DNA Extraction from Onion - Teacher Guide "Chemical version"

How do you purify DNA from cells? Students extract DNA from onions to learn about the chemical and physical properties of DNA. This activity provides a first-hand understanding of how DNA can be isolated for further analysis, such as DNA fingerprinting. Students also reinforce their understanding of cell structure and biological macromolecules.

Classroom time needed for this lab:

• one (50 minutes) or two (total time 70 minutes) class periods

Materials you will need to prepare the reagents (recipes to follow):

If you do not have the chemicals available, you can prepare solutions that use common household supplies. Refer to the alternative kitchen version of this lab.

Setting up the classroom for this lab

• 50ml conical tube (one for each pair of students)
• 15 ml conical tube containing ~10ml 50% ethanol
• Cheesecloth (need one piece for eight students)
• Funnel (need one item for eight students)
• Plastic sticks
• Onion lysis solution (in a small bottle)
• Isopropanol (in a small bottle)

Have a hot water bath in an accessible location so that students can incubate their onion mixtures. Have at least two blenders available in the classroom. You will need an ice chest filled with ice to cool down the mixtures after blending.

Recipes

Onion Lysis Solution

1. Be sure to wear a mask (sodium lauryl sulfate is an irritant). Add to a large flask or beaker (at least one liter size):
   
   

   50.0 g	Sodium laurly sulfate (SDS)
   8.8 g	Sodium chloride
   4.4 g	Sodium citrate
   0.3 g	Ethylenediamine tetraacetic acid (EDTA)

   
   
   
2. Add 800 ml deionized or distilled water to the dry ingredients. Mix the solution on a magnetic stirrer until all the ingredients are dissolved. A heated stirrer works best at dissolving the sodium lauryl sulfate.
   
   
3. Add solution to a 1000 ml graduated cylinder. Add deionized or distilled water to obtain a final volume of 1000 ml.
   
   
4. The onion lysis solution can be stored indefinitely at room temperature in a closed container.

50% Ethanol

1. Mix 200 ml of 95% ethanol with 180 ml of deionized or distilled water. Alternatively, mix 200 ml of 100% ethanol with 200 ml of deionized or distilled water.
   
   
2. The 50% ethanol solution can be stored indefinitely in a capped bottle.
   
   

Sources of laboratory materials