Nucleoid Complexity, the Nucleotypic Effect and DNA/Cell Mass Homeostasis during Bacterial Growth

DNA/cell mass homeostasis is a universal feature of living organisms. As the cell grows in response to nutrient availability, it must duplicate each chromosome once and only once each division cycle. Across the Tree of Life, cells differ in their sizes in a manner that depends directly on the amount of DNA they harbor, what has been termed the “nucleotypic effect”: cell size expands or contracts as DNA content increases or decreases. In eukaryotes, any deviation from DNA/mass homeostasis results in the dis-regulation of the developmental program and the initiation of carcinogenesis and other genetic pathologies. In bacteria, deviation from or perturbation of DNA/mass homeostasis alters important physiological features such as the cell cycle timing of DNA replication initiation and the co-ordination of initiation with replication termination and cell division. In prokaryotes, the timing of initiation occurs at a relatively constant and growth rate invariant mass, termed the initiation mass (Mi), and depends strictly on DNA replication fork rates and membrane biogenesis. Complex “machines”, frequently referred to as hyperstructures or factories, mediate the phase transitions that define the different periods of the bacterial cell cycle. The following will examine how the machines gate the phase transitions that organize the cell cycle in time and space.