HOW TO EXTRACT DNA FROM ANYTHING LIVING
TEACHER NOTES
BACKGROUND:
In this experiment, students will extract DNA from a variety of sources, using inexpensive reagents and minimal
equipment. The resulting DNA can be used for further experimentation/ investigation if desired. In order to
maximize the outcomes of this learning experience, students should already have a basic knowledge of cellular
structure and function. Students should appreciate that the process of extracting DNA from a cell is the first step
for many laboratory procedures in biotechnology. Scientists must be able to separate DNA from the unwanted
substances of the cell gently enough so that the DNA does not denature (break up).
Students will prepare DNA from sources subjected to mechanical, chemical and biological disruptions. The DNA
source is blended, treated with salt, detergent, and enzymes. The blending disperses the tissues of the DNA
source into single cells and helps to physically disrupt the cell wall in plant tissues. The salt chemically disrupts
the cell membranes/walls, and enables the DNA strands to come together and the DNA molecules to precipitate
out of solution into the alcohol layer. The detergent and enzymes break down the cell membranes by dissolving
the lipids (fatty molecules) and proteins of the cell respectively and disrupting the bonds that hold the cell
membrane together. In addition, the enzyme breaks down the chromosomal proteins to help liberate the DNA.
The detergent then forms complexes with these lipids and proteins, allowing them to be filtered out of solution,
leaving the cells' DNA in the filtrate.
ANSWERS TO STUDENT QUESTIONS:
1. Why was salt added to your DNA source?
The salt chemically disrupts the cell membranes (and walls), enables the DNA strands to come
together and the DNA molecules to precipitate out of solution into the alcohol layer.
2. How does blending help you to extract the DNA?
Blending disperses the tissues of the DNA source into single cells and helps to physically disrupt
the cell wall in plant tissues.
3. Why did you add detergent?
The detergent breaks up cell walls and membranes, releasing DNA from the nucleus.
4. Why did you place the mixture at 60-65C?
Heating makes the chemical process more efficient.
5. What is an enzyme, and how does it help in this procedure?
An enzyme is a molecule that speeds up reactions. In this procedure it helps to break down the
cellular and chromosomal proteins more efficiently.
6. Why does the alcohol need to be at least 70% and cold?
DNA is soluble in aqueous solution but precipitates as a salt in alcohol. The higher the concentration
of the alcohol and the colder it is- the greater the yield of DNA.
7. Why does only the DNA enter the alcohol layer?
The rest of the cellular components have already precipitated out of solution at the detergent/
enzyme steps and have either already been filtered out or prefer to remain in the aqueous phase.
FURTHER CHALLENGES:
There are numerous simple alterations that can be made to the procedure to allow students to use the scientific
method to investigate their effects. See Student sheet.
DNA EXTRACTION- FREQUENTLY ASKED QUESTIONS
1. I'm pretty sure I'm not seeing DNA. What did I do wrong?
First, check one more time for DNA. Look very closely at the alcohol layer for tiny bubbles.
Often, clumps of DNA are loosely attached to the bubbles.
If you are sure you don't see DNA, then the next step is to make sure that you started with
enough DNA in the first place. Many food sources of DNA, such as grapes, also contain a lot
of water. If the blended cell soup is too watery, there won't be enough DNA to see. To fix this,
go back to the first step and add less water. The cell soup should be opaque, meaning that
you can't see through it.
Another possible reason for not seeing any DNA is not allowing enough time for each step to
complete. Make sure to stir in the detergent for at least five minutes. If the cell and nuclear
membranes are still intact, the DNA will be stuck in the bottom layer. Often, if you let the test
tube of source mixture and alcohol sit for 30-60 minutes; DNA will precipitate into the alcohol
layer.
2. Why does the DNA clump together?
Single molecules of DNA are long and stringy. Each cell of your body contains 2 metres of
DNA, but it's only one hundred thousandth of a millimetre 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.
INVESTIGATIONS OF THE CHEMICAL AND PHYSICAL CHARACTERISTICS OF DNA:
The characteristics of DNA can be investigated using either the extracted DNA or commercially prepared DNA.
The following suggestions for investigations demonstrate that DNA has characteristic properties that are
conserved across phylogenetic boundaries.
Concentration- If your school has a spectrophotometer, you can demonstrate that DNA has an optical
absorption maximum at 260nm.
Viscosity- As DNA is a large linear polymer, it tends to be very viscous in solution. As the DNA molecules are
progressively broken into smaller fragments, the solution will tend to become less viscous at the same
concentration. The solution’s resistance to being pipetted by a fine bore pipette can assess changes in the
viscosity of the solution. The greater the resistance, the more viscous the solution. Students can investigate the
effects on a solution of:
 Mechanical shearing- due to rough handling/ multiple pipettings.
 Heating- lowers viscosity as DNA becomes single-stranded as base pairs detach.
 Heating and cooling- several cycles of heating and cooling will lead to denaturation of the DNA.
 Alkali treatment- increasing concentrations of NaOH (greater than 30mM) will denature and
eventually destroy the DNA molecule.
 Nuclease treatment- causes DNA degradation
Size- DNA can be run through an agarose gel and visualised by staining to allow size determination.
Enzyme Digestion Patterns- Restriction endonucleases can be used to generate DNA ‘fingerprints’ for viewing
via electrophoresis.
Other Tests- There are simple tests for the components of DNA (purines, phosphate, deoxyribose) given at
http://tipnet.taranaki.ac.nz/~mfenton. Due to the nature of the tests, they are best carried out as teacher
demonstrations.
HOW TO EXTRACT DNA FROM ANYTHING LIVING
TEACHER POINTS TO VERBALISE
Reproduced with permission from IDPG instruction Manual (Bio-Tek Services Pty Ltd).
COLLECTING AND BREAKING OPEN CELLS
Step 1. Collecting cells
The first step in DNA isolation is the collection of cells. This can be any material which is living or was
recently alive.
Step 2. Lysing the cells and dissolving the phospholipid bilayer membranes
The first step in DNA extraction is to break open the cells. This is often facilitated by blending the tissue
and putting the cells into a solution that contains detergent. Detergent dissolves oil-based molecules, and
the cell and nuclear membranes are mainly oil-based (you may have already heard of cell membranes
being composed of “phospholipid bilayers”). The process of breaking open the cells is called lysis.
REMOVING PROTEINS
Step 3. Using protease to break down cellular proteins
DNA is packaged tightly around proteins (histones) so that it doesn’t get tangled inside the nucleus, and
these proteins interfere with the precipitation of pure DNA. We can easily get rid of protein without
damaging the DNA by using a specific enzyme that digests proteins, called a protease. Protease breaks
the peptide bonds between the amino acids of proteins. By destroying all the proteins you will also
eliminate DNases- enzymes that digest DNA (because enzymes are proteins).
MAKING THE DNA VISIBLE
Step 4. Making DNA insoluble
You will add salt solution to your sample, which will cause the DNA to become less soluble in the cell
extract. DNA has a negative electrical charge due to the phosphate groups on the DNA backbone. When
the salt is added, the positively charged sodium ions of the salt are attracted to the negative charges of
the DNA, neutralizing the electrical charge of the DNA. This allows the DNA molecules to come together
instead of repelling each other.
Step 5. Precipitating the DNA with cold alcohol
To separate the DNA from the other molecules in the cell extract, you will add cold alcohol to your
sample, which causes the DNA to precipitate because it is less soluble in alcohol than in water. The
colder the ethanol is, the less soluble the DNA will be in it. This is similar to the solubility of sugar in tea
(or any drink); sugar dissolves more readily in hot tea than in iced tea.
In the presence of high salt and cold alcohol, the DNA that was released from your cells precipitates and
aggregates until it can be seen with the naked eye as long, thin white strands. The other molecules in the
cell extract, such as the amino acids and carbohydrates, remain dissolved in the alcohol and water and
will not be visible. It takes many thousands of strands of DNA to form a fiber large enough to be visible.
Each strand will have thousands of genes on it, so you will be looking at material that contains millions of
genes at once. Remember though, that you are seeing the DNA from many thousands of cells all
together.
HOW TO EXTRACT DNA FROM ANYTHING LIVING
STUDENT NOTES
All the information required to make you is stored in your DNA. In fact, the DNA in all organisms provides
instructions about how to grow and go about millions of cellular processes each day. The following procedure
will allow you to extract and visualize DNA from a variety of sources. The process of extracting DNA from a cell
is the first step for many laboratory procedures in biotechnology. Scientists must be able to separate DNA from
the unwanted substances of the cell gently enough so that the DNA does not denature (break up).
DNA! You mean I can see it? How?
Just follow these 3 easy steps:
Detergent eNzymes Alcohol
If it's that simple- tell me more!
The isolation, or extraction, can be done in about half an hour (even in your own kitchen), but it took
many years for scientists to learn how to do it.
First, you need to find something that contains DNA. Since DNA is the blueprint for life, everything living contains
DNA. There are lots of easily obtainable DNA sources, such as:
Chicken or calf liver
Fish eggs (roe)
Onions
Strawberries
Bananas
Split peas (soaked)
Ask your teacher to put the following in a blender:
Your DNA source (about 125ml or 1/2 cup)
A large pinch of table salt (about 1g or 1/4 teaspoon)
Twice as much warm water as the DNA source (about 250ml or 1 cup)
Blend on high for 15 seconds.
And now, those 3 easy steps:
1. Pour your thin cell soup through a strainer (cloth/coffee filter/filter paper) into another container.
Measure 12ml of this soup into a small beaker and add 2ml of liquid detergent and swirl to mix.
Let the mixture sit for 5-10 minutes in a container of hot tap water (60-65C),
occasionally swirling gently. Remove the mixture from the hot tap water and allow it
to cool to room temperature.
2. Add a pinch of enzymes (meat tenderiser) to your mixture and gently stir with a
toothpick for 5 minutes.
NOTE: If you can't find meat tenderiser, try using pineapple juice or contact lens
cleaning solution.
3. Pour your mixture into a test tube until it is 1/3 full. Tilt the test tube and slowly
pour ice-cold alcohol (70-95% isopropyl or ethyl alcohol) into the tube down the side
so that it forms a layer on top of the cell mixture. Do not mix the two layers
together. Pour until you have about the same amount of alcohol in the tube as cell
mixture.
DNA precipitates as a white stringy/snotty film at the water-alcohol interface and eventually will rise into the
alcohol layer from the mixture layer. Allow your test tube to sit for several minutes. The clearer your DNA is, the
fewer impurities you have. You can usually get more DNA to precipitate from the solution by using a DNAcollecting tool (such as a glass or paper clip hook) to gently lift the water solution up into the alcohol. This allows
more DNA to come in contact with the alcohol and precipitate.
When you have an acceptable amount of DNA, it can be ‘spooled’ by rotating your collecting tool and then
transferred into a clean tube. Leave the tube uncapped until the ethanol has evaporated. Your DNA can be
stored dry or water/buffer can be added. You will need to agitate the tube gently to dissolve the DNA- this may
take some time. Store your DNA in the fridge.
CONGRATULATIONS– You are now a biotechnologist!!!
The following questions will test your understanding of the procedure you have just followed.
QUESTIONS:
8.
Why was salt added to your DNA source?
9.
How does blending help you to extract the DNA?
10.
Why did you add detergent?
11.
Why did you place the mixture at 60-65C
12.
What is an enzyme, and how does it help in this procedure?
13.
Why does the alcohol need to be at least 70% and cold?
14.
Why does only the DNA enter the alcohol layer?
FURTHER CHALLENGES:
Now that you've successfully extracted DNA from one source, you're ready to experiment further! Try these
ideas or some of your own:

Experiment with other DNA sources.
Is the DNA from the different sources the same?
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?
How about shampoo or body scrub?

Experiment with leaving out or changing steps.
We've told you that you need each step, but is this true?
Find out for yourself. Try leaving out a step or changing how much of each ingredient you use.

Do only living organisms contain DNA?
Try extracting DNA from things that you think might not have DNA.