DNA Extraction from Strawberries by Michaela

Another experiment we did in Einstein’s Kitchen was extracting the DNA from strawberries, and wowed kids and adults alike!

Please do not eat anything (apart from the strawberries) in this experiment and if you’re doing this with small children please be extra vigilant to make sure they don’t put anything in their mouths.

DNA. It’s the blueprint for life. DNA spells out the instructions to all living things to tell them how to become what they are, and how their cells should work. Discovered back in 1869 as a part of the cell’s nucleus, it wasn’t until the 1950s that some very clever people realised that DNA was in fact the genetic material – the thing that carried genes and was responsible for heredity – the passage of characteristics from one generation to the next – why you look like your parents!  Since then there’s been a surge in research studying how traits are passed on and how the message in genes leads to a cell working the way it does.

DNA stands for deoxyribonucleic acid – quite a boring name that just describes the chemical structure (shown below). It’s basically a big string of units, where each unit has a different letter, A, T, G or C that the cell reads. These strands form the infamous double helix structure.

As scientists we’re interested in extracting, or getting at, DNA for a number of reasons, for example, to read the sequence of it to compare it from organism to organism, diagnosing genetic diseases, and to use it as a tool to study processes in the cell. DNA can be used from blood and other samples found at crime scenes to identify who could have been there, and the same principle is behind paternity tests.

 

To get at the DNA we scientists carry out a technique not too dissimilar to what we carried out at the festival, and that you can try yourself at home!

So here’s what you’ll need:

  • ·         A cup and a squashing instrument (like a fork)
  • ·         One strawberry
  • ·         10mL of DNA extraction buffer (add 2 tsp of salt to 50mL washing up liquid and then make that up to one litre with water).
  • ·         Cheesecloth
  • ·         A funnel
  • ·         A shot glass
  • ·         A thin stick
  • ·         10mL ice cold alcohol (we used 100% ethanol, but white rum works!). Just stick it in the freezer for a couple hours to cool it.

What you need to do is actually really easy but I’ve included an explanation for each step, so don’t be put off by the length of this:

1.          DNA as mentioned before is found inside the cell’s nucleus, so the first thing you’ll need to do is break open the cells. Simply place your strawberry in the cup and start mushing!

2.          Cells are actually really tiny things, so though after a minute or two of mushing your strawberry won’t be looking much like a strawberry, chances are that a lot of the cells are still intact! So to really help break it up add 10mL of the extraction buffer (about 2 teaspoons) and carry on stirring. Cells are held together in a sack of fatty acids called the membrane. Washing up liquid is a detergent, and in the same way as it cleans off grease it will dissolve the membrane meaning that the cell bursts open to release everything.

 

3.          This mix will contain everything inside the cell including the DNA dissolved in the liquid, as well as a lot of big bits that you’ll want to get rid of, so the next thing to do is filter the liquid away from all the big bits. To do this simply place your cheesecloth inside the funnel and pour your mushin. Give it a bit of a stir so that the liquid passes through to be collected in the shot glass.

4.          Now the next step is almost like magic, but it’s not actually. We can make the DNA appear out of the liquid! That’s because the DNA is dissolved in the liquid but we can make it so it no longer ‘likes’ water and so will turn to a solid – this is called precipitation. Slowly add 10mL ice-cold ethanol and be careful not to disturb the mush (or spray strawberry all over your kitchen). You should see a fluffy white solid forming at the junction between mush and alcohol. This is a little tricky, but bear with me: remember in your extraction buffer you added salt – salts sticks to the negative charges on DNA and neutralises them. Now in chemistry there’s a bit of a rule where things that are the same will dissolve each other but things that aren’t the same won’t dissolve in each other. Alcohol has a charge so by making the DNA unlike the alcohol by neutralising its chargesit means that it won’t dissolve any longer – it will come out of solution as a solid.

 

5.         Get your stick and fish out the white stuff – that is DNA!

 

Every living thing uses this very same chemical, and if you extracted DNA from yourself or a worm it would look exactly the same as strawberry DNA. Strawberries are great for using in this experiment because they contain so much DNA! We have two copies of each of our chromosomes, but strawberries have 8! This has mostly come about through selective breeding by farmers to get bigger strawberries! You might be wondering why it’s just a big white lump and doesn’t at all look stringy. This is because actually DNA itself is very, very small and so you’d need see a single molecule without the help of an extremely powerful microscope that uses electrons instead of light to look at very tiny things. DNA has to be very small to fit inside the cell. In Humans there’s 6ft of DNA in every single cell, and if you were to lay every strand end to end it’d reach to sun which is over 93 million miles away. Then it’d come back again… 600 times!!! That’s a lot of DNA! There are special proteins that wind up the DNA and package it all up so it fits.

Had fun making a mess? Why not send us your comments and pictures! And of course, any questions then drop us a line or comment on the blog.