The process of DNA replication is fast, accurate and complex requiring several enzymes and protein factors. This process has been worked out in detail in E.coli and its viruses over last 40 years of extensive research. DNA replication in prokaryotes as well as eukaryotes involves some basic steps which can be outlined as follows:
- Unwinding and separation of the two parental strands of DNA.
- Each parental strand then serves as a template for the synthesis of a new strand basing on the complementarity of the sequence of nucleotide of the old template strand.
- One parental strand and one new strand wind together into a double helix.
DNA replication does not begin just at any where on a DNA. It originates at specific site called replication origin and then proceeds in one or both the directions. A DNA segment specifying an origin has been isolated from E.coli and several Coli phages and plasmids as well as from Yeast and a number of eukaryotic viruses. In E.coli the origin is a unique sequence of DNA of about 245 base pair long and known as Ori C. It is A-T rich so that the two strands easily separate at the origin.
The origin is specifically recognized by a replication initiator protein which binds to the origin to begin replication. In Yeast the origin is known as Autonomous Replicating sequence (ARS) and is 150 base pair long. ARS is the binding site for Origin Recognition Complex (ORC).
The replication initiated from the origin proceeds along replication forks. So, each origin has two terminii. One origin with its two unique termini is called a rcplicon.In prokaryote like E.coli the entire circular DNA is a single replicon. But eukaryotes with larger DNA have
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Replicating and structure Appearance of structures
The DNA replication can be unidirectional or bidirectional. At the origin when the two strands separate it forms a replication eye. In unidirectional replication one of the two ends of the “eye” remains stationary while the other end moves along the replication fork. In bidirectional replication both ends move along the replication. An example of unidirectional replication is replication of mitochondrial DNA (mt DNA) in vertebrates.