Chemical modification
During interstrand cross-linking, two strands of the DNA molecule become covalently linked preventing full separation of DNA strands using heat. This can be facilitated by UV-A light and the presence of intercalating agents. Interstrand cross-linking makes DNA inaccessible to PCR detection, but this is not degradation per se. We mention this reaction because at this time we cannot differentiate non-detection resulting from true degradation versus interstrand cross-linking using PCR.