Modeling (top) shows how bacterial chromosome separation can be driven by entropy alone.

JUN/NAS

Entropy may be sufficient to drive bacterial chromosome segregation, say Suckjoon Jun and Bela Mulder (FOM Institute for Atomic and Molecular Physics, Amsterdam, Netherlands).

The rapid and abrupt nature of chromosome segregation in bacteria has led researchers to suspect eukaryotic-like mechanisms based on active cytoskeletal proteins. Jun approached the problem from a very different viewpoint: that of a polymer physicist. Polymers tend to repel each other because of the greater conformational freedom, and thus higher entropy that results if they untangle and separate. It is this tendency, which is much stronger in confined spaces such as the cell, that the Dutch group believes is the driver for chromosome segregation.

They devised a mathematical model of bacterial chromosome replication and segregation. In this model, the existing DNA is constrained within a nucleoid, and newly replicated DNA is ejected by entropic forces into a peripheral region. Based purely on a consideration of entropic states, the bacterial chromosomes segregated rapidly after replication. Supercoiling and DNA condensation are expected to increase the structure of a given chromosome and increase the entropic effects.

To test the model, better visualization of bacterial chromosome segregation is needed. Past studies have suggested that segregation may not occur until much of the chromosome is replicated, but uncertainties remain. Meanwhile, Jun hopes to create an approximation of two segregating chromosomes under controlled conditions in a microchamber.

Reference:

Jun, S., and B. Mulder.
2006
.
Proc. Natl. Acad. Sci. USA.
103
:
12388
–12393.