Can entropy save bacteria?

TL;DR

This paper applies a physical biology model to bacterial chromosomes, predicting their segregation behavior and distribution within cells, highlighting the role of physical forces and the need for active partitioning systems.

Contribution

It introduces a phase diagram model for chromosome organization in bacteria, linking physical confinement to segregation mechanisms and plasmid distribution.

Findings

Duplicated chromosomes tend to demix due to physical forces.

New DNA is likely found at the periphery during replication.

Plasmids are predicted to be randomly distributed, requiring active partitioning.

Abstract

This article presents a physical biology approach to understanding organization and segregation of bacterial chromosomes. The author uses a "piston" analogy for bacterial chromosomes in a cell, which leads to a phase diagram for the organization of two athermal chains confined in a closed geometry characterized by two length scales (length and width). When applied to rod-shaped bacteria such as Escherichia coli, this phase diagram predicts that, despite strong confinement, duplicated chromosomes will demix, i.e., there exists a primordial physical driving force for chromosome segregation. The author discusses segregation of duplicating chromosomes using the concentric-shell model, which predicts that newly synthesized DNA will be found in the periphery of the chromosome during replication. In contrast to chromosomes, these results suggest that most plasmids will be randomly distributed inside the cell because of their small sizes. An active partitioning system is therefore required for accurate segregation of low-copy number plasmids. Implications of these results are also sketched, e.g., on the role of proteins, segregation mechanisms for bacteria of diverse shapes, cell cycle of an artificial cell, and evolution.