Deoxyribonucleic acid, commonly known as DNA, serves as the fundamental instruction manual for all living organisms, carrying the genetic information that dictates their development, function, and reproduction. While often perceived as something only visible under powerful microscopes or in specialized laboratories, it is possible to extract and observe DNA using common household materials. This accessible experiment offers a tangible way to explore the building blocks of life.
Why Strawberries are Ideal
Strawberries are particularly well-suited for at-home DNA extraction due to several unique biological characteristics. Unlike many other organisms, cultivated strawberries are octoploid, meaning their cells contain eight sets of chromosomes, providing a significantly larger quantity of DNA per cell compared to organisms with fewer chromosome sets. This abundance of genetic material makes the extracted DNA more easily visible to the naked eye.
The physical structure of strawberries also contributes to their suitability for this experiment. Their soft, fleshy texture allows for easy mashing and pulverization, which helps to mechanically break down the plant’s cell walls and release the cellular contents. Furthermore, strawberries naturally contain enzymes, such as pectinases and cellulases, which assist in the breakdown of cell walls, further facilitating the release of DNA during the extraction process.
Gathering Your Materials
Collecting the necessary items for this experiment is straightforward, as most are readily available in a typical home. You will need fresh or frozen strawberries, a resealable plastic bag, and a few common kitchen ingredients: dish soap, table salt, and ice-cold rubbing alcohol (isopropyl alcohol). Essential equipment includes two small cups or test tubes, a coffee filter or a piece of cheesecloth, and measuring spoons or cups.
Each component plays a specific role in the extraction process. The dish soap helps break down cell membranes, releasing DNA. Salt helps DNA molecules clump together.
The DNA Extraction Process: Step-by-Step Guide
Begin the DNA extraction process by preparing the strawberries. Remove any green leaves or stems from two to three medium-sized strawberries and place them inside a resealable plastic bag. Carefully remove as much air as possible from the bag before sealing it tightly. Gently, but thoroughly, mash the strawberries for about two minutes to break open the cells.
Next, prepare the extraction solution in a separate cup. Combine approximately two teaspoons of liquid dish soap, one teaspoon of table salt, and a half cup of water, stirring gently until the salt is dissolved. Avoid creating excessive suds, as too many bubbles can hinder the process. Pour about two tablespoons of this extraction solution into the plastic bag containing the mashed strawberries.
Reseal the bag and gently massage the mixture for another minute, ensuring the solution is well combined with the strawberry pulp. This step allows the soap to interact with the cell membranes. After mixing, set up your filtration system by placing a coffee filter or a double layer of cheesecloth over the opening of a clean, empty cup, securing it with a rubber band.
Carefully pour the strawberry mixture from the bag into the filter, allowing the liquid to drip into the cup below. The filter will separate the solid strawberry pulp from the liquid containing the dissolved DNA. To speed up the filtration, you can gently squeeze the filter, but avoid tearing it. Once you have collected a sufficient amount of filtered liquid (about two tablespoons), remove the filter and discard the solids.
The final step involves precipitating the DNA from the filtered liquid. Slowly pour an equal amount of ice-cold rubbing alcohol down the side of the cup containing the strawberry liquid. It is important to pour slowly and avoid mixing the two layers, as the alcohol is less dense and will form a distinct layer on top.
Within a few seconds, you should observe a white, cloudy, string-like substance forming at the interface between the alcohol and the strawberry liquid. This visible material is the strawberry DNA. You can carefully use a coffee stirrer or a wooden stick to gently “spool” or collect the DNA by twirling it at the interface.
Understanding What You’ve Extracted
The white, stringy substance observed at the interface of the two liquids is indeed the DNA from the strawberry cells. While individual DNA molecules are too small to see, they typically exist as a double helix, resembling a twisted ladder. The extraction process causes thousands of these long strands to clump together, making them visible to the naked eye.
Each component used in the extraction plays a role. The dish soap works by dissolving the lipid (fat) components of the cell membranes and nuclear membranes. Cells, including those in strawberries, are encased in these protective membranes, and the DNA is housed within the nucleus, also surrounded by a membrane. Breaking down these barriers releases the DNA into the solution.
The salt added to the mixture performs two functions. DNA molecules carry a negative electrical charge, and the positive ions from the salt help to neutralize these charges. This neutralization reduces the repulsion between DNA strands, allowing them to come closer together and clump more easily. Additionally, salt helps to remove proteins bound to the DNA, ensuring primarily DNA is extracted.
Finally, the cold rubbing alcohol makes the DNA visible. DNA is soluble in water, meaning it dissolves and disperses in the watery strawberry solution. However, DNA is not soluble in alcohol. When the cold alcohol is carefully layered on top, the DNA precipitates, or separates, out of the watery solution and into the alcohol layer. The colder temperature of the alcohol further reduces DNA’s solubility, enhancing its precipitation and making the white, stringy strands more prominent.