Deoxyribonucleic acid, or DNA, is the molecule carrying the genetic instructions for all known organisms. DNA preservation is the process of protecting these molecules from damage and decay to maintain their original structure and integrity over long periods. This process allows the genetic material to be available for future analysis by using techniques designed to slow natural degradation. By stabilizing the molecule, its information is safeguarded for numerous scientific, medical, and social applications.
The Importance of Preserving DNA
In genetic research, preserved samples are used to explore inherited traits and understand evolutionary relationships. By comparing DNA from diverse populations, scientists can track genetic diversity, identify mutations, and uncover the genetic underpinnings of biological functions. This work is foundational to the study of genomics.
In medicine, preserved DNA is used to diagnose genetic disorders and predict disease risk. It allows physicians to analyze a patient’s genetic makeup to tailor treatments, a practice known as personalized medicine. Banking a person’s DNA also secures their genetic information for future therapeutic applications for themselves or their descendants.
Forensic science uses DNA from crime scenes to identify suspects, link individuals to a location, or identify human remains. The preservation of this evidence is necessary for its use in legal proceedings to help solve crimes and exonerate the wrongly accused. Methods used must ensure the sample is not degraded, which could compromise the analysis.
DNA preservation is also a component of global biodiversity and conservation. Scientists create genetic banks for endangered species, storing their DNA as a safeguard against extinction. These archives can be used to study population genetics, guide breeding programs, and potentially enable the future revival of species. This provides a genetic record of biodiversity for future study.
Techniques for DNA Preservation
One common approach is freezing samples at very low temperatures. Standard laboratory freezers operate at -20°C, while ultra-low temperature freezers maintain -80°C. For long-term storage, samples are placed in liquid nitrogen at -196°C. These temperatures slow the enzymatic and chemical reactions that cause DNA to break down.
Drying is another technique that removes water from a sample, which is necessary for reactions that degrade DNA. Lyophilization, or freeze-drying, involves freezing the sample and placing it under a vacuum to sublimate the ice into vapor. A simpler method uses desiccants, like silica gel, to absorb moisture. This approach is useful for collecting samples in field conditions.
Chemical methods offer an alternative for DNA storage at ambient temperatures. Special paper matrices, like FTA cards, are treated with chemicals that lyse cells, denature proteins, and trap the DNA. Other proprietary solutions stabilize DNA in a liquid or dried format without refrigeration. Ethanol can also precipitate DNA from a solution for storage as a pellet or in an ethanol suspension at low temperatures.
The choice of preservation technique depends on the sample type, intended applications, and logistical constraints. For instance, FTA cards are suited for collecting blood or saliva in remote locations, while ultra-low temperature freezing is standard for high-value samples in a lab. Each method aims to create a stable environment for the DNA.
Sample Types and Long-Term Storage Solutions
Biological materials like blood, saliva, and tissue can serve as sources for DNA preservation. Blood provides DNA from white blood cells, while saliva contains cells from the cheek. Tissue samples are also rich sources of DNA, and buccal swabs are used for less invasive collection from the mouth.
Other sample types are valuable in specific contexts:
- Hair follicles, if they include the root, contain cellular material for DNA extraction.
- Bone and teeth are important in archaeological or forensic cases, as their structure can preserve DNA for centuries.
- Semen is a direct source of genetic material for reproductive or genetic studies.
- Plant tissues and seeds are preserved to safeguard genetic diversity in botany and agriculture.
Proper collection is a foundational step for long-term preservation. It involves using sterile containers and minimizing contamination from external DNA. How the sample is handled immediately after collection impacts the quality and yield of the preserved DNA.
For long-term storage, samples are housed in controlled facilities called biobanks. These are organized libraries of specimens with infrastructure like automated freezer systems and tracking databases. Biobanks maintain stable environmental conditions, like constant temperature, to ensure sample integrity. Organization, clear labeling, and inventory management are fundamental to their operation.
Maintaining DNA Integrity Over Time
Maintaining DNA integrity is challenging because it is susceptible to degradation. One form is enzymatic degradation, caused by nucleases that cut DNA strands into fragments. These enzymes are present in cells and can become active if a sample is not handled or preserved properly.
Chemical degradation also poses a risk. Hydrolysis, a reaction involving water, can break bonds in the DNA backbone or cleave off nucleotide bases. Oxidation results from damage by reactive oxygen species, which can alter the chemical structure of the bases and lead to mutations or strand breaks.
Physical factors can also cause damage. Exposure to ultraviolet (UV) radiation can distort the double helix. Physical shear forces during vigorous mixing can tear long DNA strands apart. Repeated freeze-thaw cycles are also damaging, as ice crystal formation can fracture DNA molecules.
To ensure the quality of preserved DNA, regular quality control checks are performed. These checks include:
- Quantifying the amount of DNA present using spectrophotometry or fluorometry.
- Assessing sample purity by measuring absorbance ratios to detect contaminants like protein.
- Evaluating structural integrity using techniques like gel electrophoresis to reveal degradation.
- Implementing careful handling protocols, such as using nuclease-free labware and aliquotting samples to avoid freeze-thaw cycles.