Most of you won't be shocked to hear this, I know.
But have you ever wished you could see DNA? How do we really know it's a thing, if it's so small that we can't really see it?
This is a real problem for science teachers. We often are working with things that are too small, or too big, or too dangerous to show students directly. So we create models, or play videos, or show pictures...which are all good options, of course.
Take DNA as an example. When I used to teach students about DNA, I often showed them pictures of the double-helix structure in their textbook. We would view video clips of how DNA can make copies of itself using the microscopic machinery of living cells. I would have groups of students create construction paper models of the ladder-like structure of DNA.
But wouldn't it be nice to show students DNA first hand, if possible?
You have a complete set of your DNA in ever cell of your body, and if you would lay this DNA end to end, it would stretch to about six feet in length. SIX FEET. In every one of the trillions of cells in your body! (We are fearfully and wonderfully made!) The problem is that DNA is too small to be seen, of course. A strand of DNA is only a few atoms wide, after all.
The analogy I give my students is to picture me holding up a piece of thread down at the far end of the hallway. It would be mighty hard to see that thread--even a really long piece of thread--from that distance, because it's so thin...it's just hard to see it.
But what if we could ball up this thread? That would make it easier to see, right?
Or what if we had a REALLY long piece of thread that was all tangled up over itself? Then we might be able to see a "ball of string," even from a long ways away.
This is the basic idea in the lab I did with my science methods students last week. We used strawberries instead of human cells as a source of DNA, hence the name of the lab: "There is DNA in Your Smoothie!"
Basically, here's the strategy: the DNA in a strawberry cell is inside the nucleus, which is inside a cell membrane, which is inside a cell wall. If we can breakdown these barriers, and then process them "just so," we should be able to collect a sample of the DNA inside.
|Strawberries are a great source of DNA for this lab!|
Below, you can read the steps we followed. [Disclaimer: If you choose to do this activity in class with your own students, do so wisely, and at your own risk! I have done this lab successfully with middle school students and college students; it is reasonably safe, but use proper safety precautions with your students. You assume all liability if you decide to do this activity with your students, okay? I expressly disclaim any liability if you choose to do this activity.]
1. Put a strawberry in a zip top sandwich baggie and pulverize it by smashing and grinding it up with your fingers. (This step smashes the cell walls.)
|Smashing our strawberries into strawberry slurry...|
2. Add some soapy, salty water, and smash some more. I mixed up some Dawn dish detergent in water with a good dash of salt to create the solution. (This step breaks down the cell membrane and nuclear membrane, which are both made of lipids--fats. Think about how you clean that nasty pan with baked on grease...you soak it in soapy water, right?)
|Adding some soapy, salty water to the |
mixture to break down membranes
3. Once it's all mixed up really well, filter out the chunky bits by straining this strawberry slurry through a paper towel. (The pinkish liquid that drips through the paper towel has all kinds of cell fragments in it: proteins, lipids, ...and DNA.)
4. Add a little alcohol (ethanol is best, because it's non-toxic, but rubbing alcohol works too, in case you don't want to keep a bottle of Everclear in your classroom...) to the mixture, and the DNA will begin to come bubbling out of the solution! (The alcohol will form a layer on top of the strawberry slurry, and this is the key! DNA can dissolve in water, but not in alcohol, so the DNA floating at the surface of the strawberry slurry will begin to precipitate up into the alcohol in white, cottony threads.)
|Those bubbles in the clear liquid?|
5. If you're careful about it, you can use a glass stirring rod or a bamboo skewer to swirl a little of these DNA strands into a blob, and lift it out of the liquid all together. Amazing! You can actually see a blob of DNA!
|Using skewers to collect samples of DNA out of our test tubes.|
If you're curious about why I suggest using strawberries, or if you'd like more details about the steps of this lab, or how more information about how to safely do this activity with your own students (remember my disclaimer above?) you can download my lab sheet/teacher notes for this activity.
DNA is a real thing! If you are going to be learning about DNA with your elementary or middle school students, you owe it to them to show them that DNA is a real thing. An activity like this can go a long way to helping them understand the structure of the cell, and the best part: you can do it with materials easily obtainable at the grocery store.