The stability of deoxyribonucleic acid (DNA) once it leaves a living organism is a variable determined by its surrounding environment. Exogenous DNA, which includes genetic material shed from skin cells, hair, saliva, or blood, begins to degrade immediately upon release. How long it lasts depends on whether the goal is to detect any remaining fragment or to obtain a complete, usable genetic profile. DNA longevity can range from mere hours under harsh conditions to millions of years in protective natural environments. Survival is governed by the rate at which the molecular structure breaks down, a process heavily influenced by external physical and biological forces.
How DNA Molecules Degrade
Once a cell dies and releases its genetic material, the protective mechanisms that maintain its integrity cease, and chemical breakdown begins. One primary form of damage is hydrolysis, where water molecules react with the DNA structure, causing the cleavage of chemical bonds. This process attacks the glycosidic bonds connecting nucleobases to the sugar-phosphate backbone, leading to the loss of bases like adenine and guanine in a process called depurination.
The DNA backbone is also subject to hydrolytic rupture. While phosphodiester bonds are relatively stable, the loss of bases causes subsequent breaks in the sugar-phosphate structure, resulting in DNA fragmentation. Another major chemical threat is oxidation, which involves reactive oxygen species modifying the nucleobases, such as converting cytosine into uracil.
Biological agents also play a significant role in degradation through enzymatic activity, especially in the minutes and hours following the DNA’s release. Nucleases are enzymes that cleave the phosphodiester bonds, quickly breaking down the long DNA strands into shorter fragments. These enzymes can be released from the dead cells or come from environmental sources, such as bacteria and fungi that use the genetic material as a nutrient source.
External Conditions That Accelerate Breakdown
The speed of degradation mechanisms is highly dependent on the external physical and biological conditions surrounding the DNA sample. Temperature is a major factor because chemical reactions, including hydrolysis and oxidation, are accelerated by heat. Low temperatures, such as those found in frozen environments, dramatically slow down molecular motion and chemical activity, offering significant protection.
Moisture and humidity are also destructive because water is necessary for the hydrolysis reaction to occur. Dry environments are far more protective, as the lack of water prevents the chemical cleavage of DNA bonds. Conversely, damp environments promote the growth of microorganisms, which release nucleases that actively digest the genetic material.
Ultraviolet (UV) radiation from direct sunlight is particularly destructive, causing chemical changes like cross-linking and breaks in the DNA strands. Exposure to UV light can degrade DNA so severely that it becomes unusable for forensic analysis after a short period.
Microbial activity is another significant threat, as bacteria and fungi view the shed DNA as a source of carbon and phosphorus. These organisms secrete extracellular nucleases to break down the genetic material, rapidly reducing its concentration. This biological degradation is especially effective in environmental water, where DNA concentration can drop by over 90% in the first 48 hours.
Survival Timelines in Different Settings
The practical longevity of DNA is often measured by the time it remains amplifiable, meaning the genetic fragments are intact enough to be analyzed for a profile. In short-term scenarios, such as forensic surfaces, the timeline is highly variable. DNA left on dry, clean surfaces, often called “touch DNA,” can persist for hours to several weeks, but its usability depends heavily on the initial amount deposited and environmental exposure.
On metal surfaces kept indoors, touch DNA can remain usable for up to nine months, but if exposed to outdoor conditions, this timeline may drop to about six months. In contrast, a dried biological sample like blood or saliva kept in a cool, dark place can preserve DNA for years or even decades with minimal degradation. However, if the sample becomes damp or is exposed to sunlight, the DNA can be destroyed in a matter of days.
Environmental samples represent a medium-term survival scenario, often lasting months to decades. DNA in soil and sediments persists longer than in water, partly due to the protective effect of binding to mineral particles, which shields it from nucleases and chemical damage. Human DNA released into environmental water is rapidly degraded, but mitochondrial DNA may still be detectable for up to 11 days under laboratory conditions.
The longest-term survival occurs in ancient and fossilized samples under highly specific, protective conditions. Permafrost, or permanently frozen ground, is the most effective natural preservative, slowing degradation to a near halt. Usable ancient DNA has been recovered from permafrost sediment dating back as far as two million years. In bone and teeth, the DNA is protected by the dense mineral matrix, allowing it to survive for thousands of years in dry caves.