Experiments to try at home. No.1: The DNA Detective

A new book of kitchen-sink experiments, Wholly Irresponsible Science, is published this week.

We have persuaded the publishers to let us share with you a few of our favourite experiments. First - The DNA Detective:

‘A half-eaten banana was found near the door of the safe that had been blown open.’ What would Sherlock Holmes have done with that piece of evidence, found at the scene of a multi-million-pound bank robbery? Even he would have been stumped if there were no way of linking the banana with the suspect being held in the police cells. Modern detectives have a few more tools to use, and one of them is DNA analysis. DNA is the chemical ‘blueprint’ of every living thing – what makes it different from every other living thing. People leave traces of their DNA on things that they touch or eat, so the first step is to extract the DNA from the banana that the safe-cracker was eating. But how do you set about doing that? Elementary, my dear Watson.

YOU WILL NEED

• Half-eaten banana • Methylated spirits
• Food processor • Paper coffee filter
• Salt • Wooden skewer
• Cup • Clear drinking glass
• Hot water • Teaspoon
• Washing-up liquid

METHOD

1. Put the banana into the food processor.
2. Add a teaspoon of salt to half a cup of hot water.
3. Pour the salt-water mixture in with the banana and process for one minute. Rinse the spoon and cup.
4. Pour the mixture through a sieve back into the cup. Press the back of the spoon against the sieve to push the solids through.
5. Add a teaspoon of washing-up liquid. Stir occasionally for 5 minutes.
6. Set the coffee filter over the drinking glass and pour the mixture into it, so that it drips into the glass.
7. Slowly pour methylated spirits down the side of the glass until it forms a 1-cm layer on the top of the mixture.
8. A layer of fluffy white material should form between the mixture and the methylated spirits.
9. Use the toothpick to extract some of this material (which is the DNA); with any luck, you’ll be able to pick out a strand of DNA.

THE SCIENTIFIC EXCUSE

DNA, like other chemicals, is stored inside cells. Processing the banana helped break down some of these protective barriers; the washing-up liquid helped dissolve some other chemicals that were locking the DNA inside. DNA dissolves inside water-based mixtures, but doesn’t dissolve in alcohol. The fluffy material at the end of the experiment was DNA that was not dissolving in the alcohol.

TAKE CARE!

Make sure that it is an adult who handles and pours the methylated spirits.

Wholly Irresponsible Science, by Sean Connolly, is published this week by Icon Books, price £9.99 Buy the book here

Comments

[Daniel Gallichan]

So now you have the DNA of the banana.... Presumably pretty difficult to filter out the little bit of DNA that came from the saliva... So is the genetic diversity of our supermarket bananas sufficient to pin someone down based on where they do their shopping...?

Still, nice experiment!

[G. T.]

Actually, almost all commercially-grown bananas are not merely closely-related, they're cuttings of the same plant -- vegetative reproductions, clones.

[John Rose]

Our next lesson will teach you to produce 500 litres of really cheap nitro-glycerine in your very own kitchen. Wow! What a blast!