So, got any strawbs left over from that fool? As we at Planet Science always say - there’s nothing like a nice juicy strawberry … to pulverise into a squishy red paste. We must be mad. Extracting DNA? Surely that’s a little extreme? We may have eaten the odd strawberry or two (or eight) but it’s hardly a crime. Is it? Hmmmmm, it seems it is.
Strawberry Squash
Note: This activity requires adult supervision if you are giving it to youngsters to try.
You will need:
For one DNA extraction
- A zip lock plastic bag (sandwich or small food bag)
- 1 strawberry
- 2 teaspoons DNA extraction buffer
- Coffee filter paper
- Funnel (or cut the top off a plastic drinks bottle)
- Ice cold ethanol or Isopropyl rubbing alcohol (surgical spirit)
- Test tube with lid or sample tube
- Long Cocktail stick
- Black cardboard
To make the DNA extraction buffer:
- Makes 500ml (enough for 50 extractions)
- 50ml washing up liquid
- 7.5g kitchen salt (1 teaspoon)
- 450ml water
Warning: Ethanol is harmful and highly flammable. Take care when using.
What to do:
- Wash the strawberry and remove the sepals (the green leaves).
- Place the strawberry in a zip lock plastic bag and crush it with your fist. This is the bit we love!
- Add 2 teaspoons of the DNA extraction buffer to the bag, zip it up and squeeze it in your hands for 1 minute
- Place a funnel in the test tube. Place filter in the funnel.
- Pour the mixture into the filter.
- Filter the mixture into the tube.
- You should now see a clear red liquid in the tube.
- Carefully pour ice-cold ethanol into the tube, until it is about half full. The ethanol will form a layer on top of the filtrate.
- Keep the tube still at eye level; do not shake it. Watch what happens. You should see cloudy whitish strands appearing. This is the DNA.
- Scoop out the DNA with the cocktail stick.
- Spread the DNA out on a black card and leave it to dry. When it dries it looks like a snail trail.
What’s going on?
Crushing the strawberries breaks open many of the strawberry cells, where the DNA is. The soap in the washing up liquid in the extraction buffer breaks down the membranes of the cells, releasing the DNA. The salt makes the DNA molecules stick together, and separate from the proteins that are also released from the cells.
The filter paper will retain cell debris and unmashed pieces of fruit. The DNA will pass through the filter paper into the test tube.
DNA is not soluble in alcohol, so it precipitates. What you see are long, rope-like DNA molecules in the alcohol.
Why use strawberries? Strawberries are soft and easy to crush. Most interestingly, strawberries have eight copies of each chromosome – that is a lot of DNA in each cell!
This fantastic experiment comes from the Institute for Stem Cell Research.
See Extracting DNA from strawberries.
If you want to try extracting DNA from kiwi fruit then try this activity from Planet Science’s Little Book of Experiments.
Warning: Ethanol is harmful and highly flammable. Take care when using.
Since we are on the subject of DNA, fancy winning a miniDNA Model Kit from those amazing bods at 
