Deoxyribonucleic acid, or DNA, is the genetic blueprint for all living organisms, containing instructions for development, functioning, growth, and reproduction. Obtaining DNA for analysis is important in various scientific fields, and saliva has become a widely used and convenient source. Extracting DNA from saliva involves several methodical steps to isolate this genetic material.
Why Saliva is a Preferred Source
Saliva offers several advantages for DNA extraction, making it a popular choice over invasive methods like blood draws. Its non-invasive nature allows for easy, painless collection, often performed at home without medical personnel. This convenience leads to higher participant acceptance in research studies and at-home genetic testing.
Saliva samples contain sufficient cellular material for DNA isolation, primarily from shed buccal epithelial cells and white blood cells. A significant portion of DNA in saliva, up to 74%, originates from white blood cells, providing high-quality genomic DNA comparable to blood. While some bacterial DNA may be present, commercial collection methods ensure the majority is human. This makes saliva widely used in at-home genetic testing, forensic investigations, and medical diagnostics.
The Saliva DNA Extraction Process
DNA extraction from a saliva sample involves a series of precise steps, starting with collection and preparation. Individuals should rinse their mouths with water and avoid eating or drinking for at least 30 minutes before providing a sample to minimize contamination. Saliva is collected by spitting into a sterile tube, often containing a stabilization buffer that preserves DNA and inhibits bacterial growth, allowing for transport and storage.
In the laboratory, the first step is cell lysis, where cells in the saliva sample are broken open to release their DNA. This is achieved by adding a lysis buffer, a solution with detergents and enzymes like proteinase K. Detergents disrupt cell and nuclear membranes, allowing DNA to escape. Proteinase K breaks down proteins, ensuring DNA is freely available.
After cell lysis, unwanted proteins and cellular debris are separated from the DNA. A high-concentration salt solution, such as sodium or ammonium acetate, is added, causing proteins and other components to clump and precipitate. Centrifugation, a rapid spinning process, separates these solid contaminants from the liquid containing the dissolved DNA. The DNA remains in the upper liquid layer, while heavier debris forms a pellet.
The next step is DNA precipitation, where DNA is separated from the liquid solution. Ice-cold alcohol, typically ethanol or isopropanol, is added to the DNA-containing liquid. DNA is soluble in water but insoluble in alcohol, causing it to aggregate and form a visible white, stringy precipitate. This precipitated DNA can then be spooled out or pelleted by centrifugation.
Finally, the extracted DNA undergoes washing and rehydration to remove residual contaminants and prepare it for analysis. The DNA pellet is washed with an alcohol solution. After washing, the alcohol is removed, and the DNA pellet air dries briefly. The purified DNA is then rehydrated by dissolving it in a small volume of a suitable buffer, such as TE buffer or nuclease-free water, which helps maintain its stability for storage and molecular applications.
Using Extracted DNA
Once DNA is extracted from a saliva sample, it can be used for a wide array of genetic analyses. Purified DNA is used for genotyping, which examines specific genetic variations across an individual’s genome. It is also suitable for DNA sequencing, a process that determines the precise order of nucleotides in a DNA molecule, providing comprehensive genetic information.
Polymerase Chain Reaction (PCR) is a common technique that amplifies specific regions of the extracted DNA, making it possible to study minute quantities. These methods enable diverse applications, including ancestry testing, health predisposition screening, and forensic identification. Ancestry testing reveals ethnic origins and connects individuals through genetic databases. Health screening identifies genetic markers for certain conditions. In forensics, DNA from saliva links individuals to crime scenes through profiling.