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Post Replicative Repair -When DNA polymerase encounters damage in DNA, it cannot proceed. Instead it gives a gap for replication and proceeds up to 800 bp without replicating. Then again it starts replicating after synthesizing a primer by primosome. These gaps are then repaired by using one of the two mechanisms. Originally several proteins were known to facilitate the replication of DNA with lesions.

They were believed to interact with the polymerase to make it capable of using damaged DNA as a template. In E. coli, polymerase I can copy damaged DNA. Pol V is error free and can incorporate' A' opposite to thymine dimers. But sometimes, Pol V does errors for unknown reasons, especially during stress.

One possible reason for this is that the error prone polymerase may have developed by evolutionary processes. They create mutations at a time when the cell might need variability. In the second mechanism, a replication fork creates two DNA duplexes.

Thus an undamaged copy of the region with the lesion exists on the other daughter duplex. The repair takes place at a gap created by the failure of DNA replication, the process is called post-replicative repair. The RecA protein has two major properties.

First, it coats single stranded DNA and causes the coated single stranded DNA to invade double stranded DNA while displacing the other strand of that double helix. RecA continues to move the single stranded DNA along the double stranded DNA until a region of homology is found.

The RecA protein is responsible for filling a post replicative gap in newly replicated DNA with a strand from the undamaged sister duplex. The gap filling process then completes both strands. The RecA protein is responsible for the damaged single strand invading the sister duplex.

Endonuclease activity then frees the double helix containing the thymine dimer. DNA polymerase I and DNA ligase return both daughter helices to the intact state. The thymine dimer still exists, but now its duplex is intact. Another cell cycle is available for photo reactivation or excision repair to remove the dimer.

 
   
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