Tuesday, March 10, 2009

DNA Isolation

And our own DNA, at that. This lab never fails to interest kids, in my experience. I have isolated DNA from peas several times but have only once before tried human DNA isolation from cheek cells.

The pea DNA lab that I have used many times, and that never fails, comes from this site, http://learn.genetics.utah.edu/content/labs/extraction/howto/, a site with many good labs and tutorials. This website, http://www.pbs.org/wgbh/nova/teachers/activities/2809_genome.html, provided the outline for how I did the lab Monday. I am just not good at following directions as written, so of course, I altered it a bit.

Before lab, I prepared

  1. a 0.9% salt water solution, mixing 2 teaspoons of salt in a liter of water
  2. a 25% solution of dishwashing detergent, mixing 1 part detergent to 3 parts water
  3. chilled isopropyl alcohol

We started by reviewing general genetics and cell structure. DNA has to be liberated from within the nucleus of the cell, so both the cell membrane and the nuclear membrane have to be broken. These membranes are made of lipids and proteins. Soap binds them, just like it does with the lipids, or grease, on your dishes. This disrupts the membranes and the DNA is released. DNA is soluble in water but not in alcohol so we will be able to see the DNA as it separates in the alcohol.

Also before lab, I

  1. Added about a teaspoon of detergent mixture to a test tube for each participant. These were placed in a baby food jar which served as the test tube holder.
  2. Poured about a teaspoon of the salt water in a small paper cup for each participant.
  3. Poured 2 or 3 teaspoons of alcohol into 2 paper cups and placed back in the freezer. These cups can be shared.


  1. Take a toothpick and gently scrape the inside of a cheek.
  2. Then, while holding the toothpick, swish the salt water in the scraped cheek, and spit the water back into the cup.
  3. Stir the toothpick end in the cup and then pour the cup contents into the test tube. The paper cup can be folded to form a spout for easier pouring.
  4. Place a stopper in the end of the tube and, holding the tube with a thumb on the stopper, carefully rock the tube back and forth for 2 minutes to mix the solutions. This needs to be done slowly to avoid breaking the DNA strands into smaller pieces.
  5. Tilt the test tube and pour the cold alcohol down the inside of the tube to form a layer of alcohol on top of the soapy layer. Try to avoid mixing the layers.
  6. After watching the tube for a minute, put a glass rod or toothpick in the tube and gently swirl the rod. Strands of DNA should be visible and sometimes may be wrapped around the end of the rod. You may be able see the DNA if you click on the photo.
  7. The DNA may be kept in a small tube with some alcohol solution.

Science is fun. PRM


Lorna said...

That is wonderful. We must try this at home. Thank you so much for posting the pictures and directions.

Think! said...

I love studying DNA!