Q) Time required for completion of replication in E.coli is 40 minutes. How did E.coli achieve its doubling time within 20 minutes?
Basic things need to remember before answering this question
- E.coli has 4.6 million base pairs in a single circular chromosome. (For easy calculation made it to roughly 5x 106bp)
- The approximate time required for complete replication of E.coli DNA: 40 minutes
- Doubling time of E.coli: 20 minutes
- Both strands of DNA can act as template for new strand synthesis.
- DNA replication starts from a single origin of replication and proceeding around the circle bidirectionally (i.e., in both directions).
- Number of nucleotides added per second: ~1000 nucleotides / sec
- The doubling time of E.coli is much shorter than the time required for its DNA replication. (That means one round of DNA replication will not be completed within one round of cell division).
- Hence in E.coli the next round of replication does not wait for the previous one to be completed.
- So, prior to completion of one round of replication, the origin of replication of newly synthesized DNA is attacked by helicases and others to initiate the next round of replication.Figure 1: Replication pattern of rapidly growing E.coli cells
Colors used in the figure: E.coli cells (yellow) with chromosomes (blue lines) and origins (black squares)
- First cell growth and DNA replication happens. Then the cell divide and second round of replication initiate simultaneously.
Figure shows the number of replication forks and origins at different stages of the cell cycle.
1) Initiation of replication occurs at four origins at the same time as cell division
2) A young cell therefore contains four origins and six replication forks.
3) As replication proceeds, the oldest pair of forks reach the terminus and the two sister chromosomes segregate. The cell then contains four origins and four replication forks
Explanation for better understanding
- Consider E.coli under going first round of cell division. Before the cell division the DNA content must be doubled. But in case of fast growing E.coli cells first round of DNA replication will not be completed before the completion of cell division. Because the time required for DNA replication is almost double than time required for cell division.
- Each round of cell division begins with one round of DNA replication. Hence replication of both daughter DNA strands begins (even though their synthesis not completed).
Here in case of fast growing E.coli, after the first round of cell division, DNA of daughter cells has 4 OriC and 6 replication fork.
- 2 OriC (One on each parent strand) and 2 replication forks are already present in that DNA during first round of replication which is not yet completed.
- 2 OriC (one each) are present on new daughter DNA strand (synthesis not complete) and 4 replication fork on daughter DNA (2 each on one daughter strand)
- Both of them can initiate new round of DNA replication from OriC
Figure 2: Nested replication forks. The schematic shows the way in which multiple rounds of replication are taking place simultaneously in rapidly dividing E. coli cells. This picture is used to make an estimate of the time to replicate the full bacterial genome.
Rapidly dividing E.coli cells can have more than 6 origins of replication and over 10 replication forks coexisting in a single cell. (By replicating granddaughter cells genome)