Bucket Biochemistry!
Extracting DNA from a kiwi fruit
Deoxyribonucleic acid is found inside the nucleus of every living cell. It allows instructions for the functioning of the cell to be 'written' and stored in the nucleus. Until midway through the 20th Century the structure of the molecule was unknown. However, the discovery of its structure opened the field of molecular biology and allowed genetic science to reach the stage 50 years later where we know the stucture of the entire human genome.
In 1953 two researchers at the University of Cambridge, Francis Crick (an Old Millhillian) and James Watson, announced to the world that they had deduced the structure of deoxyribonucleic acid (DNA). Their discovery was the result of research by a number of workers around the world, not least Rosalind Franklin and Maurice Wilkins from King's College, University of London. The work by the scientists at King's College was especially important as it produced x-ray crystallograms of DNA crystals. Mathematical analysis of the crystallograms gave supporting evidence to Watson and Crick's model of DNA.
The research was so important that it lead to Crick, Watson and Wilkins being awarded the Nobel Prize for Physiology in 1962. Unfortunately, the other key player in the discovery, Rosalind Franklin, died in 1958. Under the rules of the Nobel Prize she could not be given an award posthumously.
Following in the footsteps of the famous DNA researchers and molecular biologists throughout the world today, the boys of St John's have been using a similar technique to the one that Rosalind Franklin would also have used.
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Why use a kiwi? Could you do this with pieces of human?
The kiwi has a particular advantage over other fruits, vegetables and, indeed, animal cells. Kiwi fruits contain enzymes called proteases. In this method the presence of protease is used to break down the protein scaffolding that helps keep the DNA tightly wound up inside the nucleus. Other organisms can have their DNA extracted but require the addition of protease enzymes to make the reaction work. The kiwi, therefore, uses a simpler method to allow DNA to be extracted.
Step One: Preparing An Extraction Mixture
Take a 250cm3 beaker
Pour in 100cm3 of water
Add 2g of sodium chloride
Add 5ml of washing-up liquid
Gently mix by stirring - try not to produce any foam
Step Two: Chop up your kiwi!
Carefully chop your kiwi fruit into small pieces.
Step Three: Add and Heat
Add the kiwi to the extraction mixture and heat in a waterbath at 60oC for 15 minutes.
Before placing the mixture into the waterbath give a gentle stir - try to avoid making a foam.
Step Four: Filter the mixture
Before you can extract your pure DNA, you need to remove all the insoluble parts of the kiwi fruit. This is done by simple filtration using the apparatus shown. By clicking here you can see the apparatus in action!
The filtrate should look something like this...
Step Five: Isolating the DNA
The DNA is soluble in water. However, by slowly running ice cold ethanol into the tube containing the kiwi filtrate the DNA will precipitate at the meniscus between the filtrate and the ethanol.
Here are three samples of kiwi DNA...
The DNA can be removed from the ethanol using a pipette. The acidic properties of DNA can be demonstrated by testing with litmus paper. Blue litmus turns red, thus DNA is an acidic compound.
