DNA EXTRACTION LAB
Name ____________________
Introduction
The process of obtaining DNA from cells is the first step in many biotechnology laboratory procedures.
Researchers must be able to separate the DNA gently from the unwanted substances in the cells so the DNA is not
broken up or sheared. The procedure for this laboratory exercise is a modified version of the Marmur preparation.
The Marmur preparation is used worldwide in biotechnology laboratories.
In this laboratory exercise you will make an extract of strawberry treated with salt, distilled water, and
detergent. The salt causes the negative phosphate ends of the DNA molecules to come closer together so they will
precipitate out of a cold alcohol solution. The detergent first disrupts the cell membranes. The detergent then forms
complexes with the lipids and proteins released from the cell, causing them to precipitate out of solution. Finally,
cold alcohol will be added to the extract to separate out the DNA.
How is DNA extraction useful to scientists? When do they use such a protocol, and why is it important?
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The extraction of DNA from a cell is often a first step for scientists who need to obtain and study a gene.
The total cell DNA is used as a pattern to make copies (called clones) of a particular gene. These copies can
then be separated away from the total cell DNA, and used to study the function of that individual gene.
Once the gene has been studied, genomic DNA taken from a person might be used to diagnose him or her
with a genetic disease. Alternatively, genomic DNA might be used to mass produce a gene or protein
important for treating a disease. This last application requires techniques that are referred to as recombinant
DNA technology or genetic engineering.
I. Materials
- Strawberries
- Salt
- Test Tube
- plastic bag
- 2 plastic cups
- Stirring rod
- Graduated cylinder
- Beaker
- Cold Isopropyl Alcohol
- Detergent
- Filter paper
II. Procedure
1. Place a strawberry and about 20 mL of water into a plastic bag. Carefully mash a one strawberry. Mash for 3-4
minutes until strawberry mixture is liquid-like.
2. Extraction solution: Measure out about 10mL of detergent, a pinch of salt, and 20mL of water into a container.
Mix well but not enough to make bubbles. Stir for 2 minutes, until salt dissolves.
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The detergent dissolves the lipids that hold the cell membranes together – this releases the DNA into the
solution. The detergent further causes the lipids and proteins to precipitate out of the solution, separating
them from the DNA.
3. Add strawberry to the extraction. Stir carefully for 7 minutes. Be careful not to make bubbles.
4. Dampen (with water) a coffee filter or thin filter paper (fold the filter into a cup-like shape) and
place it over a beaker. Make sure the filter paper does not touch the bottom of the beaker. *** Make a sketch of
this in your lab notebook ***
5. After 10 minutes or once no more liquid is flowing through your filter paper pour out filtered strawberry juice
and put it in a glass test tube. You should have a reddish liquied at the bottom of your test tube.
6. Discard of filter paper and strawberry pulp.
7. Using a pipette carefully and slowly pour 5 mL of cold alcohol down the side of the tilted test tube so that the
alcohol remains in a layer above the juice. Because the alcohol has a lower density, the alcohol will “float” on top
of the juicy solution.
8. Let the solution sit for 2-3 minutes. You will see DNA precipitate out into the alcohol layer. A white
substance will become visible at the interface where the two liquids meet – this is DNA! DNA is clear
or may appear kind of white. You may have a tough time seeing it at first. Bubbles will form on the
DNA strands, and this should help you to recognize it. *** Make a sketch of this in your lab notebook *** Label
where the alcohol is, the strawberry pulp, and the DNA.
The alcohol will form a layer on top of the cell scum. The cell scum is heavy and will remain on the bottom of the
vial. The DNA is less dense than the cell scum, but more dense than the alcohol. Thus, the DNA will stay in the
middle, where the two layers meet. Since DNA cannot remain dissolved at the alcohol concentration used here, it
begins to come out of solution or precipitate. When it does, the DNA will start to rise up into the alcohol layer.
Precipitate: to separate (a substance) in solid form from a solution, as by means of a reagent.
Optional Activity: Perform this step carefully because this will sometimes cause your DNA to break up into small
pieces. You may use a glass rod to slowly swirl the DNA in the alcohol layer (not the lower layer!). You will begin
to be able to SEE the DNA. See if you can swirl enough DNA onto your rod to pull it out of the test tube.
HOMEWORK
III: ANALYSIS
Answer the following questions in your lab notebook using complete sentences.
1. What is the purpose of the salt solution in the beginning of the procedure?
2. What is the purpose of the detergent during the extraction process?
3. Why do you think alcohol is used in this experiment to precipitate DNA and not a different solution?
4. Describe the characteristics of the extracted DNA, such as color, shape, size, and consistency.
5. Where is the DNA found in the cell?
IV: APPLICATION
Now that you've successfully extracted DNA from one source, you're ready to experiment further. Since geneticist
commonly use DNA in a variety of different ways we would like you to design controlled experiments to determine
the amounts of detergent, salt, and alcohol that would yield (produce) the most amount of DNA. Your task is to set
up the variables needed to conduct 2 controlled experiments to determine the optimal conditions to extract the
highest amount of DNA. For each experiment you must list the following:
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Purpose
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Independent Variable (Which of the 3 components are you testing? What amounts will you use?)
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Dependent Variable (How will you specifically measure your results?)
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Control Variables (List at least 3 variables that need to be kept constant and explain why each has to be
kept constant)
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Example: “Amount of the salt was kept constant because its amount might have an effect on the
amount of DNA.”
o
DO NOT use the following for control variables: student conducting experiment, number of trials,
or other pieces of equipment that do not matter.
THIS LAB WRITE - UP IS DUE FRIDAY, 2/24. BE VERY DETAILED WITH
YOUR EXPERIMENTAL DESIGN IN ORDER TO RECEIVE FULL CREDIT. A
TYPED VERSION PREFERRED BUT NOT MANDATORY.