14,874 research outputs found
A Population Genetic Approach to the Quasispecies Model
A population genetics formulation of Eigen's molecular quasispecies model is
proposed and several simple replication landscapes are investigated
analytically. Our results show a remarcable similarity to those obtained with
the original kinetics formulation of the quasispecies model. However, due to
the simplicity of our approach, the space of the parameters that define the
model can be explored. In particular, for the simgle-sharp-peak landscape our
analysis yelds some interesting predictions such as the existence of a maximum
peak height and a mini- mum molecule length for the onset of the error
threshold transition.Comment: 16 pages, 4 Postscript figures. Submited to Phy. Rev.
Large deviations for non-uniformly expanding maps
We obtain large deviation results for non-uniformly expanding maps with
non-flat singularities or criticalities and for partially hyperbolic
non-uniformly expanding attracting sets. That is, given a continuous function
we consider its space average with respect to a physical measure and compare
this with the time averages along orbits of the map, showing that the Lebesgue
measure of the set of points whose time averages stay away from the space
average decays to zero exponentially fast with the number of iterates involved.
As easy by-products we deduce escape rates from subsets of the basins of
physical measures for these types of maps. The rates of decay are naturally
related to the metric entropy and pressure function of the system with respect
to a family of equilibrium states. The corrections added to the published
version of this text appear in bold; see last section for a list of changesComment: 36 pages, 1 figure. After many PhD students and colleagues having
pointed several errors in the statements and proofs, this is a correction to
published article answering those comments. List of main changes in a new
last sectio
Finite-size scaling of the quasiespecies model
We use finite-size scaling to investigate the critical behavior of the
quasiespecies model of molecular evolution in the single-sharp-peak replication
landscape. This model exhibits a sharp threshold phenomenon at Q=Q_c=1/a, where
Q is the probability of exact replication of a molecule of length L and a is
the selective advantage of the master string.
We investigate the sharpness of the threshold and find that its
characteristic persist across a range of Q of order L^(-1) about Q_c.
Furthermore, using the data collapsing method we show that the normalized mean
Hamming distance between the master string and the entire population, as well
as the properly scaled fluctuations around this mean value, follow universal
forms in the critical region.Comment: 8 pages,tex. Submitted to Physical Review
Group selection models in prebiotic evolution
The evolution of enzyme production is studied analytically using ideas of the
group selection theory for the evolution of altruistic behavior. In particular,
we argue that the mathematical formulation of Wilson's structured deme model
({\it The Evolution of Populations and Communities}, Benjamin/Cumings, Menlo
Park, 1980) is a mean-field approach in which the actual environment that a
particular individual experiences is replaced by an {\it average} environment.
That formalism is further developed so as to avoid the mean-field approximation
and then applied to the problem of enzyme production in the prebiotic context,
where the enzyme producer molecules play the altruists role while the molecules
that benefit from the catalyst without paying its production cost play the
non-altruists role. The effects of synergism (i.e., division of labor) as well
as of mutations are also considered and the results of the equilibrium analysis
are summarized in phase diagrams showing the regions of the space of parameters
where the altruistic, non-altruistic and the coexistence regimes are stable. In
general, those regions are delimitated by discontinuous transition lines which
end at critical points.Comment: 22 pages, 10 figure
Designing biomaterials based on biomineralization of bone
In nature, organisms control crystal nucleation and growth using organic interfaces as templates.
Scientists, in the last decades, have tried to learn from nature how to design biomimetic biomaterials
inspired by the hierarchical complex structure of bone and other natural mineralised tissues or to
control the biomineralization process onto biomaterials substrates to promote the osteoconductive
properties of implantable devices. The design of synthetic bone analogues, i.e., with a structure and
properties similar to bone, would certainly constitute a major breakthrough in bone tissue engineering.
Moreover, many strategies have been proposed in the literature to develop bioactive bone-like
materials, for instance using bioactive glasses. Fundamental aspects of biomineralization may be also
important in order to propose new methodologies to improve calcification onto the surface of
biomaterials or to develop bioactive tridimensional templates that could be used in regenerative
medicine. In particular, it has been shown that some chemical groups and proteins, as well as the
tridimensional matrix in which calcification would occur, play a fundamental role on the nucleation
and growth of hydroxyapatite. All these distinct aspects will be reviewed and discussed in this paper.I. B. Leonor thanks the Portuguese Foundation for Science and Technology (FCT) for providing her a post-doctoral scholarship (SFRH/BPD/26648/2006). This work was supported by the European NoE EXPERTISSUES (NMP3-CT-2004-500283) and by the Portuguese Foundation for Science and Technology, FCT, through the projects PTDC/CTM/68804/2006, PTDC/CTM/67560/2006 and PTDC/FIS/68209/2006
The Thermal Structure of Gas in Pre-Stellar Cores: A Case Study of Barnard 68
We present a direct comparison of a chemical/physical model to
multitransitional observations of C18O and 13CO towards the Barnard 68
pre-stellar core. These observations provide a sensitive test for models of low
UV field photodissociation regions and offer the best constraint on the gas
temperature of a pre-stellar core. We find that the gas temperature of this
object is surprisingly low (~7-8 K), and significantly below the dust
temperature, in the outer layers (Av < 5 mag) that are traced by C18O and 13CO
emission. As shown previously, the inner layers (Av > 5 mag) exhibit
significant freeze-out of CO onto grain surfaces. Because the dust and gas are
not fully coupled, depletion of key coolants in the densest layers raises the
core (gas) temperature, but only by ~1 K. The gas temperature in layers not
traced by C18O and 13CO emission can be probed by NH3 emission, with a
previously estimated temperature of ~10-11 K. To reach these temperatures in
the inner core requires an order of magnitude reduction in the gas to dust
coupling rate. This potentially argues for a lack of small grains in the
densest gas, presumably due to grain coagulation.Comment: 33 pages, 11 figures, accepted by Astrophysical Journa
Análise econômica mensal sobre o setor de mandioca e derivados: maio/2010.
Menor oferta sustenta cotação em maio – O mês de maio foi caracterizado pela menor oferta de mandioca para a indústria de fécula, levando à diminuição na quantidade de mandioca processada pelas fecularias. Ainda que seja período de safra, a disponibilidade de mandioca de segundo ciclo é menor em relação a anos anteriores e, além disso, agricultores mantiveram-se retraídos em relação à colheita. Esses produtores não tiveram necessidade de caixa e aguardam preços ainda maiores nos próximos meses. O excesso de chuvas em alguns períodos de maio prejudicou a colheita, dificultando os trabalhos e diminuindo a quantidade ofertada à indústria. Vale destacara, também, que houve menor número de mão-de-obra disponível para a colheita, visto que parte dos agricultores já começou o preparo de solo para o plantio da safra 2010/11. Apesar da menor oferta em maio, o preço médio mensal da raiz de mandioca, considerando as regiões pesquisadas pelo Cepea, foi de R 0,3785/grama de amido na balança hidrostática de 5 kg), ligeira baixa de 0,5% frente à de abril. Já entre a última semana de abril e a última de maio, o preço médio subiu 18,8%, passando para R$ 237,81/t.Os preços médios mensais nos estados de São Paulo e Mato Grosso do Sul tiveram respectivas quedas de 4,9% e de 2,7% entre abril e maio.bitstream/item/23998/1/05MaiCEPEA.pd
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