Every living thing, from slime moulds to humans, contains DNA. DNA contains the instructions for how to create plants or animals and to make them work. It is at the centre of our understanding of evolution and genetics. For something so important and so famous, people rarely have a chance to see it. This week, we will take a look at some DNA.
Warning: This activity involves the use of a knife, blender and rubbing alcohol (isopropyl alcohol). Isopropyl alcohol is poisonous and knives and blenders can cut you. Have an adult assist you with this activity.
You will need
A banana (you can also use an onion or split peas)
knife and chopping board
1 teaspoon salt
1/2 cup warm water
blender or food processor
small container
1 tablespoon dishwashing detergent
paper towel
strainer
glass jar
1/4 teaspoon meat tenderiser (a white powder available from the spices section of the supermarket)
Chilled rubbing alcohol (isopropyl alcohol), available at chemists.
toothpick
magnifying glass (optional)
An adult
What to do
Peel the banana and chop it into small pieces.
Stir the salt into the water until it's dissolved.
Blend the banana and salty water in the blender until it's smooth.
Place in the small container, add the detergent and stir for five minutes.
Place the paper towel into the strainer.
Pour the banana mix through the paper and strainer into the jar. This can take a while, but the more you filter through, the more DNA you can extract.
Add about 1/4 teaspoon of meat tenderiser and mix it in.
Wait a few minutes.
Gently pour in the isopropyl alcohol so it forms a layer on top of the mixture.
I found the best way was to dribble it down the side of the jar. Don't squirt it in, or it will stir up the mixture.
You will find a gooey, cloudy substance forms where the alcohol touches the mixture. This is DNA.
You can try to lift out some DNA with the toothpick.
What's happening?
Like all living things, bananas 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 tore through some of the cell membranes. The detergent helped dissolve some of the chemicals in the cell membranes, so some of the remaining cells would break open. Pouring the mixture through the paper towel filtered out the remaining cells, leaving the water and detergent mixed with the chemicals from inside the cells.
Once the DNA was removed from the cells, we needed to separate the DNA from the other chemicals. The meat tenderiser contains an enzyme called Bromelin, which breaks up proteins, leaving the DNA molecules on their own (DNA is not a protein). Without the other chemicals, the DNA dissolved in the water.
DNA dissolves much better in water than in isopropyl alcohol, especially if the alcohol is cold. When we added the layer of isopropyl alcohol, the dissolved DNA changed back to strands of DNA. DNA molecules have a slight negative charge, which would normally make them repel each other. The salt we added 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.