- Although the mechanism of DNA replication is similar in all organisms, the overall process varies depending on the nature of the DNA molecule being copied.
- Theta replication, rolling-circle replication, and linear replication differ with respect to the initiation and progression of replication, but all produce new DNA molecules by semi conservative replication.
- Theta replication a common type of replication that takes place in circular DNA of prokaryote (eg: E.coli)
Product of theta replication: Two circular DNA molecules
Template for DNA synthesis: Circular DNA
Direction of replication: Unidirectional or bidirectional
- Theta replication does not involve breakage of nucleotide strand like that of rolling circle replication
How they got the name: Intermediate structure formed during the replication of a circular DNA molecule (prokaryote DNA) resembles the Greek letter theta (θ)
The location of origin of replication and replication terminus regions are on opposite sides of the E.coli chromosome
Mechanism of theta replication
- In theta replication, double-stranded DNA begins to unwind at the replication origin, producing single-stranded nucleotide strands.
- The unwinding of the DNA double helix generates a loop, termed a replication bubble.
- The point of unwinding, where the two single nucleotide strands separate from the double-stranded DNA helix, is called a replication fork.
- Both of the DNA strands can serve as templates on which new DNA can be synthesized.
- Lagging strand synthesis occurs discontinuously as the replication fork moves and generates number of Okazaki fragments.
- Leading strand synthesis proceeds continuously with the replication fork movement.
- If there are two replication forks, one at each end of the replication bubble, the forks proceed outward in both directions in a process called bidirectional replication, simultaneously unwinding and replicating the DNA until they eventually meet.
- If a single replication fork is present, it proceeds around the entire circle to produce two complete circular DNA molecules, each consisting of one old and one new nucleotide strand.
Figure1: Mechanism of theta mode of replication in E.coli
By theta mode of replication an E.coli can replicate it’s genome in 40 minutes. But the cell division takes 20 minutes. Then how an E.coli replicates its DNA?
- The replication always begins at an origin of replication. In bacteria there is usually one origin per chromosome or plasmid.
- The replisomes (enzymes involved in replication) assemble at the origin and then move in opposite direction around the chromosome until the meet at the termination region.
- Each replisome moves at a rate of 1000 nucleotides per second and it takes about 40 minutes to complete one round of replication.
- But coli can divide every 20 minutes. They have more than one active replisomes at a time associated with their chromosome.
- A new round of replication begins at a site which is Origin of replication, before the current round of replication is completed.
- At any given instant, there can be six or eight replication forks synthesizing DNA simultaneously inside the cell.
(This means that origins fire every 20 minutes. When the daughter chromosome segregate into daughter cells, they are already partially replicated in preparation for the next cell division)
More detailed explanation, refer the following Link