Experiment: DNA Extraction from Fruit

DNA is the hereditary material in all cells. The experiment described below allows you to isolate DNA from fruit. To understand the important principles behind this step, it is not essential to do this experiment at this stage or, in fact, at all. If you are not able to carry out this experiment, read through the details below and try to work out why the procedure works in the way it does – a few questions have been posted at the end of the method to help you think about this. You could set up a discussion about the principles behind the experiment using the Comments boxes.If you are studying this course with other people (e.g. within a school class), you might want to think about doing the experiment togetherIf you do plan to carry out this experiment, you will need to read the instructions carefully and do some planning before starting it.

Materials required

For this experiment you will need:

• Fruit – Kiwi, Strawberries, and Banana all work well
• 5 g washing up liquid
• 2 g salt
• 100 ml tap water
• 100 ml of ice cold alcohol (isopropyl alcohol can usually be found at the pharmacists); put in a freezer for at least 30 mins before starting the experiment
• Access to hot water – about 60 °C
• Sieve or coffee filter paper
• Two glass beakers (or old jam-jars)
• Several bowls of different sizes, including a large bowl for making a water bath
• A paperclip
• Safety spectacles – if desired

Experiment to purify DNA from fruit

Step 1: Mash up the fruit of your choice in a bowl. Bananas, kiwis and strawberries all work well. (Remove the skin of the bananas and kiwi, we just want the insides!)Step 2: In a separate bowl, mix the washing up liquid, salt and tap water. Stir gently trying to avoid making too many bubbles in the mixture. This is your extraction buffer.Step 3: Add the fruit to the extraction buffer and mix again. Mash your fruit sample as much as you can, but again, try to avoid making too many bubbles.Step 4: Make a water bath with a temperature of about 60 °C. (A large washing up bowl works well for this.) Leave the fruit extraction mixture to incubate for 15 minutes.Step 5: After 15 minutes, filter your fruit mixture through a fine sieve or coffee filter. This will remove all the solid material that you don’t want. You should be left with a clear(ish) liquid.Step 6: Take the ice cold alcohol and very slowly, drop by drop, pour it down the inside of the container with your fruit mixture. What you want to do is produce a layer of the alcohol floating on top of the fruit mixture.Step 7: At the interface between the alcohol and the fruit mixture, you should see a white cloud-like substance forming. Use a hook (a bent paperclip would work) to slowly draw the DNA up and out of the solution.

Questions to think about

1. Look at each step of the protocol. What are the different steps and reagents (washing-up liquid, salt, water, alcohol) used for in the experiment?
2. How are you able to see the DNA?
3. Do you think that the sample you have prepared is “pure DNA”?

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If you have the opportunity to conduct this experiment, feel free to post comments about what you observe.