Barbara Lillien                                                                                                                             Return to Biology Menu

South Shore H.S.

Summer 2001





SKILLS:                Extracting DNA from an animal cell

OBJECTIVES:     Separate and collect the DNA from liver cells

                               Describe the appearance of DNA extracted from a cell

                               Relate the location of DNA in a cell to procedures for extracting it



safety goggles                 cheesecloth (several pieces, 12cm X 12 cm)             funnel  

lab apron                         beef liver ( 2 cm square/group of 2 students)           test tubes

mortar & pestle              ice water bath                                                            fine sand

test tube rack                  graduated cylinder                                                    70% Ethanol (4ml)

SDS/NaCl solution(10ml)                                                                                  inoculating loop




Unifying Concept:                        Evidence, models, and explanation

Science & Technology:               Understanding how science and technology come together

Life, Physical, Earth Sciences:   Molecular basis of heredity, Structure and properties of matter



You are an intern working in the city’s forensics lab.  You will be assisting the forensics technician with many of her routine laboratory tests and procedures.  One procedure the technician does frequently is extract DNA from cells and purify it.  The purified DNA is used to help prepare a DNA fingerprint to help solve crimes.  To make sure you know how to do this procedure correctly, the technician has asked you to extract DNA from the cells of a piece of beef liver and spool the DNA for observation.



The extraction of DNA from cells and its purification are of primary importance to the field of biotechnology.  Extraction and purification of DNA are the first steps in the analysis and manipulation of DNA that allow scientists to detect genetic disorders, p[produce DNA fingerprints of individuals, and even create genetically engineered organisms used to produce beneficial products such as insulin, antibiotics, and hormones.

The process of extracting DNA, regardless of its original source, involves the following steps.  The first step in extracting DNA from a cell is to LYSE, or break open, the cell.   One common way to lyse cells is to grind a piece of tissue along with a mild abrasive material in a mortar with a pestle.  After the cells have been broken open, a solution containing salt (NaCl) and a detergent containing the compound SDS, or sodiumdodecyl sulfate, is used to break down and emulsify the fat and proteins that make up the cell membrane.  Finally, ethanol is added.  Because DNA is soluble in water, the addition of ethanol causes the DNA to PRECIPITATE, or settle out of solution, leaving behind all remaining cellular components that are not soluble in ethanol.  Finally, the DNA can be wound (spooled) onto an inoculating loop, and pulled from the test tube.



1. Put on safety goggles and lab coat

2. Place a piece of liver in a mortar. Add several grains of sand.

3. Pour 10ml SDS/NaCl solution in the mortar.

4. Use a pestle to grind the ingredients until they form a thick fluid. (do not overgrind mixture)

5. Place several layers of cheesecloth into a funnel. Pour the contents of the mortar through the cheesecloth into a test tube until it contains at least 2 ml of the extract. ( gently squeeze the cheesecloth to remove all the fluid from the cheesecloth.)

6. Place the test tube in an ice-water bath

7. Measure 4 ml of ice-cold ethanol in a clean graduated cylinder

8. Hold the test tube at a 45 degree angle.

a) slowly pour the ethanol slowly down the side of the tube (do not pour the ethanol too fast or directly into the liver solution)

b) as you pour the ethanol into the test tube, observe the space between the ethanol and the liver solution( the interface line), this is the DNA that you will be spooling onto the inoculating loop.

9. Gently insert an inoculating loop into the test tube as far as the interface line.

10 slowly move the loop in circles, (this motion spools the long threads of DNA around the end of the loop.

11. Lift the inoculating loop out of the solution in the test tube.  While the DNA is being pulled out of the test tube, try stretching it.  Then dip the inoculating loop again to get more DNA.

12. When spooling is complete, pull the inoculating loop from the test tube, and return the test tube to a test-tube rack.

13. Clean up your work area and wash your hands before leaving the lab.



1. Describe the appearance of the DNA you spooled from the crushed liver cells.


2. What was the purpose of adding sand to the liver in the mortar?


3. What happens to the cell when the SDS/NaCl solution is added to the liver mixture in the mortar?


4. What was the purpose of filtering the liver mixture through cheesecloth?


5. What was done to the DNA so that it could be observed and spooled?


6. How can you determine whether the material pulled from the test tube was DNA?


7. How is DNA protected inside an animal cell?  How does this location relate to the procedure you used in this lab to extract DNA?


8. Biotechnologists research DNA. How could the procedure you used to day facilitate their research?



1. Find out what a DNA fingerprint is and how it is used to compare samples of DNA from different sources.  Explain how the technique you learned in today’s lab is used in developing a DNA fingerprint.


2. find out about the Human Gnome Project.  What is this project attempting to do?  How would the procedure used in this lab be used as part of the Human Genome Project