Isolating DNA from Strawberries

Most people don’t get to see what DNA looks like, so it’s hard for them to grasp it as a tangible thing (no pun intended).  It’s microscopic, but like all tiny things, if you get a bunch of it together, it becomes something real before your eyes.

I isolated DNA from strawberries with Number 1’s 2nd or 3rd grade class a few years ago.  Although they were a bit young for it, it was still fun and very successful.

You and your kids can isolate the DNA from strawberries (or other fruit) using a few simple ingredients.  It’s a fun experiment to show what is on the inside of cells.  Strawberries work the best because every strawberry cell has 8 copies of each chromosome (human cells only have 2 copies).  This lets the plant produce fruit quickly in the spring.  Bananas or kiwi fruit can also be used, but they have less DNA.

Materials you will need:

• Fresh or frozen strawberries brought to room temperature
• Non-iodized table salt or lab grade sodium chloride
• Ziploc bags (use strong bags!)
• Dish detergent
• Water
• Funnels
• Coffee filters
• Plastic cup
• Glass cups
• Tooth picks
• Ice-cold isopropanol (rubbing alcohol; keep in freezer or on ice)
• Saran Wrap

Procedure:

1. Mix one half teaspoon of salt, one third cup of water and one tablespoon of dishwashing liquid in a glass or small bowl.  Set aside.
2. Remove the green tops from the strawberries and put 3 strawberries in each Ziploc bag. Seal the bag and squash strawberries using hands until there are no large clumps.
3. Add 3 tbsps of the detergent mix to the bag, and mix with strawberries trying not to make bubbles.
4. Put funnel in cup. Fold coffee filter to make a cone and place in funnel.
5. Lightly wet coffee filter.
6. Pour strawberry mixture into funnel and allow to seep through into cup.
7. Put the strained strawberry liquid into a glass cup.  Estimate the amount of liquid in the cup.
8. Add 2 volumes of ice-cold alcohol.
9. Slowly mix the alcohol into the strawberry liquid.  The DNA should “fall” out of solution and look like a white string or gooey glop.  Adding more isopropanol may turn the gooey glop into a white string.
10. Pull DNA out of the cup using a toothpick and put it on the saran wrap.

 How it works:

Strawberries have natural enzymes that help break down cell walls.  Detergent and salt break down lipids and proteins.  When you crush the strawberries and add the detergent/salt solution, the combination of the enzymes, soap and salt dissolves the cell wall structures, allowing the DNA to be released from the nucleus.

The DNA is released into the liquid.  Filtering through the coffee filter separates the liquid containing the DNA from the rest of the cellular debris.

The salt in the solution also makes the DNA less hydrophilic (less likely to interact with water). When you add alcohol to the DNA solution, the alcohol makes the DNA even less hydrophilic, causing it to repel water, amass together and “drop” out of the solution.

The higher the concentration of DNA you have in the solution, the more likely it will come together.  Therefore, strawberries are an excellent choice for this experiment due to their high DNA content.

Helpful hints:

• If you are going to do this with a group of kids, I would pilot the experiment first and make sure you have it working.
• The higher the percentage isopropanol you use, the better it will work.  You can buy 99% isopropanol on Amazom.com.
• Make sure the isopropanol is ice-cold.  It won’t freeze in the freezer, so keep it there if you can.

MadScientist.CrazyMom’s rating:

• Grade level:  Probably better with middle school-aged kids, but can be done with younger children.
• Difficulty rating:  ♥♥♥♥ (out of 5 hearts)

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