How to Extract DNA from a Banana

I. Jansons

 

 

What’s the point?

                                                         

To see DNA.

     

How do you do it?

 

Detergent

NaCl (salt)

Alcohol

 

What do you need/What do you do?

½ of a banana                    

1 drop of dish soap

tiny pinch of salt

3-4 Tbsp water

 

  1. Put the above in a Ziploc bag and mash it together.
  2. Let the bag sit for 5 minutes.  Use this time to either discuss what DNA might look like or why you added the salt and detergent and are mashing it.
  3. Strain the mix through a coffee filter (you may have to squeeze it gently) into a small glass beaker (50mL).  You want to have around 10mL (2tsp) for each extraction.
  4. Tilt the beaker and slowly pour ice-cold rubbing alcohol (isopropyl alcohol) down the side so you end up with an alcohol layer 2x the volume of the banana mix.  The alcohol is less dense than the banana liquid so it will float on top.
  5. Using a toothpick you can gently spool the DNA.

 

Safety

 

Isopropyl alcohol is flammable and can be a fire risk, so keep it away from any flames or sources of ignition.  It is also toxic if ingested.  Wear appropriate protection.

 

The Explanation

 

Since DNA is the blueprint for life, everything living contains DNA.  Our source of DNA is a banana.  The detergent and the mechanical ‘mashing’, break down the banana cell walls, plasma membranes and nuclear membranes to release the DNA.  The detergent emulsifies the lipid bi-layer of the membranes.  We wash our hands with soap to destroy bacteria/germ cells.  DNA has a net negative charge.  The sodium ions of salt (NaCl) neutralize/shield and stabilize the DNA molecules, allowing them to move closer together.  The alcohol dehydrates the DNA so that it precipitates out of solution.  This happens at the interface between the banana liquid and the alcohol.  The protein and grease that we broke up in the first step prefer the bottom, watery layer, while the DNA prefers the top, alcohol layer.  DNA is a long, stringy molecule that likes to clump together.

 

Why does the DNA clump together?

Single molecules of DNA are long and stringy. Each cell of your body contains six feet of DNA, but it's only one-millionth of an inch wide. To fit this entire DNA into your cells, it needs to be packed efficiently. To solve this problem, DNA twists tightly and clumps together inside cells. Even when you extract DNA from cells, it still clumps together, though not as much as it would inside the cell.

Imagine this: the human body contains about 100 trillion cells, each of which contains six feet of DNA. If you do the math, you'll find that our bodies contain more than a billion miles of DNA!

Note:  Ice-cold alcohol works best but you can also use alcohol at room temperature.

Moving On!

·        Experiment with other DNA sources. Which source gives you the most DNA? How can you compare them?

·        Experiment with different soaps and detergents. Do powdered soaps work as well as liquid detergents?

·        Experiment with leaving out or changing steps. Try changing how much of each ingredient you use.

Sources

Genetic Science Learning Center @ The University of Utah (2005)

http://gslc.genetics.utah.edu/units/activities/extraction/

 

Modified from Bilash, B. & M. Shields. A Demo a Day:  A Year of Biological Demostrations. Flinn Scientific., Inc. Batavia, Il.

 

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