When damaged bases are found in the DNA, the fidelity of DNA replication is reduced as high-fidelity polymerases (such as DNA polymerase III) are unable to translocate across these due to their tight active sites. Trans-lesion polymerases have a more flexible structure, allowing them to replicate over these damaged bases, but are not as accurate – due to their more forgiving structure.
Trans-lesion polymerases are necessary to allow DNA replication to continue, giving a greater chance of survival to the cell. Where errors are incorporated, DNA repair pathways can be activated to remove the mutated section and copy the correct form across.