A Fun Way to Introduce DNA to Any Age Group
presented by Kristin Majda, M.S., M.B.A.
Some things to remember when doing the Strawberry DNA Extraction Lab/Activity:
  • DNA has an overall negative charge which causes it to dissolve in water, which is very polar
  • The DNA extraction buffer is composed of water, detergent and salt
  • Recall that plant cells have a cell wall and a cell membrane. Crushing the strawberries helps to break open the cell walls. The detergent helps break open the cell membrane of the strawberry cells because detergent molecules have a polar end and a non-polar end, just like the phospholipid molecules that make up the membrane, so the detergent molecules help separate the phospholipids from one another
  • The salt, which is an ionic compound, dissociates in water to form NA+ ions and Cl- ions. These ions are strongly attracted to the polar water molecules. The NA+ ions also stick to the negatively charged DNA, but because water is so polar it out-competes the DNA for the Na+ ions.
  • However, when the alcohol layer is added, the DNA precipitates out of solution. This is because the alcohol is polar but LESS polar than the water so it does not out-compete the DNA for the Na + ions. Thus the Na+ ions stick to the DNA neutralizing the overall charge and making the DNA non-polar. Since non-polar molecules do not like to mix with polar molecules (think oil and water) the DNA clumps up. But if you put the DNA in water, the Na+ ions would come off and the DNA would dissolve into the water. So be sure to preserve the extracted DNA in 90% or greater alcohol for best results.
  • Reminder: I bought the 90% isopropyl alcohol from Walmart. But you can get stronger concentrations at some drug stores and most high school labs already have 99% or 100% concentrations of isopropyl alcohol.
  • If you use Amazon, here are some links to supplies that you might find useful:
    - 99% isopropyl alcohol (more expensive than buying it locally, but more convenient)
    - Pipettes (for transferring the isopropyl alcohol into the vials, stirring, and lifting out the DNA)
    - Screw-lid vials or snap-closed vials (for storing the DNA - kids love taking their DNA home and it works well with the flip books to tape them onto the DNA page, the screw top are more costly but less likely to leak) - you could also use microfuge tubes, which are pretty cheap and easy to get donated by local biotech companies (often these come unpacks of 500 so will last you a few years).
  • Small medicine cups for dispensing the alcohol to students.