Your question is ambiguous and could refer to any of the following:
- Prokaryotes employ proteins that recognise termination sites, including the 'tus' protein. These bind and act as one way gates, so that termination occurs in a predefined location. The exact mechanism of termination is unknown, but is presumed to be a simple meeting of two replication forks causing the apparatus to stop and dissociate. Replication is successful without the tus protein.
- In eukaryotes, termination of replication is poorly understood.
- Eukaryotes have linear DNA, and as such cannot replicate a short region on the end of each DNA molecule on the lagging strand, since replication requires RNA primers, and there will be nowhere for the primer to bind (it is later degraded so cannot be kept). Eukaryotes therefore use telomeress, which are GT rich repeating units that 'protect' the end of the DNA and can be placed without the use of a templace (the telomerase enzyme itself has an RNA template within it). The sequence of the telomere is species dependent. There will always be an overhang on the telomere, where telomerase added bases that could not be replicated on the other strand (as there is still nowhere to put the primer).