Events
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Seminar: Stochastic and deterministic rogue waves: from low dimensional models to chaotic dynamics in semiconductor lasers
Speaker
Jordi Zamora MuntDescription Rogue waves in the ocean are extreme events "like solid walls of water" with an amplitude many times larger than the average wave amplitude. This general concept has been recently used in a semiconductor laser with optical injection to demonstrated that rogue waves can be completely deterministic [C. Bonatto et al., PRL 107, 053901 (2011)]. However, it is still unclear the way they develop, the probability to observe them and the type of systems able to generate rogue waves. We will introduce the main concepts related with deterministic rogue waves but will also demonstrate that noise can play a crucial role in the dynamics. The main triggering mechanisms and statistical properties of rogue waves will be shown using a simple model for a semiconductor laser with optical injection.
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Seminar: Explorando la fascinante región adonde no valen las hipóteses de la mecánica estadística de Boltzmann-Gibbs
Speaker
Prof. Constantino TsallisDescription Desde hace aproximadamente 140 años verificamos cotidianamente que la entropia y la mecánica estadística de Boltzmann-Gibbs describen brillantemente aquellos sistemas físicos que satisfacen hipótesis simplificadoras tales como ergodicidad. Pero existe tambien un vasto número de fascinantes sistemas naturales, artificiales y sociales que violan este tipo de hipótesis. Podemos aplicar tambien para estes sistemas complejos los métodos de la mecánica estadística? La respuesta es afirmativa. Abordaremos en este coloquio los aspectos conceptuales centrales de esta cuestión, y presentaremos algunas de sus predicciones, verificaciones y aplicaciones recientes. Ilustraciones en experimentos actuales de altas energías en el LHC-CERN y el RHIC-Brookhaven, así como en el área de eventos financieros con riesgo, serán brevemente descriptas. BIBLIOGRAFIA: (i) C. Tsallis, Introduction to Nonextensive Statistical Mechanics - Approaching a Complex World (Springer, New York, 2009); (ii) C. Tsallis, Entropy, in Encyclopedia of Complexity and Systems Science (Springer, Berlin, 2009); (iii) S. Umarov, C. Tsallis, M. Gell-Mann and S. Steinberg, J. Math. Phys. 51, 033502 (2010); (iv) J. S. Andrade Jr., G.F.T. da Silva, A.A. Moreira, F.D. Nobre and E.M.F. Curado, Phys. Rev. Lett. 105 , 260601 (2010); (v) F. Nobre, M.A.R. Monteiro and C. Tsallis, Phys. Rev. Lett. 106, 140601 (2011); (vi) J. Ludescher, C. Tsallis and A. Bunde, EPL 95, 68002 (2011); (vii) http://tsallis.cat.cbpf.br/biblio.htm
- 2011
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Seminar: Single-molecule DNA sequencing by force: forward and backward motion of replicative polymerases and their coupling with helicases
Speaker
Vincent CroquetteDescription In the replisome the polymerase collaborates with the helicase to drive the leading strand synthesis. In vitro, the helicases alone appear to unwind the replication fork with a rate far slower than the replicative one. Polymerases alone are even worse when working on a fork substrate that is in strand displacement. On the other hand, coupling the two enzymes leads to a fast and processive synthesis. The understanding of this coupling is far from perfect and was the motivation of our study. We investigate replicative polymerases and their helicase coupling in single molecule assays using a DNA hairpin in an unzipping configuration. In this simple fork model we assist these enzymes by an external force which provides a control parameter. Modulating the force between 0 and 15pN is a mean to assist a molecular motor opening the fork and thus to replace a partner in the collaborative work. This strategy has already been used to evidence that T7 and T4 helicases display an unwinding rate increasing exponentially with the force. We have conducted this assay with different replicative polymerases, we show that strand displacement polymerization is possible with substantial assisting force level. When the force is reduced we show that the exonuclease activity is dominant. This finding leads to the development of a Cyclic Polymerase Assay (CPA) where a polymerase is periodically switched from polymerization to strand degradation by modulating the assisting force. Such an assay is very convenient to study polymerase activity. Moreover, we present here a single molecule version of the Sanger sequencing method. We shall also discuss other sequencing assay using a DNA hairpin. Finally, the same assay can also be carried out to study the coupling between helicase and polymerase. We find that the coupled system is very efficient and advances at maximum rate, in stark contrast to the case of the isolated polymerase. We explain this result by a collaborative model where both the helicase and the polymerase are described by the Betterton-Julicher helicase model (Betterton 2003). The helicase is described by a fast passive helicase while the polymerase corresponds to a slow weakly active helicase.
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Speaker
Gaspar Orriols Tubella -
Speaker
Carles PuenteDescription http://www.fractus.com