arXiv subjects

Shreerang Pande

Publications and source records attributed to Shreerang Pande.

5 recordsLinked to original sources

arXiv2026

Kinetics of segregation of topologically-modified ring polymers in cylindrical confinement

In Escherichia coli (E. coli), entropic repulsion between the two daughter DNA ring polymers under cylindrical confinement is believed to be an important factor governing chromosomal segregation. The repulsion can be enhanced by topological modifications, i.e., by the introduction of internal loops at certain locations along the contour of the circular DNA. However, the effect of topological modifications on the rate of segregation of ring polymers remains unclear. Therefore, we systematically varied the number and the contour length of loops introduced at selected locations by crosslinking monomers. The appropriate crosslinking was motivated by observations that extruded loops are located mainly near the origin of replication (ori-proximal) region of the E. coli chromosome. This resulted in the chains becoming intrinsically anisotropic. Using Langevin dynamics simulations of these topologically modified bead-spring polymers, we calculated the time required for segregation under cylinder confinement. With certain caveats, we found that increasing the number of loops resulted in a decrease in the time of segregation. In line with past work, we propose that this is due to the increase in the entropic repulsion between the polymers upon increasing the number of loops. In addition to the number of loops, the contour length of the loops and the mutual orientation of the (anisotropic) chains in the initial configurations played a role in determining the time of segregation.

cond-mat.soft↗

arXiv2025

Entropic organization of topologically modified ring polymers in spherical confinement

It has been shown that under high cylindrical confinement, two ring polymers with excluded volume interactions between monomers, segregate to two halves of the cylinder to maximize their entropy. In contrast, two ring polymers remain mixed within a sphere, as there is no symmetry breaking direction [Nat Rev Microbiol, 8, 600-607 (2010)]. Therefore, in order to observe emergent organization of ring polymers in a sphere, we can introduce an asymmetric topological modification to the polymer architecture by creating a small loop and a big loop within the ring polymer. We consider the bead-spring model of polymers where there are only repulsive excluded volume interactions between the monomers ensuring that the organization we observe is purely entropy-driven. We find that for a single topologically modified polymer within a sphere, the monomers of the bigger loop are statistically more probable to be found closer to the periphery. However, the situation is reversed when we have multiple such topologically modified polymers in a sphere. The monomers of the small loops are found closer to the walls of the sphere. We can increase this localization and radial organization of polymer segments by increasing the number of small loops in each ring polymer. We study how these loops interact with each other within a polymer, as well as with loops of other polymers in spherical confinement. We compare contact maps of multiple such topologically modified polymers in a sphere. Finally, we discuss the plausible relevance of our studies to eukaryotic chromosomes that are confined within a spherical nucleus.

cond-mat.soft↗

arXiv2024

Topology mediated organization of E.coli chromosome in fast growth conditions

Recent experiments have been able to visualise chromosome organization in fast-growing E.coli cells. However, the mechanism underlying the spatio-temporal organization remains poorly understood. We propose that the DNA adopts a specific polymer topology as it goes through its cell cycle. We establish that the emergent entropic forces between polymer segments of the DNA-polymer with modified topology, leads to chromosome organization as seen in-vivo. We employ computer simulations of a replicating bead spring model of a polymer in a cylinder to investigate the problem. Our simulation of the overlapping cell cycles not only show successful segregation, but also reproduces the evolution of the spatial organization of the chromosomes as observed in experiments. This manuscript in addition to our previous work on slowly growing bacterial cells, shows that our topology-based model can explain the organization of chromosomes in all growth conditions.

cond-mat.soft↗

arXiv2021

DNA-polymer architecture orchestrates the segregation and spatio-temporal organization of E. coli chromosomes during replication in slow growth

The mechanism and driving forces of chromosome segregation in the bacterial cell cycle of E. coli is one of the least understood events in its life cycle. Using principles of entropic repulsion between polymer loops confined in a cylinder, we use Monte carlo simulations to show that the segregation dynamics is spontaneously enhanced by the adoption of a certain DNA-polymer architecture as replication progresses. Secondly, the chosen polymer-topology ensures its self-organization along the cell axis while segregation is in progress, such that various chromosomal loci get spatially localized. The time evolution of loci positions quantitatively match the corresponding experimentally reported results, including observation of the cohesion time and the ter-transition. Additionally, the contact map generated using our bead-spring model reproduces the four macro-domains of the experimental Hi-C maps. Lastly, the proposed mechanism reproduces the observed universal dynamics as the sister loci separate during segregation. It was already hypothesized and expected that SMC proteins, e.g. MukBEF contribute over and above entropic repulsion between bacterial-DNA ring-polymers to aid the segregation of daughter DNAs in the E.coli cell cycle. We propose that cross-links (plausibly induced by SMC proteins) at crucial positions along the contour is enough to provide sufficient forces for segregation within reasonable time scales. A mapping between Monte Carlo diffusive dynamics time scales and real time units helps us use experimentally relevant numbers for our modeling.

cond-mat.soft↗

arXiv2021

The role of polymer architecture in the entropy driven segregation and spatial organization of bacterial chromosomes

Entropic repulsion between DNA ring polymers under confinement is the key mechanism governing the spatial segregation of bacterial chromosomes, although it remains incompletely understood how proteins aid the process of entropic segregation. Here we establish that `internal' loops within a modified-ring polymer architecture enhances entropic repulsion between two overlapping polymers confined in a cylinder. Moreover it also induces entropy-driven spatial organization of polymer segments as seen in-vivo. To that end, we design polymers of different architectures in our simulations, by introducing a minimal number of cross-links between particular monomers along the chain contour. This helps us to identify the underlying mechanisms which lead to faster segregation of spatially overlapping polymers as well as localization of specific polymer segments. The observed segregation dynamics of overlapping polymers is aided in our simulations by the frequent release of topological constraints, implemented by allowing chains to cross each at regular intervals. Additionally, we compare the segregation dynamics timescales with that of self-avoiding polymers and thereby highlight the significance of Topoisomerase in biological systems where DNA-strands are allowed to occasionally pass through each other. We use the blob model to provide a theoretical understanding of why or how certain architectures lead to enhanced entropic repulsive forces between loops, which in turn leads to the positional organization of segments relative to each other in confined environments. Lastly, we establish a correspondence between the C.crescentus bacterial species and our results for one particular polymer architecture.

cond-mat.soft↗