  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.
