Lesion bypass polymerases are necessary where errors are present in the DNA strand. These polymerases (IV, V) have a more forgiving and flexible active site, allowing the polymerase to extend the primer strand over the error. Lesion bypass polymerases are necessary to allow DNA replication to continue to help the cell survive for longer. However, they have low accuracy and ‘fall off’ the template strand easily. This allows the normal DNA polymerase to take over, giving high fidelity replication.
Without trans-lesion polymerases, DNA replication would not be possible. Thymine dimers and other isomers would prevent replication, causing the cell to die. Given these structures can be formed by UV radiation, it would be almost certain that life would not exist on Earth without error-prone polymerases.