Our bodies are intricate systems, and even parts that seem simple, like hair, hold complex biological information. Within each strand of human hair lies deoxyribonucleic acid, or DNA, the fundamental blueprint of life. This genetic material, present even in seemingly inert structures, can be retrieved and analyzed to reveal a wealth of information about an individual.
Where is the DNA in Your Hair?
Hair consists of two main parts: the follicle, which is embedded in the skin, and the shaft, the visible part that extends outwards. The hair follicle, often called the root, is a rich source of nuclear DNA (nDNA) because it contains living cells with intact nuclei. Nuclear DNA is inherited from both parents and is unique to almost every individual, making it highly valuable for identification purposes.
In contrast, the hair shaft is primarily composed of keratin, a protein, and does not contain intact cell nuclei. However, the hair shaft does contain mitochondrial DNA (mtDNA). Mitochondria are small organelles within cells responsible for energy production, and each mitochondrion contains its own small, circular DNA molecule. Mitochondrial DNA is inherited solely from the mother, meaning all individuals in the same maternal lineage will share the same mtDNA profile. While less individualizing than nuclear DNA, mtDNA is present in hundreds to thousands of copies per cell, making it more abundant and more resistant to degradation than nuclear DNA, especially in older or degraded samples.
Unlocking the Genetic Code: The Extraction Process
Retrieving DNA from hair involves several laboratory steps to isolate the genetic material from other cellular components and the hair’s tough protein structure. The initial step is careful sample collection and preparation, which involves handling the hair to avoid contamination and cleaning it to remove external debris. The hair is typically cut into small fragments (0.5 cm to 1 cm) to increase the surface area for subsequent processing.
The next stage is lysis, where cells are broken open to release the DNA. This is achieved by incubating the hair sample in a solution containing enzymes and detergents. Proteinase K, an enzyme, is used to digest proteins, including keratin, and break down cell membranes. Detergents like SDS (sodium dodecyl sulfate) further assist in disrupting cell membranes and denaturing proteins. This digestion process occurs at an elevated temperature (56°C) and can take from one hour to overnight.
After lysis, the DNA must be purified from digested cellular debris, proteins, and other contaminants. Purification methods include organic extraction, which separates DNA from proteins, or solid-phase extraction using silica-based columns or magnetic beads. In silica-based methods, DNA binds to a silica membrane in the presence of salts and alcohol. Contaminants are then washed away, and the purified DNA is eluted from the membrane using a low-salt buffer or water. Magnetic bead systems also bind DNA reversibly, allowing for easy separation from impurities using a magnet.
The final step is concentration and elution, where the purified DNA is collected in a smaller volume of solution, making it suitable for downstream analysis. This might involve precipitation with alcohol, such as ethanol or isopropanol, followed by centrifugation to pellet the DNA, which is then redissolved in a buffer. The concentration and purity of the extracted DNA are then measured using spectrophotometry to ensure it meets the requirements for further genetic testing.
Why Hair DNA Matters: Key Applications
The ability to extract DNA from hair has opened various avenues for its application, particularly in forensic science and ancestry research. In forensic investigations, hair samples are found at crime scenes and can be a valuable source of genetic evidence. When a hair with an intact follicle is recovered, nuclear DNA can be extracted and used to generate a DNA profile that can identify an individual with high confidence, linking them to a crime scene or a victim.
Even hair shafts, which yield limited nuclear DNA, are a source of mitochondrial DNA, making them useful in cases where nuclear DNA is absent or degraded. While mitochondrial DNA cannot uniquely identify an individual (as all maternal relatives share the same mtDNA profile), it can be used to exclude suspects or trace maternal lineages, providing investigative leads. Advancements in DNA extraction techniques are also improving the recovery of nuclear DNA from rootless hair shafts, increasing their utility in forensic contexts.
Beyond forensics, mitochondrial DNA from hair can be used in ancestry and genealogical research. By analyzing the mtDNA sequence, individuals can trace their maternal lineage back through generations, connecting them to ancestral groups and geographical regions. However, obtaining high-quality nuclear DNA from hair, especially from shed hair shafts, remains challenging. Hair DNA can be degraded, and the amount of DNA present can be low, which can limit its use in some applications compared to samples like blood or saliva.