Replication of the E. coli chromosome takes 40 to 45 minutes, but the organism has a generation time of 26 minutes. How does the cell have time to make complete chromosomes for each offspring cell?
To understand how E. coli cells manage to replicate their entire chromosome within a shorter generation time than the time required for replication, we need to consider a few factors.
Firstly, it is important to note that replication of the E. coli chromosome is a complex process involving several stages. The actual replication of the DNA molecule (synthesis of a new DNA strand complementary to each original DNA strand) is just one part of the process. Other steps include initiation, elongation, and termination.
During initiation, specific proteins bind to specific sites on the DNA molecule to start the replication process. This step is not time-consuming compared to the replication itself. Once initiation occurs, the elongation phase begins, during which the DNA molecule is duplicated. Finally, the termination step concludes replication.
The time taken for the complete replication of the E. coli chromosome includes all these stages. However, not all cells within the bacterial population go through replication at the exact same time. In a growing bacterial population, there are cells in different phases of the cell cycle. Some cells are actively replicating their DNA, while others may be in different stages such as initiation or termination.
The generation time of 26 minutes refers to the time it takes for a single E. coli cell to divide and produce two daughter cells. During this time, the cell undergoes all the stages of replication. However, it is important to note that replication is a continuous process in E. coli, meaning that replication initiation can occur even before the previous round of replication has completed.
This overlapping replication process allows the E. coli cell to start replicating its DNA before the previous round is completed. For example, while the replication is still ongoing (within the 40-45 minutes as you mentioned), the cell can initiate replication of another round, thereby ensuring that by the time it divides, each daughter cell will have a complete chromosome.
In summary, the E. coli cells manage to make complete chromosomes for each offspring cell by utilizing overlapping replication, where replication is initiated before the previous round is complete. This allows the cells to efficiently replicate their chromosome within their shorter generation time.