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Wakim JG, Sandholtz SH, Spakowitz AJ. Impact of chromosomal organization on epigenetic drift and domain stability revealed by physics-based simulations. Biophys J 2021; 120:4932-4943. [PMID: 34687722 DOI: 10.1016/j.bpj.2021.10.019] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2021] [Revised: 09/07/2021] [Accepted: 10/18/2021] [Indexed: 12/21/2022] Open
Abstract
We examine the relationship between the size of domains of epigenetic marks and the stability of those domains using our theoretical model that captures the physical mechanisms governing the maintenance of epigenetic modifications. We focus our study on histone H3 lysine-9 trimethylation, one of the most common and consequential epigenetic marks with roles in chromatin compaction and gene repression. Our model combines the effects of methyl spreading by methyltransferases and chromatin segregation into heterochromatin and euchromatin because of preferential heterochromatin protein 1 (HP1) binding. Our model indicates that, although large methylated domains are passed successfully from one chromatin generation to the next, small alterations to the methylation sequence are not maintained during chromatin replication. Using our predictive model, we investigate the size required for an epigenetic domain to persist over chromatin generations while surrounded by a much larger domain of opposite methylation and compaction state. We find that there is a critical size threshold in the hundreds-of-nucleosomes scale above which an epigenetic domain will be reliably maintained over generations. The precise size of the threshold differs for heterochromatic and euchromatic domains. Our results are consistent with natural alterations to the epigenetic sequence occurring during embryonic development and due to age-related epigenetic drift.
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Affiliation(s)
- Joseph G Wakim
- Department of Chemical Engineering, Stanford University, Stanford, California
| | | | - Andrew J Spakowitz
- Department of Chemical Engineering, Stanford University, Stanford, California; Department of Materials Science and Engineering, Stanford University, Stanford, California; Biophysics Program, Stanford University, Stanford, California; Department of Applied Physics, Stanford University, Stanford, California.
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2
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Bucci G, Gadelrab K, Spakowitz AJ. Free Energy and Dynamics of Annihilation of Topological Defects in Nanoconfined DNA. Macromolecules 2021. [DOI: 10.1021/acs.macromol.1c01164] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Giovanna Bucci
- Robert Bosch LLC, 384 Santa Trinita Ave, Sunnyvale, California 94085, United States
| | - Karim Gadelrab
- Robert Bosch LLC, 1 Kendall Square, Suite 7-101, Cambridge, Massachusetts 02139, United States
| | - Andrew J. Spakowitz
- Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States
- Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
- Department of Applied Physics, Stanford University, Stanford, California 94305, United States
- Biophysics Program, Stanford University, Stanford, California 94305, United States
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3
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Sandholtz SH, Kannan D, Beltran BG, Spakowitz AJ. Chromosome Structural Mechanics Dictates the Local Spreading of Epigenetic Marks. Biophys J 2020; 119:1630-1639. [PMID: 33010237 DOI: 10.1016/j.bpj.2020.08.039] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2020] [Revised: 08/07/2020] [Accepted: 08/17/2020] [Indexed: 10/23/2022] Open
Abstract
We present a theoretical model that demonstrates the integral role chromosome organization and structural mechanics play in the spreading of histone modifications involved in epigenetic regulation. Our model shows that heterogeneous nucleosome positioning, and the resulting position-dependent mechanical properties, must be included to reproduce several qualitative features of experimental data of histone methylation spreading around an artificially induced "nucleation site." We show that our model recreates both the extent of spreading and the presence of a subdominant peak upstream of the transcription start site. Our model indicates that the spreading of epigenetic modifications is sensitive to heterogeneity in chromatin organization and the resulting variability in the chromatin's mechanical properties, suggesting that nucleosome spacing can directly control the conferral of epigenetic marks by modifying the structural mechanics of the chromosome. It further illustrates how the physical organization of the DNA polymer may play a significant role in re-establishing the epigenetic code upon cell division.
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Affiliation(s)
| | - Deepti Kannan
- Department of Physics, University of Cambridge, Cambridge, United Kingdom
| | - Bruno G Beltran
- Biophysics Program, Stanford University, Stanford, California
| | - Andrew J Spakowitz
- Biophysics Program, Stanford University, Stanford, California; Chemical Engineering Department, Stanford University, Stanford, California; Department of Materials Science and Engineering, Stanford University, Stanford, California; Department of Applied Physics, Stanford University, Stanford, California.
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Abstract
The equilibrium conformation of a polymer molecule in an external field is often used in field theories to calculate macroscopic polymer properties of melts and solutions. We use a mathematical method called a Brownian bridge to exactly sample continuous polymer chains to end in a given state. We show that one can systematically develop such processes to sample specific polymer topologies, to confine polymers in a given geometry for its entire path, to efficiently generate high-probability conformations by excluding small Boltzmann weights, or to simulate rare events in a rugged energy landscape. This formalism can improve the polymer sampling efficiency significantly compared to traditional methods (e.g., Monte Carlo or Rosenbluth).
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Affiliation(s)
- Shiyan Wang
- Davidson School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, USA
| | - Doraiswami Ramkrishna
- Davidson School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, USA
| | - Vivek Narsimhan
- Davidson School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, USA
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5
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Abstract
The fracture and severing of polymer chains plays a critical role in the failure of fibrous materials and the regulated turnover of intracellular filaments. Using continuum wormlike chain models, we investigate the fracture of semiflexible polymers via thermal bending fluctuations, focusing on the role of filament flexibility and dynamics. Our results highlight a previously unappreciated consequence of mechanical heterogeneity in the filament, which enhances the rate of thermal fragmentation particularly in cases where constraints hinder the movement of the chain ends. Although generally applicable to semiflexible chains with regions of different bending stiffness, the model is motivated by a specific biophysical system: the enhanced severing of actin filaments at the boundary between stiff bare regions and mechanically softened regions that are coated with cofilin regulatory proteins. The results presented here point to a potential mechanism for disassembly of polymeric materials in general and cytoskeletal actin networks in particular by the introduction of locally softened chain regions, as occurs with cofilin binding.
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Affiliation(s)
- Alexander M Lorenzo
- Department of Physics, University of California San Diego, San Diego, California 92093, USA.
| | - Enrique M De La Cruz
- Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA
| | - Elena F Koslover
- Department of Physics, University of California San Diego, San Diego, California 92093, USA.
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Spakowitz AJ. Polymer physics across scales: Modeling the multiscale behavior of functional soft materials and biological systems. J Chem Phys 2019; 151:230902. [DOI: 10.1063/1.5126852] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Affiliation(s)
- Andrew J. Spakowitz
- Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA
- Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
- Department of Applied Physics, Stanford University, Stanford, California 94305, USA
- Biophysics Program, Stanford University, Stanford, California 94305, USA
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7
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Rudnicki PE, MacPherson Q, Balhorn L, Feng B, Qin J, Salleo A, Spakowitz AJ. Impact of Liquid-Crystalline Chain Alignment on Charge Transport in Conducting Polymers. Macromolecules 2019. [DOI: 10.1021/acs.macromol.9b01729] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Abstract
Predicting how epigenetic marks control the 3D organization of the genome is key to understanding how these marks regulate gene expression. We show that a physical model of a chromosome with experimentally measured local interactions segregates into euchromatin- and heterochromatin-like phases. The model reproduces many of the features of the large-scale organization of the chromosome as measured by Hi-C. Our work provides an estimate of the amount of epigenetic marking needed to segregate a gene into heterochromatin. We use a chromosome-scale simulation to show that the preferential binding of heterochromatin protein 1 (HP1) to regions high in histone methylation (specifically H3K9me3) results in phase segregation and reproduces features of the observed Hi-C contact map. Specifically, we perform Monte Carlo simulations with one computational bead per nucleosome and an H3K9me3 pattern based on published ChIP-seq signals. We implement a binding model in which HP1 preferentially binds to trimethylated histone tails and then oligomerizes to bridge together nucleosomes. We observe a phase reminiscent of heterochromatin—dense and high in H3K9me3—and another reminiscent of euchromatin—less dense and lacking H3K9me3. This segregation results in a plaid contact probability map that matches the general shape and position of published Hi-C data. Analysis suggests that a roughly 20-kb segment of H3K9me3 enrichment is required to drive segregation into the heterochromatic phase.
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Mao S, MacPherson Q, Spakowitz AJ. Polymer Semiflexibility Induces Nonuniversal Phase Transitions in Diblock Copolymers. Phys Rev Lett 2018; 120:067802. [PMID: 29481283 DOI: 10.1103/physrevlett.120.067802] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/02/2017] [Revised: 11/19/2017] [Indexed: 06/08/2023]
Abstract
The order-disorder phase transition and the associated phase diagrams of semiflexible diblock copolymers are investigated using the wormlike chain model, incorporating concentration fluctuations. The free energy up to quartic order in concentration fluctuations is developed with chain-rigidity-dependent coefficients, evaluated using our exact results for the wormlike chain model, and a one-loop renormalization treatment is used to account for fluctuation effects. The chain length N and the monomer aspect ratio α directly control the strength of immiscibility (defined by the Flory-Huggins parameter χ) at the order-disorder transition and the resulting microstructures at different chemical compositions f_{A}. When monomers are infinitely thin (i.e., large aspect ratio α), the finite chain length N lowers the χN at the phase transition. However, fluctuation effects become important when chains have a finite radius, and a decrease in the chain length N elevates the χN at the phase transition. Phase diagrams of diblock copolymers over a wide range of N and α are calculated based on our fluctuation theory. We find that both finite N and α enhance the stability of the lamellar phase above the order-disorder transition. Our results demonstrate that polymer semiflexibility plays a dramatic role in the phase behavior, even for large chain lengths (e.g., N≈100).
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Affiliation(s)
- Shifan Mao
- Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA
| | - Quinn MacPherson
- Department of Physics, Stanford University, Stanford, California 94305, USA
| | - Andrew J Spakowitz
- Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA
- Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
- Department of Applied Physics, Stanford University, Stanford, California 94305, USA
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Abstract
We present a simulation study of the equilibrium thermodynamic behavior of semiflexible diblock copolymer melts. Using discretized wormlike chains and field-theoretic Monte Carlo, we find that concentration fluctuations play a critical role in controlling phase transitions of semiflexible diblock copolymers. Polymer flexibility and aspect ratio control the order-disorder transition Flory-Huggins parameter χODTN. For polymers with low aspect ratios, fluctuations strongly elevate the phase transition χODTN at finite molecular weights. For high aspect-ratio polymers, chain semiflexibility decreases the phase transition χODTN. We find that the simulated phase behavior agrees well with our recently developed fluctuation theory based on wormlike chain configurations and a one-loop treatment of concentration fluctuations.
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Affiliation(s)
- Shifan Mao
- Department of Chemical Engineering, ‡Department of Physics, §Department of Materials Science and
Engineering, ∥Department of Applied Physics, and ⊥Biophysics Program, Stanford University, Stanford, California 94305, United States
| | - Quinn MacPherson
- Department of Chemical Engineering, ‡Department of Physics, §Department of Materials Science and
Engineering, ∥Department of Applied Physics, and ⊥Biophysics Program, Stanford University, Stanford, California 94305, United States
| | - Andrew J. Spakowitz
- Department of Chemical Engineering, ‡Department of Physics, §Department of Materials Science and
Engineering, ∥Department of Applied Physics, and ⊥Biophysics Program, Stanford University, Stanford, California 94305, United States
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11
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Krajina BA, Spakowitz AJ. Large-Scale Conformational Transitions in Supercoiled DNA Revealed by Coarse-Grained Simulation. Biophys J 2017; 111:1339-1349. [PMID: 27705758 DOI: 10.1016/j.bpj.2016.07.045] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2016] [Revised: 07/26/2016] [Accepted: 07/28/2016] [Indexed: 02/07/2023] Open
Abstract
Topological constraints, such as those associated with DNA supercoiling, play an integral role in genomic regulation and organization in living systems. However, physical understanding of the principles that underlie DNA organization at biologically relevant length scales remains a formidable challenge. We develop a coarse-grained simulation approach for predicting equilibrium conformations of supercoiled DNA. Our methodology enables the study of supercoiled DNA molecules at greater length scales and supercoiling densities than previously explored by simulation. With this approach, we study the conformational transitions that arise due to supercoiling across the full range of supercoiling densities that are commonly explored by living systems. Simulations of ring DNA molecules with lengths at the scale of topological domains in the Escherichia coli chromosome (∼10 kilobases) reveal large-scale conformational transitions elicited by supercoiling. The conformational transitions result in three supercoiling conformational regimes that are governed by a competition among chiral coils, extended plectonemes, and branched hyper-supercoils. These results capture the nonmonotonic relationship of size versus degree of supercoiling observed in experimental sedimentation studies of supercoiled DNA, and our results provide a physical explanation of the conformational transitions underlying this behavior. The length scales and supercoiling regimes investigated here coincide with those relevant to transcription-coupled remodeling of supercoiled topological domains, and we discuss possible implications of these findings in terms of the interplay between transcription and topology in bacterial chromosome organization.
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Affiliation(s)
- Brad A Krajina
- Department of Chemical Engineering, Stanford University, Stanford, California
| | - Andrew J Spakowitz
- Department of Chemical Engineering, Stanford University, Stanford, California; Department of Applied Physics, Stanford University, Stanford, California; Department of Materials Science and Engineering, Stanford University, Stanford, California; Biophysics Program, Stanford University, Stanford, California.
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12
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Mao S, MacPherson Q, Qin J, Spakowitz AJ. Field-theoretic simulations of random copolymers with structural rigidity. Soft Matter 2017; 13:2760-2772. [PMID: 28338151 DOI: 10.1039/c7sm00164a] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/21/2023]
Abstract
Copolymers play an important role in a range of soft-materials applications and biological phenomena. Prevalent works on block copolymer phase behavior use flexible chain models and incorporate interactions using a mean-field approximation. However, when phase separation takes place on length scales comparable to a few monomers, the structural rigidity of the monomers becomes important. In addition, concentration fluctuations become significant at short length scales, rendering the mean-field approximation invalid. In this work, we use simulation to address the role of finite monomer rigidity and concentration fluctuations in microphase segregation of random copolymers. Using a field-theoretic Monte-Carlo simulation of semiflexible polymers with random chemical sequences, we generate phase diagrams for random copolymers. We find that the melt morphology of random copolymers strongly depends on chain flexibility and chemical sequence correlation. Chemically anti-correlated copolymers undergo first-order phase transitions to local lamellar structures. With increasing degree of chemical correlation, this first-order phase transition is softened, and melts form microphases with irregular shaped domains. Our simulations in the homogeneous phase exhibit agreement with the density-density correlation from mean-field theory. However, conditions near a phase transition result in deviations between simulation and mean-field theory for the density-density correlation and the critical wavemode. Chain rigidity and sequence randomness lead to frustration in the segregated phase, introducing heterogeneity in the resulting morphologies.
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Affiliation(s)
- Shifan Mao
- Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.
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13
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Pilyugina E, Krajina B, Spakowitz AJ, Schieber JD. Buckling a Semiflexible Polymer Chain under Compression. Polymers (Basel) 2017; 9:polym9030099. [PMID: 30970780 PMCID: PMC6432112 DOI: 10.3390/polym9030099] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2017] [Revised: 03/02/2017] [Accepted: 03/03/2017] [Indexed: 02/07/2023] Open
Abstract
Instability and structural transitions arise in many important problems involving dynamics at molecular length scales. Buckling of an elastic rod under a compressive load offers a useful general picture of such a transition. However, the existing theoretical description of buckling is applicable in the load response of macroscopic structures, only when fluctuations can be neglected, whereas membranes, polymer brushes, filaments, and macromolecular chains undergo considerable Brownian fluctuations. We analyze here the buckling of a fluctuating semiflexible polymer experiencing a compressive load. Previous works rely on approximations to the polymer statistics, resulting in a range of predictions for the buckling transition that disagree on whether fluctuations elevate or depress the critical buckling force. In contrast, our theory exploits exact results for the statistical behavior of the worm-like chain model yielding unambiguous predictions about the buckling conditions and nature of the buckling transition. We find that a fluctuating polymer under compressive load requires a larger force to buckle than an elastic rod in the absence of fluctuations. The nature of the buckling transition exhibits a marked change from being distinctly second order in the absence of fluctuations to being a more gradual, compliant transition in the presence of fluctuations. We analyze the thermodynamic contributions throughout the buckling transition to demonstrate that the chain entropy favors the extended state over the buckled state, providing a thermodynamic justification of the elevated buckling force.
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Affiliation(s)
- Ekaterina Pilyugina
- Center for Molecular Study of Condensed Soft Matter, Illinois Institute of Technology, Chicago, IL 60616, USA.
- Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA.
| | - Brad Krajina
- Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.
| | - Andrew J Spakowitz
- Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.
- Department of Applied Physics, Stanford University, Stanford, CA 94305, USA.
- Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA.
- Biophysics Program, Stanford University, Stanford, CA 94305, USA.
| | - Jay D Schieber
- Center for Molecular Study of Condensed Soft Matter, Illinois Institute of Technology, Chicago, IL 60616, USA.
- Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA.
- Department of Physics, Illinois Institute of Technology, Chicago, IL 60616, USA.
- Department of Applied Mathematics, Illinois Institute of Technology, Chicago, IL 60616, USA.
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Mulligan PJ, Chen YJ, Phillips R, Spakowitz AJ. Interplay of Protein Binding Interactions, DNA Mechanics, and Entropy in DNA Looping Kinetics. Biophys J 2015; 109:618-29. [PMID: 26244743 DOI: 10.1016/j.bpj.2015.06.054] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2015] [Revised: 06/20/2015] [Accepted: 06/25/2015] [Indexed: 12/24/2022] Open
Abstract
DNA looping plays a key role in many fundamental biological processes, including gene regulation, recombination, and chromosomal organization. The looping of DNA is often mediated by proteins whose structural features and physical interactions can alter the length scale at which the looping occurs. Looping and unlooping processes are controlled by thermodynamic contributions associated with mechanical deformation of the DNA strand and entropy arising from thermal fluctuations of the conformation. To determine how these confounding effects influence DNA looping and unlooping kinetics, we present a theoretical model that incorporates the role of the protein interactions, DNA mechanics, and conformational entropy. We show that for shorter DNA strands the interaction distance affects the transition state, resulting in a complex relationship between the looped and unlooped state lifetimes and the physical properties of the looped DNA. We explore the range of behaviors that arise with varying interaction distance and DNA length. These results demonstrate how DNA deformation and entropy dictate the scaling of the looping and unlooping kinetics versus the J-factor, establishing the connection between kinetic and equilibrium behaviors. Our results show how the twist-and-bend elasticity of the DNA chain modulates the kinetics and how the influence of the interaction distance fades away at intermediate to longer chain lengths, in agreement with previous scaling predictions.
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16
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Córdoba A, Schieber JD, Indei T. The role of filament length, finite-extensibility and motor force dispersity in stress relaxation and buckling mechanisms in non-sarcomeric active gels. Soft Matter 2015; 11:38-57. [PMID: 25375087 DOI: 10.1039/c4sm01944j] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
After relaxing some assumptions we apply a single-chain mean-field mathematical model recently introduced [RSC Adv. (2014)] to describe the role of molecular motors in the mechanical properties of active gels. The model allows physics that are not available in models postulated on coarser levels of description. Moreover it proposes a level of description that allows the prediction of observables at time scales too difficult to achieve in multi-chain simulations for realistic filament lengths and densities. We model the semiflexible filaments that compose the active gel as bead-spring chains; molecular motors are accounted for by using a mean-field approach, in which filaments undergo transitions of one motor attachment state depending on the state of the probe filament. The level of description includes the end-to-end distance and attachment state of the filaments, and the motor-generated forces, as stochastic state variables which evolve according to a proposed differential Chapman-Kolmogorov equation. The motor-generated forces are drawn from a stationary distribution of motor stall forces. We consider bead-spring chains with multiple beads, explore the effect of finite-extensibility of the strands and incorporate into the model motor force distributions that have been measured experimentally. The model can no longer be solved analytically but is amenable to numerical simulation. This version of the model allows a more quantitative description of buckling dynamics [Lenz et. al. PRL, 2012, 108, 238107] and the dynamic modulus of active gels. The effect of finite extensibility of the filament strands on the dynamic modulus was also found to be in agreement with the microrheology experiments of Mizuno et. al., [Science, 2007, 315, 370-373].
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Affiliation(s)
- Andrés Córdoba
- Department of Chemical and Biological Engineering and Center for Molecular Study of Condensed Soft Matter, Illinois Institute of Technology, 3440 S. Dearborn St, Chicago, Illinois 60616, USA.
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Koslover EF, Spakowitz AJ. Multiscale dynamics of semiflexible polymers from a universal coarse-graining procedure. Phys Rev E Stat Nonlin Soft Matter Phys 2014; 90:013304. [PMID: 25122407 DOI: 10.1103/physreve.90.013304] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/27/2013] [Indexed: 06/03/2023]
Abstract
Simulating the dynamics of a semiflexible polymer across time and length scales that bridge the rigid and flexible regimes requires a physically sound method for generating coarse-grained representations of the polymer. Here, we study the dynamic behavior of the discrete stretchable, shearable wormlike chain model, which can be used to coarse-grain a continuous semi-elastic chain at an arbitrary discretization. We show that the dynamics of this universal model match those of the wormlike chain at length scales above the discretization length. The evolution of the stress correlation, as probed through Brownian dynamics simulations, is found to reproduce the predicted behavior in both the rigid and flexible regimes, spanning over six orders of magnitude in time scales. The coarse-graining approach employed here thus enables dynamic simulation of semiflexible polymers at lengths and times that were previously inaccessible with conventional methods.
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Affiliation(s)
- Elena F Koslover
- Chemical Engineering Department, Stanford University, Stanford, California 94305, USA
| | - Andrew J Spakowitz
- Chemical Engineering Department, Stanford University, Stanford, California 94305, USA and Biophysics Program, Stanford University, Stanford, California 94305, USA
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18
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Abstract
The idea that the dynamics of concentrated, high–molecular weight polymers are largely governed by entanglements is now widely accepted and typically understood through the tube model. Here we review alternative approaches, slip-link models, that share some similarities to and offer some advantages over tube models. Although slip links were proposed at the same time as tubes, only recently have detailed, quantitative mathematical models arisen based on this picture. In this review, we focus on these models, with most discussion limited to mathematically well-defined objects that conform to state-of-the-art beyond-equilibrium thermodynamics. These models are connected to each other through successive coarse graining, using nonequilibrium thermodynamics along the way, and with a minimal parameter set. In particular, the most detailed level of description has four parameters, three of which can be determined directly from atomistic simulations. Once the remaining parameter is determined for any system, all parameters for all members of the hierarchy are determined. We show how, using this hierarchy of slip-link models combined with atomistic simulations, we can make predictions about the nonlinear rheology of monodisperse homopolymer melts, polydisperse melts, or blends of different architectures. Mathematical details are given elsewhere, so these are limited here, and physical ideas are emphasized. We conclude with an outlook on remaining challenges that might be tackled successfully using this approach, including complex flow fields and polymer blends.
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Affiliation(s)
- Jay D. Schieber
- Center for Molecular Study of Condensed Soft Matter, Department of Chemical and Biological Engineering and Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616
| | - Marat Andreev
- Center for Molecular Study of Condensed Soft Matter, Department of Chemical and Biological Engineering and Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616
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Tree DR, Wang Y, Dorfman KD. Modeling the relaxation time of DNA confined in a nanochannel. Biomicrofluidics 2013; 7:54118. [PMID: 24309551 PMCID: PMC3820670 DOI: 10.1063/1.4826156] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/01/2013] [Accepted: 10/07/2013] [Indexed: 05/12/2023]
Abstract
Using a mapping between a Rouse dumbbell model and fine-grained Monte Carlo simulations, we have computed the relaxation time of λ-DNA in a high ionic strength buffer confined in a nanochannel. The relaxation time thus obtained agrees quantitatively with experimental data [Reisner et al., Phys. Rev. Lett. 94, 196101 (2005)] using only a single O(1) fitting parameter to account for the uncertainty in model parameters. In addition to validating our mapping, this agreement supports our previous estimates of the friction coefficient of DNA confined in a nanochannel [Tree et al., Phys. Rev. Lett. 108, 228105 (2012)], which have been difficult to validate due to the lack of direct experimental data. Furthermore, the model calculation shows that as the channel size passes below approximately 100 nm (or roughly the Kuhn length of DNA) there is a dramatic drop in the relaxation time. Inasmuch as the chain friction rises with decreasing channel size, the reduction in the relaxation time can be solely attributed to the sharp decline in the fluctuations of the chain extension. Practically, the low variance in the observed DNA extension in such small channels has important implications for genome mapping.
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Affiliation(s)
- Douglas R Tree
- Department of Chemical Engineering and Material Science, University of Minnesota-Twin Cities, 421 Washington Ave SE, Minneapolis, Minnesota 55455, USA
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