a) Termination of Replication of a circle:
There are two possible modes of replication:
- There are defined termination sequences, or
- Two growing points collide and termination occurs whenever the collision point happens to be.
termination in circular DNA
In both cases, termination might occur exactly halfway around the circle. Both termination modes have been observed. Termination has a “Topological problem”. When double – stranded circular DNA replicates semiconservatively, the result is a pair of circles that are linked as in a chain. Such a structure is called “CATENANE”. Catenated molecules have been observed in numerous systems, and evidence is accumlating to indicate that they result from replication. “DNA gyrase” is capable of decatenating two circles, which enzyme is responsible for separation of daughter molecules.
E.coli chromosome and several plasmids carry specific sequences, called “ter sites”, where TBP ( ter binding protein) or ‘Tus protein’ binds. In the termination zone of E.coli, there are three ter sites (ter A, ter D and terE) for counter-clockwise fork. These six sites are arranged in overlapping manner, laving no “Replication – free” gap on the chromosome. TBP-ter complexes formed at “ter” sites stalls the replication fork, by inhibiting the DNA helicase or DnaB. When this termination zone is deleted, replication stops, simply by the meeting of opposite replication forks, suggesting that the termination zone is not essential.
b) Termination in ‘Linear’ DNA molecules:
E.Coli phage T7 DNA replicates as a linear molecule. The origin is located 17% of the total distance from the left end of the molecule and replication is bidirectional. Initially there is a single replication bubble (molecule-I), and when the leftward fork reaches the terminus, the molecules assumes a “Y”-form (molecule-III).
termination in linear DNA
“Ribonucleases” exist that can remove this RNA but once it has been removed, either a 5’-OH group would remain at the ends of the molecule.