Biology Lab: Extracting DNA From Anything Living!                Name:__________________

                                                                                        Lab Partners:_______________________

Period: 1 2 3 4 5 6 7 8                                                                          _______________________

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Introduction:

DNA exists in the cells of all living things.  Eukaryotic cells have their DNA coiled tightly inside a nucleus and bound to special proteins.  Some eukaryotic DNA is also found outside the nucleus in mitochondria.  Extracting DNA from a sample involves breaking open the cell membranes and sometimes a cell wall.  Conditions then must be created that will allow the strands of DNA to intertwine and clump together into an observable white mass.  Since DNA molecules are negatively charged, they repel one another.  The addition of salt ions (NaCl) that are positive in charge will neutralize the DNA strands and allow them to clump.  The lipids and proteins that form the cell membranes must be disrupted to release the DNA into the solution.  Sodium lauryl sulfate, an active ingredient found in most detergents, can break the lipid bonds that hold the cell membranes together. Since DNA is less dense than water and most cellular debris; it can be precipitated into a layer of alcohol in the top of your test tube which is less dense than the soap and water.  This is sort of like looking around the room for the most comfortable seat.  Some will choose the couch; others might choose the glider chair.  In this case, the protein and fat parts find the bottom, watery layer the most comfortable place, while the DNA prefers the top, alcohol layer.  The DNA is a long, stringy, white molecule that will tend to clump together under these circumstances.

 

Objective:  To extract DNA from the cells of animals.

Safety: Wear goggles to avoid direct contact with any of the solutions.

 

Materials:  

Plastic cup                                                       Test tube

Cold water                                                      Test tube rack

8% NaCl solution                                            Rubber stopper 25%  

Ice Cold 90% Isopropyl Alcohol                     Large paper clip

Liquid Dish Detergent (green pamolive)           Your Cheek cells

10 mL graduated cylinder

 

 

Procedure: Animal Cell DNA extraction

 

Collection of cheek cells

1.      Add 20 drops of 8% NaCl solution to the test tube. Set the test tube in the test tube rack.

2.      Use a marker to draw a line on your plastic cup one cm from the bottom of the plastic cup.

3.      Fill your cup with cold tap water up to the marked line.

4.      Put the water in your mouth and swirl the water vigorously for one minute. Spit the water back into the cup. (swirling the water washed cells from the inside of your cheek into the water.)

5.      Pour about half of the water/cheek cell mixture into the test tube.

6.      THROW THE PLASTIC CUP AWAY!!  GROSS!! YOU SPIT INTO THAT CUP!!!!

Releasing  the DNA from the cheek cells

7.      Add 20 drops of 25% detergent solution to the test tube.

8.      Stopper the test tube and nix the contests be gently turning the tube upside down three times. DO NOT SHAKE THE TUBE!!

 

Precipitating the DNA

9.      Holding the test tube at a slight angle, carefully slide 5 mL of isopropyl alcohol down the side of the test tube so it forms a layer over the “cheek cell” mixture in the test tube.

10.  Hold the tube upright and observe what happens at the interface between the isopropyl alcohol and the “cheek cell” mixture. (you are looking for clouds of white strands. That is your DNA.)

11. Open the large paper clip to make a hook and try and collect the DNA by gently hooking the DNA and pulling out of the test tube. It should look long and stringy.

Questions:

 

  1. What was the source of the DNA in this experiment?

 

 

  1. What was the purpose of adding the NaCl (salt) solution to the DNA?

 

 

  1. What was the purpose of adding the detergent to the DNA?

 

 

 

  1. What did the isopropyl alcohol do to the DNA and other cellular debris in the test tube?

 

 

 

  1. Describe the appearance of the human cheek cell DNA inside of the test tube.

 

 

 

  1. How was the structure of the DNA once you pulled it out of the test tube using the paper clip similar to the structure you learned about in class? (Keep in mind that you CAN’T see the ladder structure so describe what you DO see.) 

 

 

 

  1. The cheek cells you removed from your mouth were dead, yet you were still able to extract DNA from them.  How could this be useful in science and how might this be useful in a criminal investigation?  Be sure to answer both questions.