762 research outputs found
Extension of nano-confined DNA: quantitative comparison between experiment and theory
The extension of DNA confined to nanochannels has been studied intensively
and in detail. Yet quantitative comparisons between experiments and model
calculations are difficult because most theoretical predictions involve
undetermined prefactors, and because the model parameters (contour length, Kuhn
length, effective width) are difficult to compute reliably, leading to
substantial uncertainties. Here we use a recent asymptotically exact theory for
the DNA extension in the "extended de Gennes regime" that allows us to compare
experimental results with theory. For this purpose we performed new
experiments, measuring the mean DNA extension and its standard deviation while
varying the channel geometry, dye intercalation ratio, and ionic buffer
strength. The experimental results agree very well with theory at high ionic
strengths, indicating that the model parameters are reliable. At low ionic
strengths the agreement is less good. We discuss possible reasons. Our approach
allows, in principle, to measure the Kuhn length and effective width of a
single DNA molecule and more generally of semiflexible polymers in solution.Comment: Revised version, 6 pages, 2 figures, 1 table, supplementary materia
Orientational correlations in confined DNA
We study how the orientational correlations of DNA confined to nanochannels
depend on the channel diameter D by means of Monte Carlo simulations and a
mean-field theory. This theory describes DNA conformations in the
experimentally relevant regime where the Flory-de Gennes theory does not apply.
We show how local correlations determine the dependence of the end-to-end
distance of the DNA molecule upon D. Tapered nanochannels provide the necessary
resolution in D to study experimentally how the extension of confined DNA
molecules depends upon D. Our experimental and theoretical results are in
qualitative agreement.Comment: Revised version including supplemental material, 7 pages, 8 figure
Mechanical coupling in flashing ratchets
We consider the transport of rigid objects with internal structure in a
flashing ratchet potential by investigating the overdamped behavior of a
rod-like chain of evenly spaced point particles. In 1D, analytical arguments
show that the velocity can reverse direction multiple times in response to
changing the size of the chain or the temperature of the heat bath. The
physical reason is that the effective potential experienced by the mechanically
coupled objects can have a different symmetry than that of individual objects.
All analytical predictions are confirmed by Brownian dynamics simulations.
These results may provide a route to simple, coarse-grained models of molecular
motor transport that incorporate an object's size and rotational degrees of
freedom into the mechanism of transport.Comment: 9 pages, 10 figure
Nanoconfinement-enhanced conformational response of single DNA molecules to changes in ionic environment
We show that the ionic environment plays a critical role in determining the configurational properties of DNA confined in silica nanochannels. The extension of DNA in the nanochannels increases as the ionic strength is reduced, almost tripling over two decades in ionic strength for channels around 100x100 nm in dimension. Surprisingly, we find that the variation of the persistence length alone with ionic strength is not enough to explain our results. The effect is due mainly to increasing self-avoidance created by the reduced screening of electrostatic interactions at low ionic strength. To quantify the increase in self-avoidance, we introduce a new parameter into the de Gennes theory: an effective DNA width that gives the increase in the excluded volume due to electrostatic repulsion
Diffusion mechanisms of localised knots along a polymer
We consider the diffusive motion of a localized knot along a linear polymer
chain. In particular, we derive the mean diffusion time of the knot before it
escapes from the chain once it gets close to one of the chain ends.
Self-reptation of the entire chain between either end and the knot position,
during which the knot is provided with free volume, leads to an L^3 scaling of
diffusion time; for sufficiently long chains, subdiffusion will enhance this
time even more. Conversely, we propose local ``breathing'', i.e., local
conformational rearrangement inside the knot region (KR) and its immediate
neighbourhood, as additional mechanism. The contribution of KR-breathing to the
diffusion time scales only quadratically, L^2, speeding up the knot escape
considerably and guaranteeing finite knot mobility even for very long chains.Comment: 7 pages, 2 figures. Accepted to Europhys. Let
OrthoDB v8: update of the hierarchical catalog of orthologs and the underlying free software
Orthology, refining the concept of homology, is the cornerstone of evolutionary comparative studies. With the ever-increasing availability of genomic data, inference of orthology has become instrumental for generating hypotheses about gene functions crucial to many studies. This update of the OrthoDB hierarchical catalog of orthologs (http://www.orthodb.org) covers 3027 complete genomes, including the most comprehensive set of 87 arthropods, 61 vertebrates, 227 fungi and 2627 bacteria (sampling the most complete and representative genomes from over 11,000 available). In addition to the most extensive integration of functional annotations from UniProt, InterPro, GO, OMIM, model organism phenotypes and COG functional categories, OrthoDB uniquely provides evolutionary annotations including rates of ortholog sequence divergence, copy-number profiles, sibling groups and gene architectures. We re-designed the entirety of the OrthoDB website from the underlying technology to the user interface, enabling the user to specify species of interest and to select the relevant orthology level by the NCBI taxonomy. The text searches allow use of complex logic with various identifiers of genes, proteins, domains, ontologies or annotation keywords and phrases. Gene copy-number profiles can also be queried. This release comes with the freely available underlying ortholog clustering pipeline (http://www.orthodb.org/software
Nanoconfined circular and linear DNA - equilibrium conformations and unfolding kinetics
Studies of circular DNA confined to nanofluidic channels are relevant both
from a fundamental polymer-physics perspective and due to the importance of
circular DNA molecules in vivo. We here observe the unfolding of DNA from the
circular to linear configuration as a light-induced double strand break occurs,
characterize the dynamics, and compare the equilibrium conformational
statistics of linear and circular configurations. This is important because it
allows us to determine to which extent existing statistical theories describe
the extension of confined circular DNA. We find that the ratio of the
extensions of confined linear and circular DNA configurations increases as the
buffer concentration decreases. The experimental results fall between
theoretical predictions for the extended de Gennes regime at weaker confinement
and the Odijk regime at stronger confinement. We show that it is possible to
directly distinguish between circular and linear DNA molecules by measuring the
emission intensity from the DNA. Finally, we determine the rate of unfolding
and show that this rate is larger for more confined DNA, possibly reflecting
the corresponding larger difference in entropy between the circular and linear
configurations.Comment: 21 pages, 7 figures, 1 tabl
Lipid-Based Passivation in Nanofluidics
Stretching DNA in nanochannels is a useful tool for direct, visual studies of genomic DNA at the single molecule level. To facilitate the study of the interaction of linear DNA with proteins in nanochannels, we have implemented a highly effective passivation scheme based on lipid bilayers. We demonstrate virtually complete long-term passivation of nanochannel surfaces to a range of relevant reagents, including streptavidin-coated quantum dots, RecA proteins, and RecA-DNA complexes. We show that the performance of the lipid bilayer is significantly better than that of standard bovine serum albumin-based passivation. Finally, we show how the passivated devices allow us to monitor single DNA cleavage events during enzymatic degradation by DNase I. We expect that our approach will open up for detailed, systematic studies of a wide range of protein-DNA interactions with high spatial and temporal resolution
Pressure-Driven DNA in Nanogroove Arrays: Complex Dynamics Leads to Length- and Topology-Dependent Separation
Deterministic Lateral Displacement:Challenges and Perspectives
The advent of microfluidics in the 1990s promised a revolution in multiple industries from healthcare to chemical processing. Deterministic lateral displacement (DLD) is a continuous-flow microfluidic particle separation method discovered in 2004 that has been applied successfully and widely to the separation of blood cells, yeast, spores, bacteria, viruses, DNA, droplets, and more. Deterministic lateral displacement is conceptually simple and can deliver consistent performance over a wide range of flow rates and particle concentrations. Despite wide use and in-depth study, DLD has not yet been fully elucidated or optimized, with different approaches to the same problem yielding varying results. We endeavor here to provide up-to-date expert opinion on the state-of-art and current fundamental, practical, and commercial challenges with DLD as well as describe experimental and modeling opportunities. Because these challenges and opportunities arise from constraints on hydrodynamics, fabrication, and operation at the micro- and nanoscale, we expect this Perspective to serve as a guide for the broader micro- and nanofluidic community to identify and to address open questions in the field
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