In E. coli, DNA replication and cell division are tightly coordinated processes to ensure that each daughter cell receives a fully replicated and intact copy of the genome. The time required for DNA replication, known as the doubling time, and the time required for cell division, known as the generation time, can be different.
In the scenario you described, where the generation time of E. coli is 20 minutes and the time required for DNA replication is 60 minutes, there are a few possible mechanisms that allow fully replicated DNA to be passed on to daughter cells:
- Overlapping replication cycles: E. coli cells can initiate new rounds of DNA replication before completing the previous round. This overlapping replication cycle ensures that there are multiple copies of the genome present within a single cell at any given time. When cell division occurs, each daughter cell inherits at least one fully replicated copy of the genome.
- Spatial organization: E. coli exhibits spatial organization of its DNA within the cell. The replicated DNA molecules are spatially segregated and positioned near the cell poles or other specific regions. This organization helps to prevent DNA entanglement and ensures the efficient partitioning of fully replicated DNA to daughter cells during cell division.
- Cell division regulation: The cell division process is tightly regulated to ensure that it occurs only after DNA replication is complete. A checkpoint mechanism, known as the nucleoid occlusion system, prevents cell division if replication is not finished. This system relies on the proper organization and positioning of the nucleoid (the bacterial chromosome) to prevent cytokinesis until replication is complete.
By employing these mechanisms, E. coli ensures that each daughter cell receives a complete and undamaged copy of the genome. The overlapping replication cycles, spatial organization of DNA, and cell division regulation contribute to the faithful transmission of genetic material during cell division, despite the difference in timing between DNA replication and cell division.