In prokaryotes like bacteria such a mechanism is lacking. Jacob and Brenner have proposed a model to explain the replication and separation of daughter chromosomes.

According to this model, the chromosome gets attached to the mesosome at the point of origin of replication.

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After the completion of replication, a new membrane attachment site is formed adjacent to the old one. The strand which has acted as a template remains a circle, while the newly formed one is broken.

One of the free ends of the newly formed chromosome attaches to the new binding site. During replication, the replicating fork and the enzyme system remains stationary while the DNA molecule moves along and gets replicated.

Simultaneously a new membrane is synthesized between the two and the two genomes get separated. The newly synthesized DNA becomes circular eventually.

2. Carin’s Model:

At a specific point (origin) the DNA breaks and replication begins and proceeds in two opposite directions. As the replication proceeds, the DNA strand unwinds.

This unilateral unwinding (the other cannot unwind freely) creates a pressure and it is transferred to the un replicated part resulting in super twisting. This results in the stoppage of replication.

To begin replication once again a ‘nick’ is made in one of the strands by a swiveling protein. This facilitates rotation of the strands and the break is closed subsequently.

3. Yoshikawa Model:

This is a modification of the Cairn’s model. According to Yoshikawa, the newly formed DNA strands covalently join to the ends of original chromosome.

4. Rolling Circle Model:

This is the model accepted for the replication of single stranded DNA in Viruses ( x 174). The single stranded circular DNA becomes double stranded by the synthesis of another strand starting at the origin.

One strand of this duplex ring is now cut at a point by an endonuclease. As a result of this, a linear strand with 3? and 5? end is created. The 3? end serves as a primer for the synthesis of a new DNA strand.

The unbroken strand is used for this purpose; with the parental strand itself acting as a template. The 5? end of the strand becomes attached to the membrane.

Meanwhile the original, unbroken single strand unwinds and separates leaving the tail attached to the membrane. The tail is cut off into a specific length – Progeny rod. The rod rolls and forms a circular molecule, with the enzyme ligase binding the two ends.

The newly formed circular molecule which is double stranded can in turn become rolling circles. The genetic information is preserved in the single stranded template which remains circular always.

New DNA molecules may be formed either from the single stranded original molecule or from the double stranded rolling circles. Evidence for the rolling circle model has been obtained from the replication of several viruses (M13, P2 T4, X etc).

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