Why Strawberries?
Strawberries are octoploid: eight copies of every chromosome where a diploid cell carries two. That extra DNA makes a bigger, easier-to-see clump, so the extraction works reliably with ordinary kitchen gear.
What You'll Need
The materials are ordinary kitchen items: strawberries, dish soap, salt, water, cold rubbing alcohol, a coffee filter or a strainer, a zip-top bag, and a stirrer. Chill the rubbing alcohol before you start; a freezer or a bowl of ice works.
The Step-by-Step Process
Place the strawberries in a plastic bag, seal it, and smash them for about two minutes. The mechanical disruption breaks open the cells and releases their contents, including DNA.
Mix dish detergent, salt, and water to make the extraction solution. Add the solution to the smashed strawberries and stir gently. The mixture becomes a pink slurry as the detergent begins breaking down the cell membranes.
Filter the mixture through a coffee filter or a fine strainer; both catch the pulp and let the liquid through.
Pour the cold rubbing alcohol down the side of the container into the filtrate. The alcohol layers on top because it is less dense and cold, and pouring slowly keeps the layers separate. The DNA precipitates out and rises into the alcohol layer as a whitish, snot-like material. Spool it onto a stirrer to inspect it. You can see the white clump against the pink filtrate.
If no white cloud appears, the alcohol was likely warm or added too quickly. Chill it further and pour slowly against the side of the container.
The procedure takes about 10 to 15 minutes from start to finish.
The Science Behind Each Step
The detergent does the lifting. Cell membranes and the nuclear envelope are built from lipids, fats that form a barrier around the cell and its nucleus. A soap molecule has two ends: one attracts water, the other attracts fat. When soap meets a lipid membrane, the fat-loving ends wedge between the lipid molecules and pull the membrane apart, releasing the DNA inside. The detergent molecules surround the lipids, forming micelles that carry the fats away. Dish detergent is formulated to dissolve fats, which is why it works here.
Salt helps once the DNA is free in the solution. The positive sodium ions associate with the negatively charged phosphate groups on DNA, reducing electrostatic repulsion between strands. That lets the strands come together instead of repelling each other, and it stabilizes the released DNA so it lasts long enough to collect.
Filtering removes pulp and cellular debris that would otherwise scatter light and hide the clump.
Alcohol is the last step and the one that makes DNA visible. DNA is insoluble in alcohol, so it precipitates when the alcohol mixes with the water layer. Cold improves this precipitation, pulling the DNA into a denser clump. Lower temperatures reduce solubility further, so more DNA comes out of solution. Pouring slowly keeps the layers separate, making the DNA easier to see.
What you see is not a single molecule. DNA is a double helix, two long strands wound around each other and packed tightly enough to fit inside a nucleus far smaller than the cell. Each chromosome is a single long DNA molecule, so the tangle contains whole chromosomes. Once the membrane is gone and the alcohol forces the strands out of solution, those long molecules tangle together, and the tangle is what forms the white, stringy mass.
What to Expect and Variations
Even vigorous shaking after extraction does not destroy the visible DNA. The clump is tough because it is a tangle of long molecules, not a delicate single strand.
Rubbing alcohol is flammable. Run the experiment with adult supervision, clean up spills promptly, and wash your hands afterward.
Other fruits work too. Bananas can be used for the same procedure, though their cells carry fewer DNA copies than a strawberry's.
