DNA evidence has become a critical tool for law enforcement investigations and criminal prosecutions. However, DNA evidence can be inadvertently contaminated or destroyed through improper collection, handling, and storage methods. Understanding how to preserve DNA evidence properly is essential for maintaining its integrity for testing.
What is DNA evidence?
DNA, or deoxyribonucleic acid, contains the genetic code unique to each individual. Forensic scientists can collect DNA from blood, semen, skin cells, saliva, and other bodily fluids or tissues found at a crime scene and develop a DNA profile. If this evidence DNA profile matches a suspect or sample in a criminal DNA database, it can link the suspect to the crime scene.
Some common sources of DNA evidence collected from crime scenes include:
- Blood stains
- Saliva on cigarette butts, envelopes, or bite marks
- Semen from sexual assaults
- Skin cells from touched surfaces
- Hair follicles or roots
Because of its uniqueness and ability to identify individuals with near certainty, DNA has become the gold standard of forensic evidence when connected to a crime scene or victim. However, mishandling or destruction can easily render DNA useless.
Why can DNA evidence be destroyed?
DNA begins to break down immediately after cells die. Environmental factors that cause DNA degradation include:
- Heat
- Moisture
- Bacteria
- UV radiation
- Chemicals
Crime scene DNA is also vulnerable to contamination from improper collection and storage methods. Examples include:
- Allowing different DNA samples to mix together
- Using contaminated tools or equipment to collect samples
- Degrader enzymes in bacteria mixed with samples
- Accidental introduction of unrelated DNA from handlers
Without careful handling procedures, crime scene DNA can become too degraded or contaminated to analyze accurately.
How does heat destroy DNA?
Heat causes DNA strands to break apart over time. The rate of DNA degradation from heat follows basic chemical kinetics – high temperatures accelerate the decomposition while colder temperatures preserve DNA integrity for longer.
Studies have quantified DNA degradation rates at various temperatures:
Temperature | Degradation Rate |
---|---|
Room temperature (25°C) | Measurable degradation in 1 week |
37°C (body temperature) | 50% degradation in 24 hours |
56°C | 50% degradation in 90 minutes |
95°C | 50% degradation in 30 minutes |
As shown, high temperatures rapidly accelerate DNA breakdown. Exposing evidence to heat sources – whether intentional or accidental – can quickly render DNA useless.
Examples of heat damage
Types of heat exposure that may destroy DNA evidence include:
- Leaving samples in a hot vehicle or unairconditioned storage
- Extended exposure to sun or heat in outside crime scenes
- Fire damage from arson investigations
- Attempted cleanup with hot water
- Baking or burning evidence in ovens or fires
Without proper temperature controls, heat can quickly outweigh other preservation efforts and degrade DNA before analysis.
How does moisture destroy DNA?
Water enables many of the chemical reactions that break DNA strands apart. In very dry conditions, DNA can persist intact for thousands of years. But moisture accelerates degradation processes through:
- Hydrolysis: water molecules directly cleaving DNA strands
- Bacterial growth: DNA-digesting enzymes in microbes proliferate with moisture
- Mold growth: fungi release DNA-damaging waste products
Moist or humid evidence collection and storage environments provide ideal conditions for each of these moisture-enabled breakdown mechanisms. Preventing excess moisture exposure is key to preserving DNA evidence.
Examples of moisture damage
Types of moisture exposure that may destroy DNA evidence include:
- Rainfall on outside crime scenes
- Attempted cleanup of bloody evidence with wet rags or chemicals
- Submerging evidence in water or liquids
- Leaving evidence in non-climate controlled conditions
- Long-term storage in paper or porous materials
Like heat, moisture damage can happen quickly and outweigh other preservation methods. Keeping evidence dry avoids creating ideal conditions for hydrolysis, bacteria, and mold.
How do chemicals destroy DNA?
Many chemicals are powerful solvents or reactants that break DNA apart through:
- Denaturation: altering DNA structure so it no longer functions
- Fragmentation: breaking DNA into smaller pieces
- Nucleic acid hydrolysis: severing bonds in DNA molecules
Strong acids and bases are especially damaging. But even soaps and detergents can dissolve lipids in cell membranes or disrupt protein channels that regulate DNA access.
Examples of chemical damage
Types of chemical exposures that may destroy DNA evidence include:
- Strong acids or bases like sulfuric acid, sodium hydroxide
- Bleach, peroxide, detergents
- Gasoline, oils, or other solvents
- Pesticides, herbicides, cleaning products
- Tobacco smoke residues
Many chemicals are incorporated into attempted clean-up efforts by perpetrators. But even incidental exposures during evidence collection or storage can damage DNA.
How does UV light destroy DNA?
The ultraviolet (UV) light in sunlight can directly damage DNA molecules through:
- Formation of pyrimidine dimers: Thymine bases on the same DNA strand bond together, distorting the double helix structure.
- Generation of free radicals: UV light excites electrons in DNA bases, creating highly reactive free radical molecules that damage DNA.
As little as an hour of strong sunlight each day can substantially degrade exposed DNA evidence over time.
Examples of UV light damage
Types of UV exposure that may destroy DNA evidence include:
- Sunlight exposure in outside crime scenes
- Fluorescent or halogen lights during evidence processing
- Photocopiers during documentation
- Prolonged storage near windows or lights
UV damage accumulates over time with repeated exposure. Controlling light exposure prevents free radical reactions that degrade DNA.
How does microbial growth destroy DNA?
Bacteria, molds, yeasts, and other microorganisms release DNA-digesting enzymes as waste products or when they die and burst open. Warm, moist environments accelerate microbial growth and DNA damaging secretions. Types of microbial damage include:
- Extracellular nucleases: DNA-cutting enzymes released by bacteria including Serratia marcescens.
- Aflatoxins: Toxins released by fungal growth in stored grain and legumes.
- Microbial overgrowth: Complete digestion of evidence when microbes exhaust available nutrients.
Preventing conditions favorable to microbial growth is an essential step to save DNA evidence.
Examples promoting microbial damage
Actions that encourage microbial growth and DNA destruction include:
- Prolonged storage of wet or moist evidence
- Keeping organic evidence like blood or semen at room temperature
- Using contaminated tools or collection containers
- Exposing evidence to dust or soils
Once microbes colonize evidence, they can rapidly damage DNA through active digestion and waste enzyme secretions.
How does contamination destroy DNA?
DNA evidence exposed to unrelated DNA sources becomes contaminated. Contamination introduces DNA from other sources, confusing crime scene profiles. Types of contamination include:
- Evidence mix-ups: Collecting multiple evidence items into a single container.
- Handler DNA: Shedding skin cells or hair onto evidence during collection and processing.
- Shared tools: Collecting samples with equipment containing other DNA.
Meticulous evidence-handling avoids cross-contamination. But any secondary DNA devalues evidence samples.
Examples leading to contamination
Actions promoting DNA contamination include:
- Coughing, talking, or sneezing near evidence
- Using unsterilized tools or containers
- Neglecting protective clothing like gloves and masks
- Mixing evidence items into batches
Even tiny amounts of unrelated DNA can render crime scene samples unusable. Preventing contamination is vital.
How can DNA evidence be preserved?
Despite many threats, DNA evidence can be successfully preserved using careful collection and storage protocols including:
- Wearing protective clothing like gowns, gloves, and masks
- Meticulous separation of each piece of evidence
- Using sterilized collection tools and containers
- Securely sealing evidence with tamper-proof tape
- Removing moisture with drying agents or climate control
- Rapid freezing for long-term storage
Forensic investigators take great care to maximize DNA evidence stability and prevent contamination or degradation prior to analysis.
Ideal DNA evidence handling
Proper DNA evidence preservation follows these guidelines:
- Handle as little as possible while wearing gloves and masks
- Use disposable tools and containers for each item
- Air-dry paper evidence like clothing or cigarette butts
- Store each item separately in breathable packaging
- Keep evidence in dark, cool, and dry environment
DNA evidence correctly safeguarded after collection retains integrity for forensic analysis and courtroom use.
Conclusion
DNA’s power to identify individuals means mishandling easily destroys its value. Heat, moisture, light, chemicals, and contamination all degrade fragile DNA molecules. But evidence carefully collected and stored can yield viable DNA profiles, even decades later. Following strict protocols for sealing packaging, controlling temperature, humidity, and light gives investigators the best chance of preserving DNA through crime lab analysis and trials.