Basic science: understanding experiments
Basic science: understanding experiments

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Basic science: understanding experiments

4.1.1 The experiment

Have you chosen which fruit you’re going to extract DNA from? Follow Janet’s instructions in the video (or use your activity booklet [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] ) to conduct the experiment.

Download this video clip.Video player: ou_futurelearn_experiments_vid_1007.mp4
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Transcript

JANET SUMNER
You've probably all heard of DNA. It's usually referred to as the blueprint of life. Now DNA is in all the cells of all living things, including ourselves. But it's kind of difficult to comprehend because you can't see it.
Well it turns out that with a very simple kitchen experiment, you actually can see DNA. And that's what we're going to do now. I'm going to extract the DNA from this kiwi fruit.
Now it doesn't have to be a kiwi. You can use an onion, or strawberries. Raid the fruit bowl and pick what you fancy. You can even use defrosted frozen peas if you haven't got anything fresh.
You're also going to need some ice cold alcohol. I'm using methylated spirits, and it needs to be in the freezer for half an hour before you start the experiment. You could try something like vodka or very strong white rum as well.
And you're going to need a fine sieve, or you could use coffee filter paper. I'm just using a tea strainer. The first thing I'm going to do is peel the skin off the kiwi. And that's because it's mostly dead, and it hasn't got very much DNA in it anyway.
So now I'm going to chop it up into small pieces. And then I'm going to start mashing it. This is to start breaking up the cells and to give us a bigger surface area to extract the DNA from.
The next thing you need to do, is mix together two grams of salt with 100 millilitres of water. Add this to five gram of washing up liquid. So mix the three things together. Try and do it quite gently because you don't want loads of foam and bubbles. But you do need to stir until the salt's dissolved.
This is called an extraction buffer. It doesn't really matter about the terminology. It's basically going to help to break up the cells even further, and stop the DNA from degrading. Next you're going to add this to your kiwi mix.
Keep mashing because the more you mash, the more DNA you will get. The final step is just to warm up the kiwi mix. To do that, I'm just using a bowl of warm water.
I've used boiling water from the kettle, added some cold, so it's lukewarm. I'm going to leave that sitting in there for 15 minutes. And again, this is just to help release more of the DNA.
Well that's the 15 minutes up. The next thing is to strain the kiwi mix. Oh, I'm spilling a bit of it. Now this lovely green liquid has got our kiwi DNA in it. But we still can't see it, which is where the alcohol comes in.
Well that's well and truly ice cold. Now pour the alcohol very gently down the side of the glass. It should float out over the surface of the kiwi mix because it's actually less dense.
You need to look very carefully at the join between the two liquids, and you'll see tiny white strands and filaments forming. That's the kiwi DNA. And I can just very carefully use a paper clip to start hooking come of it out. And that is DNA. The blueprint of life. Now to find out more about DNA-- what it does and why it's important-- you need to join in with the online discussion.
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Remember that vodka and rum are strongly alcoholic and, in the case of methylated spirits, also highly toxic. The ethanol in them is also highly flammable. You should also take care when you peel and chop your kiwi fruit: you don’t want any bits of finger mixed in with the fruit!

Although you are performing this experiment in your home, it actually follows the same basic principles as more advanced lab-based DNA extraction procedures, so you are, in effect, doing something that scientists in labs do every day.

The procedures you are going to carry out are:

  • the mechanical and thermal disruption of the cells
  • the liberation of the DNA with an extraction buffer
  • the precipitation of the DNA.

As you perform the experiment, you should recognise each of these stages taking place. The mashing and heating of the kiwi mixture represents the mechanical and thermal disruption of the cells. The washing-up liquid and salt water mixture forms your extraction buffer and both work to liberate the DNA, in other words remove it from the cells. The detergent works just like it does on your dishes and dissolves the lipids (fats) in the cell membranes and nucleus where the DNA currently resides.

DNA is highly soluble in water but not soluble in ethanol (alcohol). When you add the alcohol to the top of your kiwi mixture, the DNA precipitates (or deposits in a solid form) at the interface between the water and the alcohol. The salt that you added earlier helps encourage the DNA to clump together by neutralising the negatively charged phosphate groups that exist within the DNA structure.

When you have finished the experiment and extracted the DNA, move to the next section to discuss your results.

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