17,697 research outputs found
Rationalising sequence selection by ligand assemblies in the DNA minor groove : the case for thiazotropsin A
DNA-sequence and structure dependence on the formation of minor groove complexes at 5â˛-XCTAGY-3Ⲡby the short lexitropsin thiazotropsin A are explored based on NMR spectroscopy, isothermal titration calorimetry (ITC), circular dichroism (CD) and qualitative molecular modeling. The structure and solution behaviour of the complexes are similar whether X = A, T, C or G and Z = T, A, I or C, CCTAGI being thermodynamically the most favoured (ÎG = -11.1 Âą 0.1 kcal.mol-1). Binding site selectivity observed by NMR for ACTAGT in the presence of TCTAGA when both accessible sequences are concatenated in a 15-mer DNA duplex construct is consistent with thermodynamic parameters (ÎÎGÎACTAGT > ÎÎGÎTCTAGA) measured separately for the binding sites and with predictions from modeling studies. Steric bulk in the minor groove for Y = G causes unfavourable ligand-DNA interactions reflected in lower Gibbs free energy of binding (ÎG = -8.5 Âą 0.01 kcal.mol-1). ITC and CD data establish that thiazotropsin A binds the ODNs with binding constants between 106 and 108 M-1 and reveal that binding is driven enthalpically through hydrogen bond formation and van der Waals interactions. The consequences of these findings are considered with respect to ligand self-association and the energetics responsible for driving DNA recognition by small molecule DNA minor groove binder
Statistics of shared components in complex component systems
Many complex systems are modular. Such systems can be represented as
"component systems", i.e., sets of elementary components, such as LEGO bricks
in LEGO sets. The bricks found in a LEGO set reflect a target architecture,
which can be built following a set-specific list of instructions. In other
component systems, instead, the underlying functional design and constraints
are not obvious a priori, and their detection is often a challenge of both
scientific and practical importance, requiring a clear understanding of
component statistics. Importantly, some quantitative invariants appear to be
common to many component systems, most notably a common broad distribution of
component abundances, which often resembles the well-known Zipf's law. Such
"laws" affect in a general and non-trivial way the component statistics,
potentially hindering the identification of system-specific functional
constraints or generative processes. Here, we specifically focus on the
statistics of shared components, i.e., the distribution of the number of
components shared by different system-realizations, such as the common bricks
found in different LEGO sets. To account for the effects of component
heterogeneity, we consider a simple null model, which builds
system-realizations by random draws from a universe of possible components.
Under general assumptions on abundance heterogeneity, we provide analytical
estimates of component occurrence, which quantify exhaustively the statistics
of shared components. Surprisingly, this simple null model can positively
explain important features of empirical component-occurrence distributions
obtained from data on bacterial genomes, LEGO sets, and book chapters. Specific
architectural features and functional constraints can be detected from
occurrence patterns as deviations from these null predictions, as we show for
the illustrative case of the "core" genome in bacteria.Comment: 18 pages, 7 main figures, 7 supplementary figure
Emergent Properties of Tumor Microenvironment in a Real-life Model of Multicell Tumor Spheroids
Multicellular tumor spheroids are an important {\it in vitro} model of the
pre-vascular phase of solid tumors, for sizes well below the diagnostic limit:
therefore a biophysical model of spheroids has the ability to shed light on the
internal workings and organization of tumors at a critical phase of their
development. To this end, we have developed a computer program that integrates
the behavior of individual cells and their interactions with other cells and
the surrounding environment. It is based on a quantitative description of
metabolism, growth, proliferation and death of single tumor cells, and on
equations that model biochemical and mechanical cell-cell and cell-environment
interactions. The program reproduces existing experimental data on spheroids,
and yields unique views of their microenvironment. Simulations show complex
internal flows and motions of nutrients, metabolites and cells, that are
otherwise unobservable with current experimental techniques, and give novel
clues on tumor development and strong hints for future therapies.Comment: 20 pages, 10 figures. Accepted for publication in PLOS One. The
published version contains links to a supplementary text and three video
file
Essential guidelines for computational method benchmarking
In computational biology and other sciences, researchers are frequently faced
with a choice between several computational methods for performing data
analyses. Benchmarking studies aim to rigorously compare the performance of
different methods using well-characterized benchmark datasets, to determine the
strengths of each method or to provide recommendations regarding suitable
choices of methods for an analysis. However, benchmarking studies must be
carefully designed and implemented to provide accurate, unbiased, and
informative results. Here, we summarize key practical guidelines and
recommendations for performing high-quality benchmarking analyses, based on our
experiences in computational biology.Comment: Minor update
The Genomic HyperBrowser: inferential genomics at the sequence level
The immense increase in the generation of genomic scale data poses an unmet
analytical challenge, due to a lack of established methodology with the
required flexibility and power. We propose a first principled approach to
statistical analysis of sequence-level genomic information. We provide a
growing collection of generic biological investigations that query pairwise
relations between tracks, represented as mathematical objects, along the
genome. The Genomic HyperBrowser implements the approach and is available at
http://hyperbrowser.uio.no
DNA cruciform arms nucleate through a correlated but non-synchronous cooperative mechanism
Inverted repeat (IR) sequences in DNA can form non-canonical cruciform
structures to relieve torsional stress. We use Monte Carlo simulations of a
recently developed coarse-grained model of DNA to demonstrate that the
nucleation of a cruciform can proceed through a cooperative mechanism. Firstly,
a twist-induced denaturation bubble must diffuse so that its midpoint is near
the centre of symmetry of the IR sequence. Secondly, bubble fluctuations must
be large enough to allow one of the arms to form a small number of hairpin
bonds. Once the first arm is partially formed, the second arm can rapidly grow
to a similar size. Because bubbles can twist back on themselves, they need
considerably fewer bases to resolve torsional stress than the final cruciform
state does. The initially stabilised cruciform therefore continues to grow,
which typically proceeds synchronously, reminiscent of the S-type mechanism of
cruciform formation. By using umbrella sampling techniques we calculate, for
different temperatures and superhelical densities, the free energy as a
function of the number of bonds in each cruciform along the correlated but
non-synchronous nucleation pathways we observed in direct simulations.Comment: 12 pages main paper + 11 pages supplementary dat
Biosynthetic potentials of metabolites and their hierarchical organization
Peer reviewedPublisher PD
Sensitivity analysis of oscillator models in the space of phase-response curves: Oscillators as open systems
Oscillator models are central to the study of system properties such as
entrainment or synchronization. Due to their nonlinear nature, few
system-theoretic tools exist to analyze those models. The paper develops a
sensitivity analysis for phase-response curves, a fundamental one-dimensional
phase reduction of oscillator models. The proposed theoretical and numerical
analysis tools are illustrated on several system-theoretic questions and models
arising in the biology of cellular rhythms
Force-induced rupture of a DNA duplex
The rupture of double-stranded DNA under stress is a key process in
biophysics and nanotechnology. In this article we consider the shear-induced
rupture of short DNA duplexes, a system that has been given new importance by
recently designed force sensors and nanotechnological devices. We argue that
rupture must be understood as an activated process, where the duplex state is
metastable and the strands will separate in a finite time that depends on the
duplex length and the force applied. Thus, the critical shearing force required
to rupture a duplex within a given experiment depends strongly on the time
scale of observation. We use simple models of DNA to demonstrate that this
approach naturally captures the experimentally observed dependence of the
critical force on duplex length for a given observation time. In particular,
the critical force is zero for the shortest duplexes, before rising sharply and
then plateauing in the long length limit. The prevailing approach, based on
identifying when the presence of each additional base pair within the duplex is
thermodynamically unfavorable rather than allowing for metastability, does not
predict a time-scale-dependent critical force and does not naturally
incorporate a critical force of zero for the shortest duplexes. Additionally,
motivated by a recently proposed force sensor, we investigate application of
stress to a duplex in a mixed mode that interpolates between shearing and
unzipping. As with pure shearing, the critical force depends on the time scale
of observation; at a fixed time scale and duplex length, the critical force
exhibits a sigmoidal dependence on the fraction of the duplex that is subject
to shearing.Comment: 10 pages, 6 figure
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