|
This experiment is best done in groups of 4, but can be accomplished by a single person. The protocol below is designed to involve 4 people, but it can be tailored to accommodate smaller groups. The first protocol is a detailed example of how to incorporate all 4 member of the group, a simplified version is also supplied below. Make sure you have sufficient supplies for each group, please see the supplies list at the end for details. I found that it was easiest have the reagents made in advance, and have the teacher assist you in making aliquots prior to each class.
Reagent 1 is the Soap and Salt mix used to break apart the cell membranes, and assist in DNA precipitation.
Reagent 2 is the 95% Ethanol used to make the DNA fall out of solution. DNA is repelled by the hydrophobic environment of the alcohol causing it to stick together. The salt helps the DNA clump together and fall out of solution.
Procedure:
-
Break students into groups of 4 and have them count off 1-4.
Be sure and remind them that while you aren't using any dangerous chemicals, it is important to treat all scientific reagents as though they were poisons, and that eating/drinking is prohibited in the lab.
-
Student number 1 places the strawberry in a Ziplock bag and smashes it to a liquid pulp. ( ¼ to ½ of a strawberry is sufficient for each group.)
-
The same student then adds the 1ml aliquot of Reagent 1 to the Ziplock bag/strawberry pulp and continues smashing. The goal is to make sure Reagent 1 evenly mixes with the pulp.
-
Student number 2 will take the coffee filter and hold it in funnel like position over the empty Reagent 1 tube (held either in a rack, or by student 3). Student 1 opens the ziplock bag pouring the mix of Strawberry Pulp/Reagent 1 through the filter and back into the Reagent 1 tube. The filter should collect all of the large leftover strawberry chunks while allowing the Reagent 1 and DNA to pass through the filter.
-
Student number 4 will add the 2.5 ml of Pre aliqouted Reagent 2 to the filtered strawberry pulp/Reagent 1. Student 4 then takes a glass rod and spins it in circles, and a white stringy precipitate will form like cotton candy around the rod (that is the DNA!). The DNA can be removed from the tube using the glass rod. Make sure the students pass it around the group so everyone can get a closer look.
If the lab is equipped with microscopes it can be fun to let the students observe the DNA, alternatively, you can take a picture of the DNA using a microscope camera and show it to the class (this will save time and complications of mounting the DNA
etc. It depends on the age group you are dealing with.)
Abridged Procedure:
-
Place ½ - ¼ of a strawberry into a zip lock back and crush into pulp
-
Add 1ml of Reagent 1 to the pulp and continue crushing and mixing in the bag.
-
Strain the strawberry pulp/Reagent 1 through a coffee filter into a test tube (you can reuse the tube that contained the 1ml of Reagent 1).
-
Add 2.5 ml Reagent 2 (95% EtOH) to the Strawberry flow through / Reagent 1
-
Use a glass rod to mix the contents together until the white precipitate forms. Spinning the rod will make the DNA wind around like cotton candy on a stick.
Supplies needed per group:
2 Test tubes
1 ziplock bag
1 coffee filter
1 glass rod
½ - ¼ of a strawberry
1 ml Reagent 1
2.5 ml Reagent 2
Reagent 1: 3M NaCl, 5% Dish soap (87.6 g NaCl, 25ml Dish Soap in 500 ml H2O)
Reagent 2: 95% Ethanol (95ml 100% Ethanol + 5ml H2O)
Extra Notes:
I found it helpful to have the teacher aliquot the reagents while I talked to the class about the experiment. That way the students don't have to worry about measuring or getting the correct ratios of the reagents together.
|
