CHEN SCIENCE
T
I
K
DNA EXTRACTION
RECIPE
• 1 re-sealable bag
• 2 strawberries (fresh or frozen)
• 2 tsp dish detergent
• half cup of water
• 2 plastic cups
• 1 coffee filter
• 1 scientist
• half cup rubbing alcohol
• 1 stirrer
1. Pull off any green leaves on
the strawberry that have not
been removed yet
3. Put the strawberries into the
plastic bag, seal it and gently
smash if for about two
minutes. Completely crush
the strawberries. This starts
to break open the cells and
release the DNA.
3. In the plastic cup, make your
DNA extraction liquid: mix
together 2 teaspoons of
detergent, 1 teaspoon of
salt and half cup of water.
4. Add 2 teaspoons of the DNA
extraction liquid into the bag
with the strawberries. This will
further break open the cells.
5. Reseal the bag and gently
smash for another minute (avoid
making too many soap bubbles)
6-7. Place the coffee filter inside
the other plastic cup. Open
the bag and pour the
strawberry liquid into the
filter. You can twist the filter
just above the liquid and
gently squeeze the
remaining liquid into the cup.
8. Next, pour down the side of
the cup an equal amount of
cold rubbing alcohol as there
is strawberry liquid. Do not mix
or stir. You have just isolated
the DNA from the rest of the
material contained in the cells
of the strawberry.
9. Within a few seconds, watch
for the development of a white
cloudy substance (DNA) in the
top layer above the strawberry
extract layer.
10. Tilt the cup and pick up the
DNA using a plastic coffee
stirrer or wooden stick.
Every living thing has DNA in it. It will code for what you look like on the outside as well as how you
work on the inside. DNA is found in every cell of your body except for red blood cells because they
do not contain a nucleus.
In the first step of DNA extraction, you crush the strawberries to open up the cells so you can have
access to the nucleus. The nucleus, like all organelles in the cell, has contents that are contained within
a membrane – in this case, DNA. You need to get rid of this membrane to get to the DNA, so you use
soap to break it down. You also add salt at this stage – I’ll get to this later.
After filtering your mixture, you add ice-cold alcohol. This is when a solid is formed from a liquid, known
as a precipitate, and this is able to happen because DNA is insoluble in alcohol. This solid is the DNA.
The salt that you added earlier helps the DNA clump together in this step. The alcohol sits on top of the
mixture you’ve made because alcohol is less dense than water.
The process that you just followed is quite similar to what scientists do whenever they extract DNA in
the lab.
Why do we use strawberries for this experiment? We can use any fruit or vegetable, but strawberries
contain more DNA because they are an octoploid. This means that they have eight sets of chromosomes in every cell. In comparison, we are diploid, which means we have two sets.
How can you take it further?
• Try different fruits and vegetables to see how they compare.
• Why not try different soap –does one work better than the other?
• What happens if the alcohol is not cold?
• What if you did this experiment in a different order?
• Why not try extracting your own DNA? You can use your own saliva.