The Chemistry of Decay
DNA is a remarkably stable molecule under physiological conditions, but once removed from the protective environment of living cells, it begins an inexorable process of chemical degradation. The primary mechanisms are hydrolysis (cleavage of the sugar-phosphate backbone by water), oxidation (damage to purine and pyrimidine bases by reactive oxygen species), and enzymatic digestion by endogenous and microbial nucleases. Each mechanism has distinct kinetics and environmental dependencies.
Temperature and the Arrhenius Relationship
The rate of DNA degradation follows Arrhenius kinetics — the rate constant approximately doubles for every 10°C increase in temperature. At 37°C, the depurination rate alone produces roughly one lesion per 100,000 bases per day. At 4°C, this rate drops by roughly 30-fold. This explains why refrigerated and frozen samples preserve DNA far better than those stored at ambient temperature, and why permafrost specimens can yield DNA sequences after millennia.
Humidity, UV, and Environmental Factors
Water is essential for hydrolytic cleavage — desiccated samples degrade much more slowly than moist ones. UV radiation causes direct photochemical damage, creating cyclobutane pyrimidine dimers and strand breaks. The combination of heat, humidity, and sunlight creates the most destructive environment for DNA. Indoor crime scene samples typically degrade much more slowly than outdoor evidence exposed to the elements.
Forensic Implications: From Full to Partial Profiles
As DNA fragments shorten, larger PCR amplicons fail first — a predictable pattern exploited by the degradation index. Standard STR kits with amplicons up to 400 bp are sensitive to moderate degradation, while mini-STR kits with amplicons under 150 bp can recover information from heavily degraded samples. Understanding degradation kinetics helps forensic scientists set realistic expectations for DNA recovery and choose appropriate analytical strategies for compromised evidence.