Science corner

23rd May 2003 at 01:00
Inside information: Sophie Duncan shows a way to see DNAin the classroom.

This beautiful object is a model made by Crick and Watson, the people famed for discovering the double helix structure of deoxyribonucleic acid (DNA) 50 years ago.

You can see DNA for yourself by doing the following experiment, but do take care and use proper precautions when using surgical spirit.

Select a piece of fruit such as a banana, strawberry or kiwi fruit - or try an onion - and cut it into small pieces. Add four tablespoons of salt and four tablespoons of washing up liquid, and stir. Add 400 millilitres of warm water and mix well. Leave it for 10 minutes, and then filter the mixture through filter paper, using a coffee filter and funnel. Your aim is to collect as much liquid as possible, and this can take up to six hours.

Pour the liquid into a clear beaker. Take some surgical spirit that's been cooled in a fridge and carefully pour it down the side of the beaker so it floats on top of the liquid. Create a layer four centimetres thick. This should take at least two minutes.

During the next 10 minutes white strands will form where the two liquids meet and this is DNA. You can remove it using a cocktail stick.

The cells in the fruit are broken up by chopping. The membranes, which are made up of fats and protein are broken up by the washing-up liquid. You can see how effective washing-up liquid is at breaking down fats when you are doing the washing up.

The salt binds with the released DNA and remains in the liquid when the rest of the fruit is filtered out. When you add the surgical spirit the DNA precipitates - takes solid form - and you can see it.

Sophie Duncan is project manager for science at the BBC www.bbc.co.ukscience

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