The DNA molecule is chemically unstable, giving rise to DNA lesions of various kinds. That is why DNA damage detection, signaling and repair, collectively known as the DNA damage response, are needed. The DNA damage response is immensely important, for example, for ensuring the highest possible quality of DNA before replication prior to cell division. If the damaged DNA is replicated, the risk of cancer and other diseases increases significantly due to mutations. All in all this may lead to the death of a cell itself.
See more: http://phys.org/news/2015-03-dna.html