Capstone: Extract DNA from a Strawberry

What you are about to do

Every step of this protocol is something you have already studied. You are going to break open cells, dissolve two lipid membranes, neutralise the charge on the DNA backbone, and then exploit the fact that DNA is soluble in water but not in alcohol. Nothing here is a trick; it is Units 0 to 2, applied with kitchen equipment.

The result is genuinely visible. Not a stain, not a colour change — actual strands of DNA you can lift out on a cocktail stick.

What you need

• 2–3 strawberries (fresh or frozen and thawed)
• A zip-lock bag
• 1 tsp washing-up liquid
• ½ tsp table salt
• 100 ml water
• A coffee filter or piece of muslin, and a clear glass
• 50 ml surgical spirit or vodka, kept in the freezer for at least an hour — it must be properly cold
• A cocktail stick or thin glass rod

Why a strawberry

Because strawberries are octoploid — eight complete sets of chromosomes per cell, where you have two. Gram for gram there is far more DNA in a strawberry than in most tissue, and they mash easily. Bananas and kiwis also work; a strawberry works best.

Why each step works

Mashing breaks cell walls mechanically. Detergent dissolves the phospholipid bilayer of both the cell membrane and the nuclear membrane — the same chemistry that lifts grease off a plate, applied to the structure from Unit 0.2. Salt supplies Na⁺ ions that shield the negative charges along DNA’s phosphate backbone, so the strands stop repelling one another and can clump together instead. Filtering removes debris.

The final step is the one worth savouring. Cold alcohol is poured gently on top so it does not mix. DNA dissolves in water but not in alcohol, so at the boundary between the two layers it precipitates out — coming out of solution as white, stringy threads that gather at the interface within seconds. Cold matters because it slows the enzymes that would otherwise chop the DNA into invisible pieces.

What you are looking at

The white mass on your stick is not one molecule — it is the tangled DNA of millions of cells, clumped together. A single DNA molecule is about 2 nm wide and completely invisible. What makes this collection visible is sheer number, the same way one strand of hair is thin but a ponytail is not.

And it is the real thing: the same molecule, using the same four bases and the same genetic code, that you spent Unit 2 learning to read. A strawberry’s genes and yours are written in the identical alphabet.