42 research outputs found
Dynamics of a Semiflexible Polymer or Polymer Ring in Shear Flow
Polymers exposed to shear flow exhibit a rich tumbling dynamics. While rigid
rods rotate on Jeffery orbits, flexible polymers stretch and coil up during
tumbling. Theoretical results show that in both of these asymptotic regimes the
tumbling frequency f_c in a linear shear flow of strength \gamma scales as a
power law Wi^(2/3) in the Weissenberg number Wi=\gamma \tau, where \tau is a
characteristic time of the polymer's relaxational dynamics. For flexible
polymers these theoretical results are well confirmed by experimental single
molecule studies. However, for the intermediate semiflexible regime the
situation is less clear. Here we perform extensive Brownian dynamics
simulations to explore the tumbling dynamics of semiflexible polymers over a
broad range of shear strength and the polymer's persistence length l_p. We find
that the Weissenberg number alone does not suffice to fully characterize the
tumbling dynamics, and the classical scaling law breaks down. Instead, both the
polymer's stiffness and the shear rate are relevant control parameters. Based
on our Brownian dynamics simulations we postulate that in the parameter range
most relevant for cytoskeletal filaments there is a distinct scaling behavior
with f_c \tau*=Wi^(3/4) f_c (x) with Wi=\gamma \tau* and the scaling variable
x=(l_p/L)(Wi)^(-1/3); here \tau* is the time the polymer's center of mass
requires to diffuse its own contour length L. Comparing these results with
experimental data on F-actin we find that the Wi^(3/4) scaling law agrees
quantitatively significantly better with the data than the classical Wi^(2/3)
law. Finally, we extend our results to single ring polymers in shear flow, and
find similar results as for linear polymers with slightly different power laws.Comment: 17 pages, 14 figure
Coupling of transverse and longitudinal response in stiff polymers
The time-dependent transverse response of stiff polymers, represented as
weakly-bending wormlike chains (WLCs), is well-understood on the linear level,
where transverse degrees of freedom evolve independently from the longitudinal
ones. We show that, beyond a characteristic time scale, the nonlinear coupling
of transverse and longitudinal motion in an inextensible WLC significantly
weakens the polymer response compared to the widely used linear response
predictions. The corresponding feedback mechanism is rationalized by scaling
arguments and quantified by a multiple scale approach that exploits an inherent
separation of transverse and longitudinal correlation length scales. Crossover
scaling laws and exact analytical and numerical solutions for characteristic
response quantities are derived for different experimentally relevant setups.
Our findings are applicable to cytoskeletal filaments as well as DNA under
tension.Comment: 4 pages, 3 figures, 1 table; final versio
Tension dynamics and viscoelasticity of extensible wormlike chains
The dynamic response of prestressed semiflexible biopolymers is characterized
by the propagation and relaxation of tension, which arises due to the near
inextensibility of a stiff backbone. It is coupled to the dynamics of contour
length stored in thermal undulations, but also to the local relaxation of
elongational strain. We present a systematic theory of tension dynamics for
stiff yet extensible wormlike chains. Our results show that even moderate
prestress gives rise to distinct Rouse-like extensibility signatures in the
high-frequency viscoelastic response.Comment: 4 pages, 1 figure; corrected typo
Exploring the miRNA Regulatory Network Using Evolutionary Correlations
Post-transcriptional regulation by miRNAs is a widespread and highly conserved phenomenon in metazoans, with several hundreds to thousands of conserved binding sites for each miRNA, and up to two thirds of all genes under miRNA regulation. At the same time, the effect of miRNA regulation on mRNA and protein levels is usually quite modest and associated phenotypes are often weak or subtle. This has given rise to the notion that the highly interconnected miRNA regulatory network exerts its function less through any individual link and more via collective effects that lead to a functional interdependence of network links. We present a Bayesian framework to quantify conservation of miRNA target sites using vertebrate whole-genome alignments. The increased statistical power of our phylogenetic model allows detection of evolutionary correlation in the conservation patterns of site pairs. Such correlations could result from collective functions in the regulatory network. For instance, co-conservation of target site pairs supports a selective benefit of combinatorial regulation by multiple miRNAs. We find that some miRNA families are under pronounced co-targeting constraints, indicating a high connectivity in the regulatory network, while others appear to function in a more isolated way. By analyzing coordinated targeting of different curated gene sets, we observe distinct evolutionary signatures for protein complexes and signaling pathways that could reflect differences in control strategies. Our method is easily scalable to analyze upcoming larger data sets, and readily adaptable to detect high-level selective constraints between other genomic loci. We thus provide a proof-of-principle method to understand regulatory networks from an evolutionary perspective
Inverse Ising inference with correlated samples
Correlations between two variables of a high-dimensional system can be
indicative of an underlying interaction, but can also result from indirect
effects. Inverse Ising inference is a method to distinguish one from the other.
Essentially, the parameters of the least constrained statistical model are
learned from the observed correlations such that direct interactions can be
separated from indirect correlations. Among many other applications, this
approach has been helpful for protein structure prediction, because residues
which interact in the 3D structure often show correlated substitutions in a
multiple sequence alignment. In this context, samples used for inference are
not independent but share an evolutionary history on a phylogenetic tree. Here,
we discuss the effects of correlations between samples on global inference.
Such correlations could arise due to phylogeny but also via other slow
dynamical processes. We present a simple analytical model to address the
resulting inference biases, and develop an exact method accounting for
background correlations in alignment data by combining phylogenetic modeling
with an adaptive cluster expansion algorithm. We find that popular reweighting
schemes are only marginally effective at removing phylogenetic bias, suggest a
rescaling strategy that yields better results, and provide evidence that our
conclusions carry over to the frequently used mean-field approach to the
inverse Ising problem.Comment: 18 pages, 6 figures; accepted at New J Phy
Longitudinal Response of Confined Semiflexible Polymers
The longitudinal response of single semiflexible polymers to sudden changes
in externally applied forces is known to be controlled by the propagation and
relaxation of backbone tension. Under many experimental circumstances,
realized, e.g., in nano-fluidic devices or in polymeric networks or solutions,
these polymers are effectively confined in a channel- or tube-like geometry. By
means of heuristic scaling laws and rigorous analytical theory, we analyze the
tension dynamics of confined semiflexible polymers for various generic
experimental setups. It turns out that in contrast to the well-known linear
response, the influence of confinement on the non-linear dynamics can largely
be described as that of an effective prestress. We also study the free
relaxation of an initially confined chain, finding a surprising superlinear
t^(9/8) growth law for the change in end-to-end distance at short times.Comment: 18 pages, 1 figur
The prebiotic evolutionary advantage of transferring genetic information from RNA to DNA.
In the early 'RNA world' stage of life, RNA stored genetic information and catalyzed chemical reactions. However, the RNA world eventually gave rise to the DNA-RNA-protein world, and this transition included the 'genetic takeover' of information storage by DNA. We investigated evolutionary advantages for using DNA as the genetic material. The error rate of replication imposes a fundamental limit on the amount of information that can be stored in the genome, as mutations degrade information. We compared misincorporation rates of RNA and DNA in experimental non-enzymatic polymerization and calculated the lowest possible error rates from a thermodynamic model. Both analyses found that RNA replication was intrinsically error-prone compared to DNA, suggesting that total genomic information could increase after the transition to DNA. Analysis of the transitional RNA/DNA hybrid duplexes showed that copying RNA into DNA had similar fidelity to RNA replication, so information could be maintained during the genetic takeover. However, copying DNA into RNA was very error-prone, suggesting that attempts to return to the RNA world would result in a considerable loss of information. Therefore, the genetic takeover may have been driven by a combination of increased chemical stability, increased genome size and irreversibility
Periodic vs. intermittent adaptive cycles in quasispecies co-evolution
We study an abstract model for the co-evolution between mutating viruses and
the adaptive immune system. In sequence space, these two populations are
localized around transiently dominant strains. Delocalization or error
thresholds exhibit a novel interdependence because immune response is
conditional on the viral attack. An evolutionary chase is induced by stochastic
fluctuations and can occur via periodic or intermittent cycles. Using
simulations and stochastic analysis, we show how the transition between these
two dynamic regimes depends on mutation rate, immune response, and population
size.Comment: 5 pages, 3 figures, 11 pages supplementary material; updated
formatting; accepted at Phys. Rev. Let
Stress-Energy Tensor for the Massless Spin 1/2 Field in Static Black Hole Spacetimes
The stress-energy tensor for the massless spin 1/2 field is numerically
computed outside and on the event horizons of both charged and uncharged static
non-rotating black holes, corresponding to the Schwarzschild,
Reissner-Nordstrom and extreme Reissner-Nordstr\"om solutions of Einstein's
equations. The field is assumed to be in a thermal state at the black hole
temperature. Comparison is made between the numerical results and previous
analytic approximations for the stress-energy tensor in these spacetimes. For
the Schwarzschild (charge zero) solution, it is shown that the stress-energy
differs even in sign from the analytic approximation. For the
Reissner-Nordstrom and extreme Reissner-Nordstrom solutions, divergences
predicted by the analytic approximations are shown not to exist.Comment: 5 pages, 4 figures, additional discussio