Cairns
Model
of
Replication - According to this model replication begins by denaturation of the DNA double strands at a specific site called the origin. Two growing points are established and there is bidirectional DNA synthesis. Both strands of DNA are replicated. As the growing points move apart, unwinding of the DNA double strand takes place. This unwinding creates torque since the parental DNA strands cannot unwind freely.
The torque is transmitted to the unreplicated part of the molecule which consequently becomes supertwisted. Supertwisting brings about a conformational strain on the DNA molecule and prevents it from replicating further. To counteract this effect a temporary break ('nick') is brought about on one of the strands by a swiveling protein (II).
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