Index >> Replication of DNA >> Rolling Circle Model of Replication

The chromosome first becomes double stranded by the synthesis of a negative strand. Synthesis is presumed to begin at a specific initiation point on the template ring. One strand of the parental duplex ring is now cut at a specific point by an endonuclease. This enzyme recognizes a particular sequence at this point. As a result of the cut ('nick') a linear strand with 3' and 5' ends is created.

The 5' end of the broken strand becomes attached to the plasma membrane of the host bacterium. Such replicating phage DNA is commonly found associated with bacterial membranes. The unbroken parental strand rolls and unwinds as synthesis proceeds, leaving a 'tail' which is attached to the membrane.

New DNA is also synthesized in the tail region in discontinuous segments in the 5'à3' direction. This synthesis is presumably preceeded by the synthesis of an RNA primer under the catalytic action of RNA polymerase. The tail is cut off by a specific endonuclease into a unit length progeny rod.