To replicate an E. coli cell, its ~5,000,000 base-pair circular chromosome needs to be replicated. The rate of DNA synthesis of the DNA replication machinery is approximately 1000 nucleotides per second. The replication (DNA synthesis) is bidirectional and can only be initiated from only one location in the genome (i.e. it has only one origin of replication). So, it would take ~2,500 seconds or ~42 minutes to replicate the genome. However, it can be experimentally demonstrated that an E. coli cell can replicate itself in ~20 minutes. Explain how this could be possible. Hint: think about possible mechanism(s) that would allow the complete replication of the genome in 20 minutes of the cell division cycle.

E. coli is a bacterium with a circular chromosome that is approximately 5 million base pairs long. When it replicates, DNA synthesis occurs bidirectionally from a single origin of replication. At a rate of 1000 nucleotides per second and with two replication forks moving away from the origin, it should take around 2500 seconds, or about 42 minutes, for the entire chromosome to be copied. However, experimental observations show that E. coli can divide and complete genome replication in just about 20 minutes under optimal conditions.

Here's how this can be possible:

Multifork Replication: E. coli uses a process called multifork replication to achieve this faster rate. Before the first replication cycle has been completed, a second round of replication can begin at the origin of replication. This means the replication machinery for the next generation is already set in motion before the first generation's replication is complete. This overlapping of replication cycles leads to faster completion times.
Rapid Cell Division: While each individual replication round might still take 42 minutes, the actual cell division cycle time can be reduced to 20 minutes by starting new rounds of replication before the previous one is completed. This allows for continuous and efficient production of daughter cells under favorable growth conditions.
Optimal Growth Conditions: E. coli's ability to divide every 20 minutes is often observed under ideal laboratory conditions where nutrients are abundant and environmental stresses are minimal. These conditions encourage both rapid cell growth and efficient DNA replication.

Thus, the combination of multifork replication and favorable growth conditions enables E. coli to replicate its DNA much faster than would be possible through a simple calculation of replication time based solely on the rate of nucleotide addition and the size of the genome.