Most people think that DNA is some kind of mysterious substance buried away in our cells never to be seen — but that is not the case. Here is a recipe for the extraction of DNA to make in your own kitchen — not unlike the type of experiment Rosalind Franklin would have performed in her lab at King’s college, London. She had in fact tried oranges which were not at all suited to this form of experiment and proved useless. Kiwi fruit is best suited to making
DNA because of its structure.
You will need:
1. A ripe Kiwi fruit.
2. Table salt.
3. Non concentrated washing up liquid.
4. Knife and a chopping board.
5. Measuring scales.
6. A measuring jug.
7. A teaspoon (5 ml).
8. Small bowl.
9. A large sauce pan full of hot water (not boiling — about 60 degrees).
10. A large sauce pan full of ice.
11. Coffee filter and filter funnel.
12. A tall, thin plain glass.
13. Some thin wire (fuse wire).
What to do:
1. Put the bottle of methylated spirits in the ice.
2. Make up a solution of 3 grams of salt, 10 ml of washing up
liquid and 100ml of water. Measure 3 grams of salt .
You may find that your scales will weigh out a minimum of 25 grams. If this is
the case add 80 ml of washing up liquid (16 measuring spoons) and make
up to just less than one litre. Stir your mixture thoroughly (avoid froth) to
dissolve the salt.
3. Peel and chop one Kiwi fruit. Scoop this into the small bowl and add
100 ml of the salt — detergent mix.
4. Put the bowl containing the mixture into the saucepan of hot water and leave for 15 minutes.
5. After 15 minutes pour the mixture into the coffee filter, and catch the liquid
that filters through in the tall glass. You will need about one fifth of a glass.
6. Very carefully drizzle the ice cold meths down the inside of the
glass so that it forms a purple layer on top of the green layer.
When there is about a fifth of a glass of meths (i.e. two fifths including the
green liquid) put the glass on the table and watch what happens.
7. After a few moments you should see a white layer beginning to
form at the boundary between the green and the purple layers. If you have
done it correctly you will see that the layer is made up of a filament — DNA
from Kiwi fruit.
8. Try scooping the DNA out with a loop of thin wire — purple white goo - the
molecules of life.
What is happening:
DNA is found in every cell of every living thing, but it is difficult to get it out
and disentangle the DNA from the protein inside the cells.
The composition of Kiwi fruit allows the DNA to be extracted without a great
deal of effort, but not all things do. By chopping the Kiwi fruit and letting it
soak in the detergent and salt the first part of the problem is solved. The
detergent strips away the cell membranes that cover the inside of the cell
and lets the internal goo escape. The kiwi fruit needs to be chopped fine
enough to get the cells broken open, but this should not be overdone or the
DNA will be smashed.
The second part of the problem, getting rid of the protein that has stuck to the
DNA is all done by the Kiwi fruit. The Kiwi fruit already contains a lot of a
special enzyme called a proteinase. Enzymes are like small molecular
machines that do many things — the enzymes in washing powder
digest fat for example. The proteinase enzyme attacks the proteins clinging
to the DNA and breaks them up thereby releasing the DNA. Rosalind
Franklin tried oranges, but oranges do not contain enzymes and therefore
she failed to make DNA from them. She and Raymond Gosling finished up
having a food fight with the residue of their efforts.
