Deoxyribonucleic acid, or DNA, serves as the genetic instruction set for all living organisms, from the smallest bacterium to the largest whale. This molecule dictates the development, functioning, and reproduction of every cell. While often viewed as something confined to advanced laboratories, DNA can be easily isolated and observed from common fruit using simple household items, offering a direct look at the molecular foundation of biology.
Gathering Your Materials
The experiment requires fruit, equipment, and chemical agents. Strawberries are an excellent choice because they are octoploid, meaning they contain eight sets of chromosomes, which dramatically increases the amount of visible DNA extracted. For the physical process, you will need a resealable plastic bag for mashing, two small clear cups, a funnel, and a coffee filter or cheesecloth for straining.
The necessary chemical agents are standard kitchen staples, including liquid dish soap, table salt, and cold rubbing alcohol (isopropyl or ethanol). The alcohol must be kept chilled in a freezer for several hours, as low temperatures are necessary for the precipitation stage. Mixing the soap, salt, and water creates the extraction solution needed to free the DNA from the cells.
Step-by-Step DNA Extraction
The first step involves mashing the fruit inside the resealable bag for about two minutes, which mechanically breaks apart the tough cell walls. Once the fruit is pulped, add the prepared extraction solution containing the soap and salt mixture to the bag. Gently knead the mixture for another minute to ensure the solution penetrates the fruit pulp.
The resulting slurry must then be filtered to separate the large fruit pieces from the dissolved cellular contents, including the DNA. Place a coffee filter or cheesecloth inside a funnel and pour the fruit mixture into a clean cup, allowing the liquid filtrate to slowly drip through. Avoid squeezing the filter too forcefully, as this can push unwanted debris into the liquid, contaminating the final DNA sample.
Finally, add the ice-cold rubbing alcohol to the filtered liquid. Pour an amount of cold alcohol roughly equal to the volume of the filtrate down the side of the cup to create a distinct top layer. The alcohol will float on top, and within a few minutes, white, cloudy strands will begin to appear at the boundary between the two layers.
The Chemistry Behind the Process
The mechanical action of mashing breaks down the rigid cellulose cell walls of the plant tissue. Following this, the dish detergent in the extraction solution initiates lysis. Detergents disrupt lipid bilayers by dissolving the lipid molecules that compose the cell and nuclear membranes, effectively rupturing them and releasing the cellular contents, including the DNA.
The salt added to the extraction buffer neutralizes the DNA molecule. DNA has a highly negative charge due to the phosphate groups along its sugar-phosphate backbone, allowing it to dissolve easily in water. When salt is added, the positively charged sodium ions bind to the negative phosphate groups, neutralizing the overall charge. This neutralization causes the DNA strands to clump together and become less hydrophilic, preparing them for the final stage.
The addition of cold alcohol causes the DNA to precipitate, making it visible. DNA is insoluble in alcohol, and the neutralized DNA is forced out of the liquid suspension when introduced to the less polar alcohol solution. Using ice-cold alcohol further reduces the solubility of the DNA, maximizing the amount that collects as a visible mass where the two liquid layers meet.
What to Expect from Your Experiment
The isolated DNA appears as a white, translucent, cloudy, or mucous-like substance accumulating at the interface between the alcohol layer and the fruit extract. This visible material consists of millions of clumped DNA molecules tangled together with some residual protein. The appearance is often compared to wet cotton wool, confirming the success of the extraction.
Once enough material has collected, you can gently spool the DNA onto a wooden stick or toothpick. Slowly rotate the stick at the interface of the two liquids, and the long, sticky DNA strands will adhere to the wood, allowing you to lift them out. If no precipitation occurs, ensure the rubbing alcohol was sufficiently cold or that the fruit was thoroughly mashed to break open the cell walls.