If DNA is very heavily damaged by UV light such that it contains many thymine dimers, then light and excision repair may not be able to correct all of the dimers and the cells will die. Therefore, bacteria have a mechanism, termed SOS repair, that is a last ditch mechanism to bypass the damaged DNA and allow replication to continue. The damaged DNA induces the SOS system, which comprises about 20 genes. One of the most important activated gene products is a new DNA polymerase that is able to bypass the damaged DNA. However, unlike the standard DNA-replicating machinery which is relatively error-free, but cannot copy past the lesion, this newly activated poly-merase makes many mistakes and incorporates the wrong bases in the DNA strand it is synthesizing. As a result, mutations arise.
Table 8.2 summarizes the key features of the major DNA repair systems in bacteria. Note that the cell cannot repair all types of mutations, such as insertion mutations caused by transposition.
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