Deoxyribonucleic acid, or DNA, serves as the complete instructional manual for life in every living organism. While the iconic double helix structure is often associated with high-tech laboratories, the molecule itself can be made visible using only common household products. The banana is a particularly useful specimen for this experiment because its cells naturally contain multiple sets of chromosomes (polyploidy), which provides an abundant quantity of genetic material. By carefully breaking down the cellular structures, we can isolate enough of this genetic blueprint to see it with the naked eye.
Preparation and Necessary Supplies
Proper preparation requires gathering materials from your kitchen and medicine cabinet. The alcohol must be chilled in the freezer for at least 30 minutes before the experiment begins, as its low temperature is necessary for the isolation process.
You will need the following supplies:
- One ripe banana.
- A quarter cup of table salt.
- Approximately half a cup of liquid dish soap or detergent.
- About one cup of 70% or 91% isopropyl or rubbing alcohol.
- A sealable plastic zip-top bag for mashing.
- A clear glass or test tube.
- Measuring spoons.
- A filtration apparatus, such as a paper coffee filter or cheesecloth, and a funnel or second glass.
Step-by-Step Guide for DNA Extraction
Start by peeling the banana, placing about half of it into the zip-top bag, and adding half a cup of tap water. Mash the fruit thoroughly within the sealed bag for several minutes until the mixture is a smooth pulp with no large chunks remaining. This initial action physically ruptures the tough outer cell walls of the plant cells.
Prepare the extraction solution by gently mixing two teaspoons of liquid dish soap, a quarter teaspoon of table salt, and two tablespoons of water in a separate container. Stir slowly to dissolve the salt without creating excessive foam. Combine two tablespoons of the mashed banana mixture with two tablespoons of the prepared extraction solution, and stir gently for five to ten minutes.
Separate the liquid containing the DNA from the larger solid particles. Place a coffee filter or cheesecloth over a clean glass and pour the banana-soap mixture into it, allowing the liquid (the filtrate) to slowly drip through. The filter traps the larger pulp while the filtrate contains the dissolved DNA and small cellular components. Do not squeeze the filter, as this can tear the material and force unwanted solids into the clean filtrate.
Finally, add the ice-cold rubbing alcohol to the filtrate to visualize the DNA. Slowly pour an equal volume of the cold alcohol down the side of the glass so it forms a distinct, separate layer on top of the banana liquid. After several minutes, a white, cloudy, stringy substance will begin to precipitate and rise into the alcohol layer at the boundary between the two liquids. This visible, thread-like material is the collected banana DNA, which can be carefully spooled out using a thin wooden stick or skewer.
Understanding the Chemical Roles
This procedure relies on a sequence of chemical reactions designed to systematically break down the cell and isolate the genetic material. While mashing performs the mechanical work of cracking the cell walls, chemical intervention is needed to reach the DNA housed inside the nucleus. The liquid dish soap acts as a lysis buffer, playing a primary role in this process.
The membranes surrounding the entire cell and the inner nucleus are composed of lipids. The detergent contains surfactants that disrupt and dissolve these lipid bilayers, effectively breaking them open to release the DNA into the surrounding solution. This action is comparable to how soap emulsifies grease when washing dishes.
Once the DNA is released, the dissolved table salt, containing positively charged sodium ions, takes on a distinct function. DNA has a negative electrical charge due to the phosphate groups along its backbone. The positive sodium ions neutralize this negative charge, reducing the repulsion between DNA strands. This neutralization causes the long strands of DNA to condense and clump together, making them visible.
The final step involves the addition of cold alcohol, which exploits a difference in solubility to make the DNA precipitate. DNA is soluble in the water-based extraction mixture, but it is insoluble in alcohol. When the alcohol is introduced, the DNA instantly comes out of the solution, forming a visible precipitate. Using cold alcohol helps to further reduce the solubility of the DNA, ensuring the maximum amount of genetic material clumps together as a white, cotton-like mass.