Isolating DNA

Isolating DNA from Strawberries

All creatures contain DNA, the information storage molecule.  In fact, all organisms share at least 500 of the same genes. Changes in DNA lead to biological evolution. In this activity, you can isolate DNA and see what it looks like! Scientists use variations of this method to isolate DNA from all kinds of organisms.

Objectives:

  • Students learn how to extract DNA
  • They learn various properties of DNA:
    • DNA is a long molecule
    • DNA is negatively charged and dissolves in water
    • DNA precipitates in alcohol
Materials

  • Freezer Baggie – pint size
  • Extraction solution
    • 1 cup water
    • 1 Tbsn shampoo containing EDTA, citric acid, Lauryl sulfate (Pantene works well)
    • 1/2 tsp salt
  • 1 Strawberry – fresh
  • Paper towels to be used as filters (Melita Coffee filters work also)
  • clear plastic cup
  • ice bath
  • ice-cold 95% ethanol (room temperature 91% isopropanol at room temperature works well, too)
  • Glass rod, loop or straw
  • Eppendorf tube
  • 95% ethanol

 

Procedure

  • Put strawberry into freezer baggie
  • Mash for 2 minutes
  • Add 10 ml DNA extraction solution into baggie
  • Gently mix strawberry mash solution for 1 minute
  • Cool baggie in ice bath (to precipitate proteins)
  • Filter mashed solution through filter into clear plastic cup (that is in ice bath if using ethanol.)
  • Allow filtered DNA solution to cool a minute.
  • Carefully pour alcohol along side of cup. Use 2x the volume alcohol as DNA solution.
  • Let the two layers of liquid settle 3 minutes.
  • Extract DNA by rotating a glass rod in solution and reeling out DNA strands onto rod.

Background

DNA is abundant in strawberries and relatively easy to isolate from strawberries because they are octaploid, that is, each cell contains 8 sets of chromosomes.

Questions to Ponder

Q:What was the purpose of mashing up the strawberry?
A: To break down the cell wall, cellular and nuclear membranes.

Q: Where is DNA found in strawberry cells?
A: In the nucleus.

Q: What is the purpose of detergent in the extraction solution?
A: Soap dissolves cell membranes so that DNA is liberated.

Q: What is the purpose of EDTA in the extraction solution?
A: EDTA prevents DNA breakdown. It removes (“chelates”) minerals that are needed by the enzyme, DNAse, which breaks down DNA.

Q: Why is the solution filtered?
A: This filters out larger particles, such as cell walls, etc.

Q: What happened when alcohol was added to the solution?
A: The DNA is precipitated out of the solution and formed long threads. DNA is negatively charged and remains soluble in water. DNA precipitates out in non-polar solvent, such as alcohol.

Q: If DNA is so small, how come we can see it?
A: Many threads of DNA bundle together so that we can see it.

 

 

Adapted from Carl Vermeulen at scienceprojects.com and from Berry Full of DNA by http://www.mrsortbiology.com/berryfullofdnalab.doc.

“DNA is an abbreviation for deoxyribo-
nucleican- tidisestablish-
mentarianism, a complex string of syllables.”
~ Dave Barry