3. DNA Extraction - Lesson Plan

Lesson Plan

  1. Have students write down a few sentences to describe what DNA is and what they think DNA looks like. After this lab or the series of DNA modeling activities, they will come back to this naive description to revise their answers with a more scientific one.
  2. Draw a diagram on the board showing DNA (as a long tangled thread) within the nucleus of a cell. Label the DNA, nucleus, cell membrane, and cell wall. Remind (or teach) students about basic cell structure.
  3. Tell students that they will be extracting the DNA from a strawberry and will then be able to look at the DNA. Briefly describe the process explaining the purpose of each of the steps.
  4. Pass out ziplock bags and strawberries. Tell students there are strawberries to eat after the lab is cleaned up.
  5. Students should put the strawberry in the bag, squeeze out most of the air and seal the bag. The strawberry can then be crushed into juice and pulp. Try to squish all of the chunks into an even, smooth puree. Warn students not to pound the strawberry on the table or risk the bag bursting and getting strawberry pulp all over themselves and the classroom.
  6. Next, open the bag and add 10 ml of extraction buffer (approximately 10 eyedroppers full). Seal the bag again and gently mix the strawberry juice with the extraction buffer. Warn students not to mix too vigorously or it will generate a lot of bubbles and can’t be filtered effectively. Use a gentle tilting back and forth motion while lightly squeezing the bag.
  7. Set up a filtration system. I had students wrap a paper towel around their finger then put their paper-wrapped finger into the mouth of the 15 ml tube or 5 oz cup. When you remove your finger, the paper towel should form a well into which the strawberry juice can be poured.
  8. Carefully pour the extract into the well in the paper towel. Allow the juice to filter through the towel into the container below. Let it drip for 3-5 minutes. Do not squeeze the towel or you will create lots of bubbles, disrupting the interface needed in the next step.
  9. The paper towels can be put inside the ziplock bags and thrown away.
  10. Carefully transfer liquid from the 15 ml tube or cup into the clear test tube until the test tube is about a third full.
  11. Slowly add 3 ml (3 eye droppers full) of ice cold alcohol to the test tube. The alcohol should be added so that it trickles down the side of the tube before pooling on top of the strawberry extract. You should end up with a red bottom layer and a clear top layer.
  12. Have the students make observations of anything going on in the clear alcohol layer. You may wish to have students write down observations at this point.
  13. After 2-3 minutes, a skewer or stirring rod can be inserted into the tube and gently swirled around. This will spool the DNA around the stick. The DNA can be pulled out of the tube and stored in a microcentrifuge tube filled with some alcohol. Students may safely touch the DNA although the DNA should NOT be tasted under any circumstances.
  14. By trapping a piece of string in the lid of the microcentrifuge tube, students can wear their DNA home as a necklace.
  15. Have students clean up their areas. Nicely cleaned tables and washed hands may be rewarded with a piece of strawberry to eat.