DNA is present in all organisms, from the tiniest bacteria to the largest plants and animals. In fact, each nucleus in a human cell contains a quantity of DNA that would measure approximately two meters, if unwound. With a sample of purified DNA, scientists can test patients for genetic disease, establish criminal identity, and trace ancestral roots. But before DNA can be analyzed for these and other purposes, it must first be extracted from cell or tissue samples.
The interactive activity demonstrates the DNA extraction process using a plant leaf. Extracting DNA from human cells uses the same fundamental steps, but varies somewhat in the details. Because the epithelial tissue that lines the inside of our mouths loses thousands of cells every day, cheeks are a good and noninvasive source for human cells. After rubbing a swab against the inside surface of the cheek and placing the sampling end of the swab into a microfuge tube, a lab technician initiates extraction by adding lysis solution to the tube. This solution contains detergent, which breaks down cell membranes by disrupting the phospholipid bilayer, thus releasing the DNA. It also contains an enzyme that digests proteins. These proteins, called histones, bind the DNA.
The next step, incubating the solution in a hot water bath, weakens the cytoplasmic enzymes also released from the cell. These enzymes, which include deoxyribonuclease, would otherwise break down the DNA being isolated. Adding a salt solution to the tube and placing the tube in a centrifuge separates the DNA from the cellular debris. DNA dissolves in ionic solutions while fats, carbohydrates, and many proteins do not. Centrifugation separates the DNA from these other components, which sink to the bottom of the liquid.
The next step involves carefully adding some isopropyl alcohol or ethanol to the tube. Since DNA will not dissolve in these alcohols, millions of strands will collect into a clump when centrifugation is complete. Because DNA is less dense than what remains in the solution, the DNA clump floats to the top of the alcohol layer. It appears to the naked eye like a clumped, string-like substance. Note: The double helix shape will not be visible—it's too small to be seen with the naked eye.
Extraction procedures will result in one of the following: DNA that appears as thin strands; DNA that appears fluffy, which indicates that it has sheared in the extraction process; or no DNA at all. Poor results most commonly occur when the alcohol is added too quickly to the tube and the layers of liquid mix together rather than forming two distinct layers, or if the sample tube is shaken too violently in the mixing process. DNA is a relatively sturdy molecule, but its length leaves it prone to breaking outside of its protective nucleus. If the DNA is broken into shorter fragments, it cannot be spooled onto a glass rod or pipette tip and removed from the solution.