Banana DNA

Extracting DNA – a classroom experiment

You can extract DNA in your classroom from a piece of fruit or vegetable – peas, onions and bananas usually work pretty well. Follow the instructions below … the way in which scientists extract bacterial DNA is slightly different, but it uses the same principles (cell wall disruption, salty environment, protein breakdown and alcohol precipitation) …

Banana DNA (left) and bacterial DNA from a sediment sample (right). Both have been beaten to release DNA from cells, prepared in a salty environment, had their proteins removed, and have been precipitated in alcohol. Photos: Bonnie Laverock.
Banana DNA (left) and bacterial DNA from a sediment sample (right). Both have been beaten to release DNA from cells, prepared in a salty environment, had their proteins removed, and have been precipitated in alcohol. Photos: Bonnie Laverock.

 

Step 1: Dissolve 3 grams of salt in about 100 millilitres of warm water (distilled water is best if you have access to a lab).

Why? The salt provides a “good environment” for the DNA so that it is more likely that the DNA will come out of the cells and into solution (DNA is negatively charged, and salt is positively charged)

 

Step 2: Add a mashed up banana to a food blender, pour the salty water over the top, and blend for 5 to 10 seconds. Pour through a sieve into a glass.

Why? The blender helps to break open the cells which contain the DNA.

 

Step 3: Add 2 to 3 teaspoons of normal washing-up liquid and stir gently (we don’t want bubbles!).

Why? Washing-up liquid works by breaking up the fat and grease stuck to our dishes. We have fatty molecules in our body – they are called lipids, and they hold cells together. The washing-up liquid helps to break open cells by attacking these lipids – this releases the DNA from the cells.

 

Optional extra step: If you want, you can also add pineapple juice to your mixture!

Why? The pineapple juice contains a compound called bromelain, which is an enzyme that breaks down proteins. Adding this to the mix prevents your DNA from being contaminated by cell proteins.
 
Step 4: Use “rubbing alcohol” (or ethanol if you’re in a lab) and pour it very slowly down the side of the glass so that you can see it floating on top of the soapy mixture. Let it sit for a few minutes and watch what happens. The stringy whitish stuff you can see rising upwards is the banana’s DNA.
 
Why? DNA dissolves in water, so you cannot see it when it is in the soapy mixture. When you add the alcohol, the DNA rises upwards because it is less dense than water, soap, and the other cell goo. The reason you can see it now is that DNA does not dissolve in alcohol, so it becomes a gloopy solid as it leaves the watery layer.
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