When DNA is copied, sometimes a new, incorrect base-pairing appears within the structure of a new DNA molecule replacing original base pairs. This mistake is called a base-pair mismatch due to replication error. This is known as DNA damage.
These kind of mismatch errors can be corrected by DAM (DNA adenine methylation) repair system. This repair system recognizes one of the two mismatched nucleotide to be corrected by methylation pattern. The non-methylated DNA strand in the hemi-methylated DNA progeny is recognized as the strand to be corrected by DAM repair system.