideradas de qualidade fisiológica inferior as demais cultivares. O sequenciamento de DNA permitiu identificar a espécie Fusarium thapsinum como agente patogênico na cultura do sorgo sacarino, sendo comprovada sua transmissão via sementes.

La croissance hétérotrophe de l’hypocotyle est une étape clé pour la réussite de la levée. La présente étude est focalisée sur le déterminisme génétique de l’allongement de cet organe à l’obscurité chez Medicago truncatula en analysant le nombre et la longueur des cellules de l’épiderme, tissu gouvernant l’allongement des organes. Une grande variabilité génétique du nombre de cellules a été révélée dans les graines de 15 génotypes représentatifs de la diversité génétique de l’espèce. La stabilité de ce caractère dans des graines provenant de différentes productions suggère qu’il est sous contrôle génétique fort. Il a été montré que ce nombre de cellules, préétabli dans les graines, est le principal déterminant de la variation génotypique de la longueur de hypocotyle en conditions optimales de croissance. Par contre, l'élongation cellulaire devient le déterminant majeur des différences génotypiques observées sous stress abiotiques (basse température, déficit hydrique).Des loci contrôlant le nombre de cellules de l’épiderme et leur longueur maximale à basse température ont ensuite été identifiés dans une population de lignées recombinantes. Ceux ayant un impact sur l’élongation de l’hypocotyle à basse température ont été mis en évidence. Enfin, deux génotypes présentant un nombre de cellules similaire mais des capacités d’allongement cellulaire contrastées ont été plus finement comparés. Des protéines ayant un rôle dans la formation et l’organisation du cytosquelette et dans la modification des parois cellulaires ont été révélées en lien avec les différences d’allongeme / The heterotrophic growth of hypocotyl is a crucial process for successful seedling emergence. The present study is focused on the genetic determinism of its elongation in darkness in Medicago truncatula by analyzing the number and the length of cells of epidermis, the tissue controlling organ elongation.A large genetic variability of the epidermal cell number of the hypocotyl in seeds of 15 genotypes representative of the genetic diversity of the species was revealed. The stability of this trait in the seeds collected from different productions suggests it is under strong genetic control. This cell number was shown to be the main contributor of genotypic variation of hypocotyl length in optimal conditions. On the other hand, cell elongation becomes the major determinant of the genotypic differences observed under abiotic stresses (low temperature, water deficit).Quantitative Trait Loci (QTLs) controlling the number of epidermal cells and their maximal length at low temperature were then identified using a recombinant inbred lines population, and those impacting hypocotyl elongation at low temperature were highlighted.Finally, two genotypes sharing a similar cell number but contrasted capacities of cell elongation were compared more in detail. Proteins playing a role in the formation and organization of cytoskeleton and in the modification of the cell wall were revealed in connection with the differences in cellular elongation between genotypes. Moreover, differences in the cell wall sugar composition, in the degree of methylation of pectins and in a potential inhibito

The reliability of modern gas turbine engines is largely due to careful damage tolerant design a method of structural design based on the assumption that flaws (cracks) exist in any structure and will continue to grow with usage. With proper monitoring, largely in the form of periodic inspections at conservative intervals reliability and safety is maintained. These methods while reliable can lead to the early retirement of some components and unforeseen failure if design assumptions fail to reflect reality. With improvements to sensor and computing technology there is a growing interest in a system that could continuously monitor the health of structural aircraft as well as forecast future damage accumulation in real-time. Through the use of two-dimensional and three-dimensional numerical modeling the initial goals and findings for this continued work include: (a) establishing measurable parameters directly linked to the health of the blade and (b) the feasibility of detecting accumulated damage to the structural material and thermal barrier coating as well as the onset of damage causing structural failure.

La capacité d’une plante à réguler sa croissance racinaire est une composante importante de l’acclimatation aux stress environnementaux. A l’échelle cellulaire, cette régulation est effectuée via le contrôle de la division et de l’élongation des cellules mais les rôles respectifs de chaque processus et leurs interactions sont peu connus. Notamment, l’activité de production de cellules du méristème apical racinaire (RAM) est trop souvent négligée. Dans cette thèse, l’analyse spatiale de la croissance le long de l’apex racinaire et l’analyse temporelle des trajectoires de croissance des cellules ont été couplées pour comprendre les liens existants entre division et élongation cellulaire. Pour cela, j’ai développé un système de phénotypage de la croissance à haute résolution spatio-temporelle qui a été appliqué à l’étude de racines d’un peuplier euraméricain (Populus deltoides × Populus nigra) en réponse à différents stress (stress osmotique, impédance mécanique). Une forte variabilité du taux de croissance racinaire entre individus ainsi que des variations individuelles cycliques de la croissance ont été observées malgré des conditions environnementales contrôlées. L’utilisation de cette variabilité couplée à la quantification de l’activité du RAM a mis en évidence l’importance du taux de production de cellules pour soutenir la croissance racinaire. Ces travaux analysent une nouvelle échelle de variations spatiales et temporelles de la croissance peu prise en compte jusqu’à présent. Hautement applicable à d’autres questions scientifiques, l’analyse du devenir des cellules une fois sortie du RAM est également discutée pour des conditions de croissance non stables / Regulation of root growth is a crucial capacity of plants for acclimatization to environmental stresses. At cell scale, this regulation is controlled through cell division and cell elongation but respective importance of these processes and interactions between them are still poorly known. Notably, the cell production activity of the root apical meristem (RAM) is often excluded. During this thesis, spatial analyses of growth along the root apex were coupled with temporal analyses of cell trajectories in order to decipher the links between cell division and cell elongation. This required the setup of a system for phenotyping root growth at a high spatiotemporal resolution which was applied to study the growth of roots from an euramerican poplar (Populus deltoides × Populus nigra) in response to different environmental stresses (osmotic stress or mechanical impedance). An important variability of root growth rate between individuals as well as individual cyclic variations of growth along time were observed despite tightly controlled environmental conditions. Use of this variability coupled with quantification of the RAM activity led us to a better understanding of the importance of the cell production rate for sustaining root growth. This work analyses a new spatiotemporal scale of growth variability poorly considered. Widely applicable to others scientific questioning, temporal analyses of cell fate once produced in the RAM is also discussed for non-steady growth conditions

Gene expression of human immunodeficiency virus type-1 (HIV-1), which causes Acquired Immunodeficiency Syndrome (AIDS), is regulated at the transcriptional level, where negative factors can block elongation that is overcome by HIV Tat protein and P-TEFb. P-TEFb, a positive elongation transcription factor with two subunits, CDK9 and Cyclin T1 (CycT1), catalyzes Tat-dependent phosphorylation of Ser-5 in the Pol II C-terminal domain (CTD), allowing production of longer mRNAs. Ser-5 phosphorylation enables the CTD to recruit mammalian mRNA capping enzyme (Mce1) and stimulate its guanylyltransferase activity. This dissertation demonstrates that stable binding of Mce1 and cap methyltransferase to template-engaged Pol II depends on CTD phosphorylation, but not on nascent RNA. Capping and methylation doesn't occur until nascent pre-mRNA become 19-22 nucleotides long. A second and novel pathway for recruiting and activating Mce1 involved direct physical interaction between the CTD, Tat and Mce1. Tat stimulated the guanylyltransferase and triphosphatase activities of Mce1, thereby enhancing the otherwise low efficiency of cotranscriptional capping of HIV mRNA. These findings imply that multiple mechanisms exist for coupling transcription elongation and mRNA processing at a checkpoint critical to HIV gene expression. To elucidate P-TEFb's function in human (HeLa) cells, RNA interference (RNAi) was used to degrade mRNA for hCycT1 or CDK9. Down-regulation of P-TEFb expression by RNAi can be achieved without causing major toxic or lethal effects and can control Tat transactivation and HIV replication in host cells. High-density oligonucleotide arrays were used to determine the effect of P-TEFb knockdown on global gene expression. Of 44,928 human genes analyzed, 25 were down-regulated and known or likely to be involved in cell proliferation and differentiation. These results provide new insight into P-TEFb function, its potent role in early embryonic development and strong evidence that P-TEFb is a new target for developing AIDS and cancer therapies. To fulfill the promise of RNAi for treating infectious and human genetic diseases, structural and functional mechanisms underlying RNAi in human cells were studied. The status of the 5' hydroxyl terminus of the antisense strand of short interfering RNA (siRNA) duplexes determined RNAi activity, while a 3' terminus block was tolerated in vivo. A perfect A-form helix in siRNA was not required for RNAi, but was required for antisense-target RNA duplexes. Strikingly, crosslinking siRNA duplexes with psoralen did not completely block RNAi, indicating that complete unwinding of the siRNA helix is not necessary for RNAi in vivo. These results suggest that RNA amplification by RNA-dependent RNA polymerase is not essential for RNAi in human cells.

The aim of this master thesis is to show a connection among nuclear fuel safety, experiments led in research reactors and calculation codes. This thesis focuses on the calculation code Transuranus. There are represented four experiments, which were calculated in Transuranus. The fission gas release, elongation and growth of fuel were particularly monitored. Is is possible to set differences among versions v1m1j09 and v1m3j12 from achieved results, as well as the influence of selected Transuranus parameters on the results, so the thesis may bring new pieces of knowledge for improvement of safety analysis calculation by Transuranus.