Activity: Extracting DNA

Getting DNA from an onion.

On SCOPE's genetics episode, you saw Nikita extract DNA from an onion. Here’s how she did it. Watch the clip.

What you need

• Meat tenderiser
• Coffee filter paper
• Chilled isopropanol (also called rubbing alcohol) or methylated spirits
• Onions
• Salt
• Washing-up liquid
• Wooden skewer
• Blender.

What to do

1. Chop an onion and put it in a blender with a teaspoon of salt and half a cup of warm water.
2. Blend, add a few squirts of washing-up liquid and blend again.
3. Strain the mixture, keeping the liquid.
4. Add meat tenderiser to the juice, stir through and let mixture settle.
5. Slowly pour in the chilled isopropanol. It should form a layer on top.
6. After a while you should see DNA clumping at the boundary between the two liquid layers. It kind of looks a bit like snot. Try and scoop it out with a wooden skewer.

What's happening?

Extracting DNA from food is easy. We used onions, but many other fruits and veggies will also work well.

The DNA is inside the nucleus of the onion cells, so to release it the cell membranes or walls have to be broken up. This was done with the blender. The detergent helped to dissolve some of the parts of the cell walls. Straining it meant we kept just the dissolved components, which includes DNA, and got rid of the rest. Adding the meat tenderiser, which is an enzyme, cuts up the proteins in the mixture and stops them contaminating your DNA.

DNA is insoluble in alcohols, and the isopropanol is less dense than the water, so it will form a layer on top into which the DNA will precipitate (or undissolve). The proteins will stay in the bottom watery layer.

It only took about half an hour to do this in the kitchen, but it took scientists years to learn how to do it in the first place.