33. In E. coli grown under nutrient rich conditions, replication of entire genome takes about 40 min., yet it can divide every 20 min. This is so because:

(1) While E. coli divides every 20 min, equal transfer of genetic material occurs only in the alternate rounds of cell divisions.

(2) A second round of genome replication begins before the completion of first round of replication, and by the time cell is ready to divide, two copies of the genome are available.

(3) Genome replication cell division are not

coordinated with each other.

(4) During cell division, only one of the strands of the genome whose synthesis can be achieved in 20 min, is transferred to the daughter cell.

Introduction

Escherichia coli (E. coli), a fast-growing bacterium, exhibits a fascinating ability to divide approximately every 20 minutes under nutrient-rich conditions, even though its entire genome takes about 40 minutes to replicate. This apparent paradox is resolved by the bacterium’s strategy of overlapping rounds of DNA replication, allowing it to initiate a new replication cycle before the previous one completes. This article explains this mechanism and why it is essential for rapid bacterial proliferation.

The Paradox: Division Time vs. Replication Time

• Genome replication time: ~40 minutes

• Cell division time (generation time): ~20 minutes

At first glance, it seems impossible for E. coli to divide every 20 minutes if it takes 40 minutes just to replicate its DNA. However, E. coli overcomes this by initiating multiple rounds of replication simultaneously.

Overlapping Rounds of DNA Replication

• Before the first round of replication finishes, E. coli initiates a second round of replication at the origin (oriC).

• This means that at the time of cell division, the cell already has two partially replicated chromosomes in progress.

• By the time the cell divides, two complete copies of the genome are ready, one for each daughter cell.

This strategy is known as multifork replication or overlapping replication rounds.

How Overlapping Replication Enables Rapid Division

• The replication forks from the first round continue progressing while new forks start at the origin for the next round.

• This results in multiple active replication forks within a single cell, increasing the DNA synthesis rate.

• The cell can thus maintain a doubling time shorter than the time required for a single round of replication.

Why Other Options Are Incorrect

1. Equal transfer of genetic material only in alternate divisions:

   • Incorrect; genetic material is duplicated and segregated every division, not only alternate ones.

2. Genome replication and cell division are not coordinated:

   • Incorrect; replication and division are tightly coordinated to ensure genome integrity.

3. Only one DNA strand is transferred during division:

   • Incorrect; complete double-stranded genomes are transferred to daughter cells.

Supporting Evidence from Research

• Studies show that overlapping rounds of replication are a hallmark of fast-growing E. coli cells in rich media.

• This mechanism ensures that DNA replication does not limit the bacterial growth rate.

• The initiation of replication is tightly regulated to prevent over-replication and maintain genome stability.

Summary Table

Final Answer

(2) A second round of genome replication begins before the completion of the first round, and by the time the cell is ready to divide, two copies of the genome are available.

Conclusion

E. coli’s ability to divide every 20 minutes despite a 40-minute genome replication time is due to overlapping rounds of DNA replication. This efficient strategy allows the bacterium to initiate new replication cycles before the previous ones finish, ensuring rapid growth and faithful genome duplication.