Grade 11 · 🧬 Biology · Genetics · BIO-G11-G1.1
Extract real DNA from a strawberry
Use soap, salt and cold alcohol to pull visible strands of DNA out of a strawberry — the experiment from our first school session with The Science Basement.
Why this matters
Researchers at the Ethiopian Biodiversity Institute sequence the DNA of Ethiopian crops to find genes for drought tolerance and disease resistance. Doctors testing for sickle cell disease are reading one letter in one gene. It all starts by getting DNA out of cells — exactly what you'll do today.
Watch the demo
Video by NHGRI (genome.gov)
The science
What is DNA made of?
DNA is a long chain of nucleotides. Each has a sugar, a phosphate group and one of four bases — A, T, G or C. The order of the bases is the genetic code.
The double helix
DNA is two strands twisted into a double helix. A always pairs with T, and G always pairs with C.
Where DNA lives
In plant and animal cells, DNA is inside the nucleus, wrapped by membranes made of fats. To extract it you must break those membranes open.
You'll need
- 1 ripe strawberry (or 2–3 small ones)↳ Banana or kiwi also work, with less DNA
- A zip-lock bag or any clean plastic bag
- 1 teaspoon dish soap
- A pinch of table salt (about ¼ teaspoon)
- About 100 ml clean water
- A coffee filter or a clean piece of cloth
- A clear cup or small glass
- Ice-cold rubbing alcohol, kept in the freezer for 30 minutes↳ From a pharmacy. An adult may use strong araki if needed.
- A toothpick or thin wooden skewer
Safety first
- Rubbing alcohol is flammable and poisonous. Keep it away from flames and never drink it. An adult should handle it.
- Work in a well-ventilated room.
Step by step
0/7 doneWhat you should see
A white, stringy, cloudy substance appears at the boundary between the juice and the alcohol and floats up into the alcohol layer.
Why it happens
That white substance is DNA — billions of molecules tangled together so they're visible. The soap broke open the fatty cell and nuclear membranes. The salt neutralised the negative charges on DNA so the strands clump. The cold alcohol pulled the DNA out of solution, because DNA does not dissolve in alcohol. Strawberries have eight copies of every chromosome, so they give a lot of DNA.
Didn't work? Try this
No white strands appeared.
The alcohol must be really cold — freeze it for at least 30 minutes. Also check you poured it slowly so it formed a separate layer.
The layers mixed together.
Pour the alcohol even more slowly down the tilted side of the cup, or along the back of a spoon.
I only see tiny bubbles, not strands.
Squish the strawberry more thoroughly in step 2, and wait 2–3 minutes for the DNA to rise.
Still stuck? Ask the lab assistant
Check your understanding
1. What was the job of the dish soap?
2. In DNA, which bases pair with each other?
Your lab notebook
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