DNA from a Kiwi fruit
Here's how we extract DNA from a kiwi fruit with kids (age 10-15) while explaining what genetics is all about.
- 1/2 kiwi fruit (preferably overripe)
- plastic spoon
- ziploc bag
- pinch of salt
- 2 drops dish (preferably cheap) soap
- clear cup
- coffee filter
- ice cold alcohol or hard liquor (yes, this is a workshop for preteens, openly discuss the dangers involved to the kids)
- plastic fork
Put the kiwi contents in a ziploc bag and squeeze it
How: Empty the juicy contents of the kiwi into a ziploc bag. Close the ziploc bag carefully. Press and squeeze the fruit for a few minutes to turn it into kiwi mush. Don't blow up the bag of kiwi mush!
Why: DNA is in all the cells of the kiwi fruit. We want to get the DNA out of the cells of the fruit. So first we will separate the cells of the kiwi fruit by applying brute force. We also start breaking up some of the cells so that their contents and DNA spills out in the kiwi soup. We preferably use overripe kiwi fruits, since the kiwi contains a protein that breaks apart the cells while ripening, which makes our job to crack the DNA out of the cells' shell easier.
Add a pinch of salt and 2 drops of dish soap and squeeze it some more
How: Open the ziploc bag. Add a pinch of salt. Add 2 drops of liquid dish soap. Close the ziploc bag. Squeeze for a few more minutes.
Why: The dish soap contains molecules that try to fit into the cell walls (they are similar molecules) and disrupt the membrane structures, spilling out the contents of the kiwi cells into a fine kiwi sap. The salts neutralises the DNA's charge so they can precipitate. They come together to form a bigger ball of DNA, as DNA acts as repulsing magnets without the added salt. The soap and salt will also allow proteins (machinery of the cell) and carbohydrates (nutrients of the cell) to be come together.
Filter the kiwi juice through a coffee filter
How: Push a coffee filter down a clear plastic cup or glass. Pour the contents of the ziploc bag into the coffee filter. GENTLY squeeze the filter to extract the kiwi juice, drop by drop. The filter easily breaks, so apply as little force as possible and take your time. If the filter fails and fruit solids such as seeds spill out, try again with whatever you can save. You only need a few millimeters of liquid in your cup. Throw out the coffee filter and kiwi solids.
Why: We try to extract as much of the cell contents from the solids of the kiwi fruit. The little balls of DNA are small enough to get though the microscopic holes in the filter paper.
Add ice cold alcohol to the kiwi juice
How: Take ice cold alcohol (I get mine at 94% volume ethanol, but you could also use an ice cold vodka with high alcohol content) from the freezer. Tilt the cup and SLOWLY pour a tiny stream of alcohol into the cup. Use the inner side of the cup to slowly stream the alcohol on top of the kiwi juice. You'll need about as much alcohol as kiwi juice, so add just a few millimeters of alcohol in the glass. You'll see that the ice cold alcohol floats on top of the kiwi juice.
Pick up the DNA!
How: After pouring the alcohol on top of the kiwi, a web of white strands will appear at the separation of the kiwi juice and the alcohol. This white web is the kiwi fruit's DNA! Use a fork to gently pick up the strands, without disturbing kiwi juice too much. The snotty substance you're getting out of your cup is DNA! isn't it amazing how much DNA there is in only half a kiwi and the bit of juice you pulled through the coffee filter?
Why: DNA is one of the few molecules that prefers to be in alcohol over being in the watery kiwi juice. As the DNA reaches into the ethanol, it prefers to unfold its molecular structure. The little balls of DNA stretch out and fill the whole cup with strands that clotter together. As you lift this web from the cup, you're mostly picking up DNA. DNA should look white, if you end up with a blob of green snot, the web of DNA strings picked up some of the kiwi juice, but that's OK. The fact that the end result looks like snot proves that something formed long very long strings to form a net, and that is exactly what DNA does.