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Strawberry DNA Extractions
Objectives: Participants will be able to…..
Explain how DNA is present in the cells of every living thing.
Use an extraction procedure to isolate DNA from the cells of a strawberry
Examine the visual characteristics of a large amount of DNA precipitate.
Take home a sample of extracted strawberry DNA.
Background:
Deoxyribonucleic acid (DNA) is the chemical inside our cells that contains the
genetic code. Each DNA molecule is a long polymer of smaller chemical units called
nucleotides. There are 4 different types of nucleotides nicknamed A,T, G and C. These
occur in a particular sequences along the length of the DNA molecule in each
individual’s cells. The order of the nucleotide sequence is slightly different in each
organism in a particular species, making that individual unique. The order of nucleotides
in DNA is what forms the genetic code. The instructions in the genetic code tell the
embryo how to divide and develop into each particular organism and how each cell
should carry out the biochemical reactions that keep us alive and allow our bodies to
function.
All living things are made of cells and each cell contains a full set of genetic
instructions in the form of several long molecules of coiled DNA called chromosomes. If
the DNA molecules from each chromosome were uncoiled and laid end to end, each cell
in a human would have a total of nearly 6 feet of DNA. This DNA is very neatly supercoiled into chromosomes and packaged into a compartment called the nucleus in every
cell.
The procedure we will use today will allow the DNA to escape the boundaries of
the nucleus and the cell membrane and precipitate into a visible form we can observe
with the naked eye. The white fibers we will see in our preparation today are actually
thousands of DNA strands wrapped around each other. An individual DNA strand is so
small, it can only be imaged by the most sophisticated and specialized electron
microscope.
Scientists who study DNA and the gene sequences contained in the DNA
molecules begin their study with an isolation procedure similar to the one you are
performing today. They would follow up by additional purification steps to remove all
the protein and contaminating materials (including enzymes that can attack and destroy
DNA in storage unless removed). They would then use additional procedures to isolate
the particular region of DNA that contains the genes (DNA segments) they are intending
to study.
Procedure notes:
Today we will isolate DNA from strawberry cell. Ripe fruit is a good material for
isolating plant DNA since the cell walls are already weakened by the ripening process.
Commercial strawberries 8 sets of each type of chromosome (this is called octoploid).
Wild strawberries have only 2 sets of chromosomes (diploid). This makes grocery store
strawberries a particularly good material to use. The procedure used today must
accomplish several things. First, the cells must be broke open. The nuclear membrane
must also be broken open and the DNA allowed to escape into the extraction buffer. The
cell and nuclear membranes are made of fats and proteins. These are chemically very
different from the nucleotide chemical that makes up DNA. The detergent and salt in the
extraction buffer do a good job of separating the proteins and fats from the nucleus and
the rest of the cell contents. The DNA is dissolved in the water in the extraction buffer
and needs to be precipitated so we can see it. This is accomplished by using alcohol. By
carefully placing the alcohol layer on top of the detergent layer, the proteins and fats in
the cell debris will stay in the lower layer and the DNA will rise to the top and form
insoluble threads in the alcohol layer. By using a tool such as a bamboo skewer to
carefully wind up the DNA threads, you can lift the DNA out of the test tube, and remove
it for study. The DNA can be dried and re-dissolved in water, or kept as a precipitate in
alcohol in a closed tube.
Illustrations of experimental apparatus
Extraction set up
Precipitation set up
Materials
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Ripe strawberry
Ziploc bag (quart size)
Small beaker or glass tumbler
Small funnel
coffee filter trimmed to fit funnel (As an alternative to a funnel: Use an
accordion style coffee filter and use rubber band to secure to top of glass
container)
15 ml. clear glass or plastic centrifuge tube with cap
graduated cylinder or liquid measuring cup
non-iodized salt (iodized salt will work fine)
distilled water
95% ice cold ethanol (can use Everclear liquor, but may need to modify amounts
to see a DNA precipitate)
Ivory, Dawn or Palmolive liquid dishwashing detergent (minimal additives)
6 “ square of waxed paper
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1.5 mL Eppendorph test tube with snap lid (optional)
Extraction buffer solution recipe
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50 mL liquid detergent
15 g salt
add 950 mL distilled water for a total of 1000 ml
Use 10 mL of extraction buffer per large strawberry
Protocol
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Select one medium to large ripe strawberry (approx. 30 gm.) and remove stem.
Place inside Ziploc bag.
Press out as much air out of the bag as possible and seal it.
Gently mash the strawberry in the bag for 2 minutes until large chunks are no
longer visible.
5. Add 10 mL extraction buffer to the bag, press the air out and reseal the bag.
6. Mash the strawberry and buffer mixture for 1 minute taking care to avoid
bubbling and foaming as much as possible.
7. Keep foaming to a minimum if possible.
8. Place funnel with coffee filter into beaker.
9. Pour liquid contents of Ziploc bag into filter.
10. Collect filtrate in beaker
11. Pour approximately 3 mL filtrate into test tube (2 cm. or 1/8th tube volume if tube
has no volume markings.) Try to avoid pouring foam into tube.
12. Obtain 9 mL ice cold 95% ethanol.
13. Pour ethanol slowly down side of tube, taking care not to disturb interface
between extraction bufferlayer and alcohol layer .
14. Note the white thread-like material that accumulates at the interface between the
alcohol and buffer layers. This is DNA.
15. Let the mixture sit undisturbed for a minute or two.
16. The DNA will float in the alcohol. The DNA of the strawberry cells will be long
threads that easily spool.
17. Use a wooden bamboo skewer or plastic inoculating loop to carefully spool the
DNA from the interface and remove it from the tube onto the waxed paper square.
18. Observe the visible characteristics of the DNA once removed from the solution.
19. The DNA can be allowed to dry to a near transparent film on the waxed paper,
or….
20. If desired, the DNA mass can be placed in a 1.5 mL plastic tube while still wet
and covered with alcohol for safekeeping.
Strawberry DNA protocol developed using modifications of extraction protocols found at
the following sites:
http://www.mrsortbiology.com/berryfullofdnalab.doc
http://www.science-projects.com/onionDNA.htm
http://gslc.genetics.utah.edu/units/activities/extraction/
http://www.exploratorium.edu/ti/human_body/dna.pdf
http://www.mrsortbiology.com/berryfullofdnalab.doc
http://www.accessexcellence.org/AE/AEC/CC/DNA_extractions.html
Equal quantities of any of a number of different soft fruits or vegetables works equally
well.
Pre-Lab Questions:
1. Define and draw a picture of a gene, DNA, and chromosome using the
rubberband model.
2. Ethanol is a type of __________________________.
3. Draw a pipet.
4. Why use a cheesecloth? Why use a wooden stick?
5. What exactly are we going to do in the Strawberry DNA extraction lab?
6. How do you make a buffer?