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7.3: DNA Extraction

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  • Introduction

    DNA is the instruction manual for living things. By far, the greatest amount of DNA is located in the nucleus of eukaryotic cells and described as a double-helix. The double-stranded genetic blueprint, runs antiparallel, with bases bonding in a complementary fashion, ensuring that with every round of replication or transcription, a parent (or template) strand remains.  This semi-conservative replicative strategy ensures the integrity of the code, for the proteins that result from the nucleotide sequence are vital to the cell.   Every cell that comprises a living organism contains the complete genetic blueprint of that organism, what enables the specialization of a particular cell in a particular area of the body is control over which genes are expressed and when. 


    As every cell contains DNA, isolating the genetic sequence of an organism from a subset of cells, such as blood, sweat, and skin, to name a few. Once a sample has been obtained, the DNA must be released from the nucleus, which involves the physical disruption of the cell.  After the cells have broken open, a salt solution (NaCl) and a detergent solution containing the compound SDS (sodium dodecyl sulfate) are added, to breakdown the cell membrane. Finally, ethanol is added causing the DNA to precipitate (settle out) of the solution, leaving behind all the cellular components that aren't soluble in alcohol. The resulting DNA can be spooled (wound) on a stirring rod and pulled from the solution at this point.


    The extraction and purification of DNA are of primary importance to the field of biotechnology and forensics. DNA extraction allows for analysis including the detection of genetic disorders, identification of individuals using DNA fingerprints and studies involving GMOs (genetically modified organisms). The genetic code is universal, which means, traits from one organism can be expressed in other, nonrelated organisms. Genetic engineering is the directed addition of new nucleotide sequences to an organism's genome, which can be employed to increase nutrient content, make animals more resilient, or better adapted for a region. While, often the subject of heated debate regarding safety, restrictions, and ethics, genetically engineered organisms could revolutionize agriculture and healthcare. Modifications could result in bacteria capable of producing insulin, antibiotics, and hormones.


    Part 1: DNA Extraction

    1. Obtain several models to compare, such as fruit, meat, and cheek cells.

    2. Put the first sample into a plastic Ziploc bag, seal thoroughly and gently smash the sample for about two minutes.

    3. In a plastic cup, prepare the extraction solution: mix together 2 teaspoons of detergents, 1 tsp of salt and ½ c water.

    4. Add approximately 2 teaspoons of the extraction solution to the specimen bag.

    5. Reseal the bag and gently smash for another minute (avoid making too many soap bubbles).

    6. Obtain a second cup and line with a filter. 

    a. Pour the liquid into the filter. 

    b. Filter can be compressed to squeeze any remaining liquid into the cup.

    7. Add an equal amount of alcohol to the cup

    a. Pour down the side of the cup to facilitate layering of solutions. 

    b. Do not mix or stir.

    8. Watch for the development of a white cloudy substance (this is the DNA) in the top layer above the extract layer.

    a. Tilt the cup and pick up the DNA using a wooden stirring stick or tweezers.

    b. Set aside.

    9. Repeat procedure using several alternative food sources.


    Part 2: Extract Human DNA

    1. Mix the bottled water with the salt in a cup. Stir until salt is dissolved.

    2. Gargle the salt water solution for approximately 1 minute.

    a. Don't swallow it!

    b. Spit the water back into salt solution cup.

    3. Add one drop of dish soap to the salt water.

    a. Stir gently.

    b. Try not to create any bubbles.

    4. Add an equal amount of alcohol (plus optional food coloring) into the salt water cup.

    a. Pour down the side of the cup to facilitate layering of solutions. 

    b. Do not mix or stir.

    5. Wait for 2.5 minutes. You should see white clumps and strings forming.

    a. Tilt the cup and pick up the DNA using a wooden stirring stick or tweezers.

    b. Set aside.




    Sample Extract Positive or Negative for DNA (+ or -)



    1. Describe the role of the detergent in DNA extraction.






    2. Describe the role of the salt solution in DNA extraction.






    3. Describe the role of alcohol in DNA extraction.