Affinity maturation versus diversity generation.

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SmartCell: a framework to simulate cellular processes that combines stochastic approximation with diffusion and localization.

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From pollen tube growth to pollen tube orientation.

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Parvovirus B19 outbreak in three Portuguese schools.

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Evolvability is a selectable trait.

PNAS (2004) 101:11531

Explaining the Lognormal distribution through stochastic protein synthesis.

In cell populations that are qualitatively homogeneous in their qualitative gene expression profile there are significant quantitative differences in protein levels. This cell population heterogeneity is an ubiquitous phenomena in biology. This project aims to understand the origin and function of this heterogeneity. The heterogeneity in protein level has been explained by randomness in gene transcription, which predicts a symmetrical distribution because it is basically a sum of birth and death processes or Poisson events. This prediction, however, is in blunt contrast with the observed distributions of protein levels that are skewed and well fitted by a lognormal distribution. Here, we propose a model of protein synthesis where fluctuations of intervenient molecules are taken in account. This model explicitly includes the multistep nature of protein synthesis. We show that this process always converges to a lognormal distribution, irrespectively of the nature of the fluctuations that we impose on the intermediate steps. We explored some consequences of such distributions on responses of cell populations. In particular, we show that all sigmoidal response curves obtained from titration assays can be explained by the intrinsic heterogeneity of the population, thereby indicating that frequent assertions about response cooperativity based on such data might be unwarranted.