Our genomes possess an intrinsic level of instability, resulting from the misincorporation of RNA, the chemical sister of DNA. ~ English biochemist Keith Caldecott
While DNA is the universal storage medium for genetic information, cells are awash in RNA, as RNA is employed in decoding the instructions locked away in DNA. There is an operational tension between the two. RNA may mistakenly be incorporated during DNA synthesis. So, there are several quality control and repair processes that ensure DNA integrity.
The notion that the purity of DNA is an intrinsic property of its synthesis is wrong. Effort is required by cells to ensure that genetic material retains its DNA identity. ~ Keith Caldecott