Extracting DNA in your kitchen

Follow these instructions to learn how to extract DNA from an onion.

• 29 May 2006 | Updated 14 October 2011

• What you need 
• What to do
• What's happening  

Warning: This activity involves the use of a knife, blender and methylated spirits. Methylated spirits is poisonous and knives and blenders can cut you. Have an adult assist you with this activity.

All the information required to make you is stored in your DNA. In fact, the DNA in all organisms provide instructions about how to grow and go about millions of cellular processes each day.

To find out how these instructions are communicated, researchers separate the DNA from the rest of the cell and examine how it interacts with proteins and other parts of the cell. The isolation, or extraction, can be done in about half an hour in your kitchen, but it took many years to learn how to do it.

What you need

To do this activity you will need to gather:

• half an onion
• salt
• washing-up detergent
• methylated spirits
• warm water
• coffee filter paper
• a toothpick or wooden skewer
• a knife and chopping board
• a strainer or sieve
• a blender or food processor
• a clear drinking glass.

What to do

1. Ask an adult to help you roughly chop the onion and put it in the food processor.
2. Dissolve one teaspoon of salt in half a cup of warm water, add to the onion and process for one minute.
3. Strain the mixture through the sieve into a cup, pressing the liquid out using the back of a spoon.
4. Add one teaspoon of washing-up liquid and stir. Leave for five minutes, stirring occasionally.
5. Pour the mixture through the coffee filter into a clear glass. Let it drip through; you can squeeze the filter gently to speed this up.
6. With an adult's help, carefully pour enough methylated spirits down the side of the glass to form a one-centimetre layer on top of the onion solution.
7. You should see a white fluffly layer form between the onion solution and the methylated spirits. Gently swirl the toothpick or skewer through the mixture and you will see little white strands moving around. These are bits of onion DNA!
8. You can also twist the toothpick and, if you are careful enough, you should be able to lift out some blobs of DNA. It looks almost exactly like white snot!

Try extracting DNA from dried peas (the type used for split pea soup), yeast (available in the baking section of supermarkets), cod roe (fish eggs), bananas (without the skin) and raw liver (from a chicken or calf). Is the DNA from the different organisms the same?

What's happening

Like all living things, onions store their DNA inside their cells. Cells also contain other chemicals, such as proteins, carbohydrates and lipids, surrounded by a cell membrane. In this activity, we separated the DNA from the cells and the other chemicals in them.

To start with, we had to break the cells open. The blender broke the onion into single cells. The detergent helped dissolve some of the chemicals in the cell membranes (the skin around the cell), letting the DNA and other chemicals out.

Once the DNA was removed from the cells, we needed to separate the DNA from the other chemicals. DNA dissolves in water, but it doesn't dissolve in alcohol. Adding methylated spirits (which is almost pure alcohol) causes the DNA to stop being dissolved in the onion liquid and become solid, forming the white strands you saw.

DNA molecules have a slight negative charge, which would normally make them repel each other. The salt we added at the beginning of the experiment contains positively charged ions, which neutralized the charge in the DNA and let them clump together. If you're lucky, you will be able to pick up the long strands of DNA.

This activity and others like it are featured in Scientriffic: a science magazine for ages 7+.