We have described some of the features of bacterial mismatch repair, and apologize for other reported features that we may have omitted. There are at least two kinds: Long-patch repair that plays a major role in the maintenance of fidelity and in the avoidance of genetic exchange between related sequences that have experienced substantial sequence divergence, and short-patch repair that also plays a role in the maintenance of fidelity; these include the MutY system for its action in avoiding sequence changes introduced by the oxidation of guanine. These are prominently C to A transversions. The other repair system acts to avoid sequence errors that could be introduced by the spontaneous deamination of postreplicatively methylated cytosine, thus avoiding T to C transitions.
This array of observations provide a glimpse of the processes that are active in the preservation of fidelity during the DNA replication process that is required for the preservation of the desirable sequences that have evolved.