72,971 research outputs found
On the Origin of Logarithmic-Normal Distributions: An Analytical Derivation, and its Application to Nucleation and Growth Processes
The logarithmic-normal (lognormal) distribution is one of the most frequently
observed distributions in nature and describes a large number of physical,
biological and even sociological phenomena. The origin of this distribution is
therefore of broad interest but a general derivation from basic principles is
still lacking. Using random nucleation and growth to describe crystallization
processes we derive the time development of grain size distributions. Our
derivation provides, for the first time, an analytical expression of the size
distribution in the form of a lognormal type distribution. We apply our results
to the grain size distribution of solid phase crystallized Si-films.Comment: four pages, one figur
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Prediction of regulatory targets of alternative isoforms of the epidermal growth factor receptor in a glioblastoma cell line.
BackgroundThe epidermal growth factor receptor (EGFR) is a major regulator of proliferation in tumor cells. Elevated expression levels of EGFR are associated with prognosis and clinical outcomes of patients in a variety of tumor types. There are at least four splice variants of the mRNA encoding four protein isoforms of EGFR in humans, named I through IV. EGFR isoform I is the full-length protein, whereas isoforms II-IV are shorter protein isoforms. Nevertheless, all EGFR isoforms bind the epidermal growth factor (EGF). Although EGFR is an essential target of long-established and successful tumor therapeutics, the exact function and biomarker potential of alternative EGFR isoforms II-IV are unclear, motivating more in-depth analyses. Hence, we analyzed transcriptome data from glioblastoma cell line SF767 to predict target genes regulated by EGFR isoforms II-IV, but not by EGFR isoform I nor other receptors such as HER2, HER3, or HER4.ResultsWe analyzed the differential expression of potential target genes in a glioblastoma cell line in two nested RNAi experimental conditions and one negative control, contrasting expression with EGF stimulation against expression without EGF stimulation. In one RNAi experiment, we selectively knocked down EGFR splice variant I, while in the other we knocked down all four EGFR splice variants, so the associated effects of EGFR II-IV knock-down can only be inferred indirectly. For this type of nested experimental design, we developed a two-step bioinformatics approach based on the Bayesian Information Criterion for predicting putative target genes of EGFR isoforms II-IV. Finally, we experimentally validated a set of six putative target genes, and we found that qPCR validations confirmed the predictions in all cases.ConclusionsBy performing RNAi experiments for three poorly investigated EGFR isoforms, we were able to successfully predict 1140 putative target genes specifically regulated by EGFR isoforms II-IV using the developed Bayesian Gene Selection Criterion (BGSC) approach. This approach is easily utilizable for the analysis of data of other nested experimental designs, and we provide an implementation in R that is easily adaptable to similar data or experimental designs together with all raw datasets used in this study in the BGSC repository, https://github.com/GrosseLab/BGSC
Exploitation dynamics of fish stocks
I address the question of the fluctuations in fishery landings. Using the
fishery statistics time-series collected by the Food and Agriculture
Organization of the United Nations since the early 1950s, I here analyze
fishing activities and find two scaling features of capture fisheries
production: (i) the standard deviation of growth rate of the domestically
landed catches decays as a power-law function of country landings with an
exponent of value 0.15; (ii) the average number of fishers in a country scales
to the 0.7 power of country landings. I show how these socio-ecological
patterns may be related, yielding a scaling relation between these exponents.
The predicted scaling relation implies that the width of the annual per capita
growth-rate distribution scales to the 0.2 power of country landings, i.e.
annual fluctuations in per capita landed catches increase with increased per
capita catches in highly producing countries. Beside the scaling behavior, I
report that fluctuations in the annual domestic landings have increased in the
last 30 years, while the mean of the annual growth rate declined significantly
after 1972.Comment: 27 pages, 19 figure
A jump-growth model for predator-prey dynamics: derivation and application to marine ecosystems
This paper investigates the dynamics of biomass in a marine ecosystem. A
stochastic process is defined in which organisms undergo jumps in body size as
they catch and eat smaller organisms. Using a systematic expansion of the
master equation, we derive a deterministic equation for the macroscopic
dynamics, which we call the deterministic jump-growth equation, and a linear
Fokker-Planck equation for the stochastic fluctuations. The McKendrick--von
Foerster equation, used in previous studies, is shown to be a first-order
approximation, appropriate in equilibrium systems where predators are much
larger than their prey. The model has a power-law steady state consistent with
the approximate constancy of mass density in logarithmic intervals of body mass
often observed in marine ecosystems. The behaviours of the stochastic process,
the deterministic jump-growth equation and the McKendrick--von Foerster
equation are compared using numerical methods. The numerical analysis shows two
classes of attractors: steady states and travelling waves.Comment: 27 pages, 4 figures. Final version as published. Only minor change
Universal expressions of population change by the Price equation: natural selection, information, and maximum entropy production
The Price equation shows the unity between the fundamental expressions of
change in biology, in information and entropy descriptions of populations, and
in aspects of thermodynamics. The Price equation partitions the change in the
average value of a metric between two populations. A population may be composed
of organisms or particles or any members of a set to which we can assign
probabilities. A metric may be biological fitness or physical energy or the
output of an arbitrarily complicated function that assigns quantitative values
to members of the population. The first part of the Price equation describes
how directly applied forces change the probabilities assigned to members of the
population when holding constant the metrical values of the members---a fixed
metrical frame of reference. The second part describes how the metrical values
change, altering the metrical frame of reference. In canonical examples, the
direct forces balance the changing metrical frame of reference, leaving the
average or total metrical values unchanged. In biology, relative reproductive
success (fitness) remains invariant as a simple consequence of the conservation
of total probability. In physics, systems often conserve total energy.
Nonconservative metrics can be described by starting with conserved metrics,
and then studying how coordinate transformations between conserved and
nonconserved metrics alter the geometry of the dynamics and the aggregate
values of populations. From this abstract perspective, key results from
different subjects appear more simply as universal geometric principles for the
dynamics of populations subject to the constraints of particular conserved
quantitiesComment: v2: Complete rewrite, new title and abstract. Changed focus to Price
equation as basis for universal expression of changes in populations. v3:
Cleaned up usage of terms virtual and reversible displacements and virtual
work and usage of d'Alembert's principle. v4: minor editing and correction
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