DNA from a Kiwi fruit
Here's how to extract DNA from a kiwi fruit with a group preteen kids (age 10-15) while explaining what genetics is all about. A workshop takes 1h15.
- 1/2 kiwi fruit (preferably overripe)
- (plastic) spoon
- ziploc bag
- pinch of salt
- 2 drops dish (preferably cheap) soap
- clear cup or glass
- 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
- Some paper towels to keep the kids' hands clean
- Paper table cloth
- Permanent markers to make drawings and explain stuff
Prep: What do you know about genetics?
- Think: Why do people often say "You look like your dad/mom/brother/sister?" Why don't you look like a monkey, a dog, a snake or ... a kiwi fruit?
- Our bodies (animals, plants, bacteria, everything that lives) are made up of cells.
- All of our cells are little factories and machines that build your body (muscle cells, skin cells, brain cells are all different machines).
- These machines work with a blueprint/building plan that is the same for each of your cells. You get half of this building plan from mom and the other half from dad, which makes you a copy of your parents' blueprints, but also very unique.
- These blueprints of the cells are called DNA (shorthand for "Deoxyribonucleic acid"). DNA molecules are long strings of molecular letters (4 "nucleotides" A, C, G, T) that acts as "computer code" for the cell.
- DNA contain instructions for the cell machinery to do its thing. For example, when cells construct the colour molecules of your eyes, the cells will look up the specific buidling instructions in your DNA. For some people the cells then build blue molecules, while for others brown or green molecules are constructed.
- Such locations of genetic code are called genes. They are parts of the DNA that have specific instructions to build proteins and to operate the machinery of the cell.
- Single letters that are changed (mutations) in the DNA code can have grave effects.
- Whether you like coriander or think it tastes like soap (many people have this) is caused by a single letter that is different between those who like coriander and those who don't.
- A single changed letter can also make you vulnerable to diseases, such as cancer.
- Think: How many cells are in your finger, in your body? A preteen kid's body consists of ~20 trillion cells. How many zeroes is that? A single cell DNA is folded up into a neat but tiny little ball in the cell core. When we would stretch out a single human cell's DNA, it would reach 1.8 meters long! The amount of information encoded in DNA in our body is HUGE! One kid's DNA all stretched out reaches to the moon, ONE HUNDRED THOUSAND TIMES! Or to the sun, 240 times!!!
- Think: Here's a kiwi fruit. Is it alive? (the seeds definitely are!) Does it contain DNA? (yes!) Where? (everywhere!) How much is there? Let's find out!
Step 1: Put the kiwi contents in a ziploc bag and squeeze it
How: Empty the juicy contents of half a kiwi fruit 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 or you'll spill kiwi puree all over!
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 from each other 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, as the kiwi fruit contains a protein that breaks apart the cells while ripening, which makes our job to crack the DNA out of the cells' shell easier.
Step 2: 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 the molecules can stick together ("precipitate" - The DNA molecules act as repulsing magnets without the added salt), the DNA molecules come together to form a bigger DNA balls. The soap and salt will also allow proteins (machinery of the cell) and carbohydrates (nutrients of the cell) to clot together.
Step 3: 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. Hold the filter in your hands and 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, so a lot of the DNA ends up in the cup while a lot of the structural material of the kiwi fruit remains stuck in the coffee filter.
Step 4: 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.
Step 5: 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 in the cell 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, as the strands prefer to be exposed to the ethanol molecules over sitting next to each other. The little balls of DNA stretch out and fill the whole cup with long DNA strands that clot together and form a visible web. As you lift this web from the cup, you're picking up the DNA. DNA should look white, so 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 very long strands to form a net, and that is exactly what DNA does.
- All the kids have now earned a cookie for all their hard work.
- Think: Is there DNA in the cookie? It's not alive, it's not a plant or animal, right? But, what goes into a cookie? Check the ingredients of the cookie and ponder which ingredients definitely have DNA (flour, eggs, milk - ingredients that don't have (a lot of) DNA are for example salt, sugar and additives).
- Think: Which foods do not contain DNA? Almost all of the food we eat comes from animals and plants, so they contain DNA. Sweets may be a candidate as they mainly contain sugars, but most additives, such as ingredients for colour, texture and taste will have DNA.