DNA
Extraction
Students extract their own DNA.
TECHNOLOGY TOPICS
Systems
Environmental Effects
Consequences, Ethics
Controls
PROCESS SKILLS
Observing
Following Instructions
Measuring
Safely Using Tools
GRADE LEVELS
5-12
TIME REQUIRED
Advance Preparation
15 minutes
Set Up
Activity
Clean Up
5 minutes
45 minutes
10 minutes
SUPPLIES
1 Tablespoon transparent liquid dish soap or
shampoo (such as Palmolive) per student
About 2 Tablespoons ethanol (rubbing alcohol) per
student
Small transparent cup or tube for each student
Pipette or medicine dropper for each student
About ¾ cup water per student
¼ teaspoon of salt per student
Stirring rods (e.g. coffee stirrers), 1 per student
For Optional Extension:
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Thermometer that will measure 60°C (140°F)
Strainer that will fit in measuring cups
Ice water bath (or large bowl with ice)
Sauce pan or hot water bath
Burner or source of heat for hot water bath
¼ of a kiwifruit
Knife to cut kiwi
ADVANCE
PREPARATION
Put ethanol in the freezer for at least 24 hours before
use.
Carefully dilute the liquid soap, without forming suds.
Use 2 parts water to 1 part soap.
SET UP
Set out a test tube or small clear cup half full of water
for each student.
Have ready:
• Salt
• Soap
• Ethanol
• Droppers (or pipettes or straws)
INTRODUCING THE
ACTIVITY
DNA is found in every living thing. It is what determines
the traits of each individual organism. DNA determines
the color of your hair and eyes. Scientists can mix the
DNA of different organisms with special technology,
giving some organisms traits they otherwise would not
have. This is called genetic engineering. Genetic
engineering is one form of biotechnology. Biotechnology
is the application of technology and engineering to the life
sciences.
The first step in genetic engineering, and in many other
processes of biotechnology, is extracting DNA from an
organism. Today we are going to extract DNA from
ourselves.
CLASSROOM
ACTIVITY
Each student follows the directions below.
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Procedure for DNA Extraction
1
Add ¼ tsp. of salt to your water.
2
Swish the salt water in your mouth, scraping against the inside of your
cheeks with your teeth.
•
The salt helps collect cells.
3
Spit the salt water back in the cup.
4
Add 1 teaspoon of dilute soap to each cup, and mix with a stirrer.
•
5
The soap destroys the cell wall and the nucleus, exposing the DNA.
Carefully roll a dropperful of ethanol down the sides of the cup, so it
creates a layer on top of the soap/salt/spit solution. The DNA will
precipitate between the two layers.
Spend 5-10 minutes discussing DNA, genetic engineering, and related topics,
(See Class Discussion below) then look at the results of the experiment. It takes
at least 5 minutes to get the DNA to appear in visible quantities. It should be
white and stringy.
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CLASS
DISCUSSION
Ask for student
observations.
There is no correct
answer. Let
students guide the
discussion and
present their
hypotheses before
discussing
explanations.
What does the DNA look like?
What parts of this procedure were easy? What parts
were difficult? Why? Could you invent something to
make the process simpler?
Every living thing has DNA. DNA has information about
different traits, like hair and eye color, the shape of a leaf,
or how fast bones should grow.
What else has DNA?
Anything alive. Plants, animals, viruses, bacteria.
What is something that does not have DNA?
Anything not alive, like rocks, water, air.
EXPLANATION
In-depth background information for teachers
and interested students.
You just performed the first step of a common biotechnology process. To use this
DNA for genetic engineering, the next step is to identify a specific gene (a piece
of DNA that codes for a particular trait, like blue eyes or red hair). Then you put
the gene into another organism to transfer the trait carried by the gene.
There was a lot of chemistry in this experiment. Each ingredient had a different
role in the extraction of the DNA:
•
Soap- breaks down fatty (lipid) cell membrane to release DNA into solution by
disrupting the polar interactions that hold the cell membrane together. The
soap forms compounds with the lipids which causes them to precipitate out of
solution.
•
Salt- shields the negative phosphate ends of the DNA, allowing DNA to come
together and precipitate out of cold alcohol solution.
•
Heat- precipitates proteins and extraneous lipids out of solution leaving DNA
behind. Destroys enzymes that could break down DNA (DNAase).
•
Alcohol- precipitates DNA while leaving other substances in solution.
•
Cold- slows the rate of DNA breakdown while it precipitates.
•
Water- dilutes solution to allow uniformity of reactions.
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Genetic engineering is controversial. Most of us have eaten food made with
genetically modified plants: usually corn, soy or canola. Genetically modified
plants are safe to eat. All our food has DNA. DNA from the food we eat does not
mix with our DNA. This is no different with genetically engineered DNA. In
Europe, food made with genetically engineered plants must be labeled. In the
US, labels are not required. If something is labeled “organic” in the US, it was
not genetically engineered.
There is a possible environmental hazard from genetically engineered plants.
They could mix with natural DNA, and spread throughout the world, changing the
environment in ways we can’t predict. Engineers have tried to prevent it, by
making genetically modified plants sterile, or keeping them in greenhouses
instead of open fields. This hasn’t worked. The DNA escapes into the
environment, and we don’t know what effect that might have. (It’s not as
impressive as dinosaurs, but it’s the same idea as in Jurassic Park.)
Some companies that make genetically engineered plants compare the process
to selective breeding and hybridization. Hybrids are created when you breed two
different strains to get the strengths of both in one plant. By making hybrids,
humans are essentially just accelerating the natural breeding process, mating
cows with cows and different strains of corn together with other corn. Given
millions of years, the strains of corn we have bred as hybrids could have
eventually occurred naturally. Selective breeding happens when you only let the
best plants or animals breed, to strengthen a particular trait. When hybrids were
first introduced, many people called it unnatural, and were slow to use it. They
are now common. There is something fundamentally different about genetic
engineering, because it allows us to introduce genes from unrelated species.
This would never happen in nature.
"You can put a fish and a strawberry in the same room together for a million
years, and they're never going to cross breed…" -Craig Culp
OPTIONAL
EXTENSIONS
Advanced Fruit DNA Extraction
(For secondary classes with a chemistry or biology laboratory)
1. Extract DNA from kiwi fruit.
a.
Peel kiwis and slice into fourths. Start hot
water bath (fill saucepan with tap water and set on
burner) to maintain a constant temperature between
55-60° C. Do not let water heat above 60°, as this
may denature (break down) the DNA. Combine 1
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Tablespoon transparent liquid dish soap with ¼ tsp
salt in beaker.
b.
Add 100mL distilled water to the beaker. Stir
slowly to dissolve salt and soap. Avoid making
bubbles or foaming.
c.
Add ¼ kiwi.
d.
Put the beaker of solution into hot water bath
(don’t mix hot water with solution!) for 12 minutes.
Mash the kiwi with a spoon against sides of beaker.
(Note: don’t leave beaker in hot water bath for more
than 15 minutes, as it will denature the DNA!)
e.
Gently pour kiwi solution through the strainer
into a second beaker.
f.
Put kiwi solution into test tubes (one per
student or one per group). The test tubes should be
about 1/3 full.
g.
With medicine dropper, add cold ethanol to the
test tube until it is about 2/3 full.
h.
Let test tube sit undisturbed in ice bath for 5
minutes. The white cloudy mucous that precipitates is
the DNA.
2. Try the same experiment with a different fruit, such as
a banana. Is there a difference in the smoothness/ease
of the process? Are certain steps easier than others? Do
you need to adjust the technique (process) to get the
same results?
3. Try measuring the same initial volume of kiwi and
banana used, and then see which yields more DNA. This
may be done by:
a. Visual analysis
b. More precise measurements of volumes and a final
wet weight comparison
c. Briefly centrifuging test tubes to spin down DNA,
decanting solutions, drying DNA for 30 minutes or
overnight (in refrigerator if overnight), then weighing
final dried precipitate in test tube.
Is there a difference? If so, what could account for the
difference? Is it more likely to be something related to
laboratory procedure (i.e. more thorough straining, or
some accidental spillage somewhere along the way), or a
biological difference in the two types of fruit?
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Sources:
Food Safety Network, University of Guelph (2003) Kiwi
DNA Extraction Activity, Retrieved May 2003 from
http://www.foodsafetynetwork.ca/biotechres/activityextrac
tion.pdf
Video at
http://www.foodsafetynetwork.ca/articles/703/DNAxtract.
mov
Office of Biotechnology, Iowa State University. (2002)
DNA Extraction from a Kiwi. Retrieved May, 2003 from
http://www.biotech.iastate.edu/publications/lab_protocols/
DNA_Extraction_Kiwi.html
CROSS-CURRICULAR CONNECTIONS
SOCIAL STUDIES
Have students research and prepare a debate on
the increasingly public issue of genetically
engineering food crops. Have half the students
research each side of this issue (and be able to
identify which claims are valid and the claims that
seem less valid based on current findings):
1. What are the claims of the biotech companies
who are selling these products?
2. What are the concerns/arguments of people
opposed to this application of genetic engineering?
Which points appear more valid based on the
students’ research? What results have been seen
to date?
Have students research non-food uses of genetic
engineering, like xenotransplantation and drug
manufacturing.
Extra credit: have students research how genetic
engineering technology is currently being used.
The majority of genetic engineering is done with
bacteria to produce or consume substances in bulk
(including making cheese, polymers, insulin or
pharmaceuticals; consuming toxic waste spills; and
making more DNA in the process of medical and
genetic research).
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LANGUAGE ARTS
Have students read Mary Shelley’s Frankenstein,
or Michael Crichton’s Jurassic Park, which is
explicitly based on Frankenstein. Watch a movie of
one of these books, or another movie that takes up
similar topics, like Soylent Green. Discuss the
ethics of creating new science without
understanding the possible consequences.
CHEMISTRY
Have students make a drawing or model of the
chemical structure of DNA. DNA stands for
deoxyribonucleic acid—break down the name to
show different parts of the compound.
MATH
Try converting the metric quantities to English
measurements, or vice versa. For example, convert
the alcohol temperature from Celsius to Fahrenheit
or vice versa.
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