Episode TwoDNA Extraction

Lesson Goal Visualize the physical nature of DNA


  • .9% NaCl (saline)
  • Dixie cups
  • 10 ml graduated cylinder
  • Test tube and rack
  • 25% soapy water solution
  • Ethanol (in a squeeze bottle)
  • Toothpicks
  • Strawberries and Kiwis
  • Ziploc baggies
  • Table salt (dry NaCl)
  • Gauze to filter smushed fruit mix
  • Funnels
  • Scissors

Step One – Cheek Cell DNA Extraction

  • Vigorously swish 10ml of .9% saline solution (9 grams of dry NaCl in 1 liter of water) in your mouth for 30-60 seconds. The Dixie cups have 10ml of saline solution in them already.
  • Think & Write about this:
    Why did you use saline instead of plain water?
  • Gently spit the saline/spit mix into your cup.
  • Select a test tube. Note the number . You’ll use this test tube again.
  • Carefully pour the saline/spit mix into your clean test tube and place it in a test tube rack.
  • Measure 5 ml of soapy water and carefully pour into the test tube.
  • Hint: use the graduated cylinder that’s marked for soap so you don’t have to wash it.
  • Think & Write about this:
    Why did you use soap?
  • Place your thumb over the test tube opening and gently tilt the test tube back and forth several times (10 times works well) to mix well.
  • Using a squeeze bottle, run about 1/2” of chilled ethanol down the inside edge of the test tube – be careful so the two liquids don’t mix. You’re trying to create two separate layers.
  • You should observe (see) a white stringy substance forming at the place where the two layers meet. Don’t be alarmed if some of the material rises to the top of the ethanol – that’s okay.
  • Twirl a toothpick or wooden stirrer in the DNA to extract the DNA. The “strings” will wind around the toothpick/stirrer. Cool, hunh?
  • Think & Write about this:
    What are you seeing? What does it look like?
  • Wash your test tube and take it with you to the next station (there’s more work to be done!).

Step Two – Fruit DNA Extraction

  • Select a piece of fruit (always a good place to start).
  • Hint: Strawberries work really well because most of them are octoploid. This means that they have eight (octo means eight) copies of each chromosome. In comparison, humans are diploid (di means two), having two copies of each chromosome.
  • Place a small (1-inch cube) piece of the chosen fruit into a Ziploc baggie Press the air out and seal the bag. Mash the bagged fruit really well with your fingers.
  • Think & Write about this:
    Why are you mashing the fruit?
  • Add 10ml of the 25 % soap solution and a pinch of salt to the baggie. Press the air out and seal the bag. Mash the fruit, soap and salt for another minute.
  • Place a gauze or filter-lined funnel into your test tube while it’s still in the test tube rack.
  • Use scissors to cut a corner of the baggie and squeeze the fruit/soap/salt mixture into a gauze or filter-lined funnel, collecting the liquid in your test tube.
  • Throw away the fruit pulp and filter in the trash or provided container.
  • Slowly drip about a 1/2” of chilled ethanol along the side of the test tube using the squeeze bottle. Remember: the goal is to form a new layer of ethanol on top of the liquid.
  • You should observe a white filamentous (stringy) substance forming at the place where the two layers meet. Did you get more DNA material this time? Why/why not, do you think?
  • Twirl a toothpick/wooden stirrer in the DNA to extract the DNA. The “strings” will wind around the toothpick. Still cool, hunh?
  • Wash your test tube!
  • Think & Write about this:
    Did your cheek cell DNA extraction look like the fruit DNA? Why do you think it looked the same and/or different?


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