DNA Spooling from Thymus Gland

IHS / IB Biology
DNA Spooling from Strawberries
I saw DNA. From that moment on, I understood why molecular biologists are so enraptured
by their subject... I thought that if I grabbed the DNA, I could roll it in my hands, twist it, pinch it,
shape it into little animals, even drop it on the floor, and still it would jiggle merrily...”
- N. Angiers, New York Times Reporter and author of Natural Obsessions (1988)
Background
In this activity you will extract or "spool" DNA from plant cells. Strawberries are a good choice for this activity as
their genome is octoploid, meaning they have 8 copies (!) of each chromosome (humans are diploid – we have 2
copies of each chromosome per cell). Most of the unusual physical properties of DNA result from it being a
very long, thin molecule. Each of your cells, for example, contains approximately two meters (six feet) of
very thin, compact DNA!
Purpose: To isolate DNA from the cells of strawberries, and wind it up on a stick (“spooling”)
Materials
strawberries (fresh or frozen, 1 or 2 per student)
sealable plastic bag (preferably freezer quality)
extraction buffer (50 ml dishwashing detergent, 2 tsp. non-iodized salt, fill to 1 L with H2O)
tube rack
cheesecloth
95% ethanol, chilled
funnel
large test tube
test
stirring rod
Procedure
• HINT Any molecule as long as DNA is vulnerable to cutting by enzymes on your skin called DNases. So use only scrupulously
clean glassware.
1.
Remove the green sepals from the strawberry.
2.
Place the strawberry into the plastic bag, push out as much air as possible, and seal shut.
3.
Squish the bag for a few minutes to completely squash the fruit.
4.
Add about 10 mL of extraction buffer (soapy salty water) to the bag and squish for a few more minutes.
Try to minimize the amount of soap bubbles.
5.
Filter the strawberry mixture through a funnel lined with cheesecloth and collect into a test tube. If chunks
of strawberry tissue remain, filter again. The filtrate now contains DNA that has been released from the
strawberry cells.
Stop & think: Where is DNA located in a cell? How does the extraction buffer help release the DNA into the filtrate?
6.
Perform this step VERY CAREFULLY: add about 2 volumes of ice cold ethanol to the strawberry liquid in the
test tube by pouring the ethanol SLOWLY down the SIDE of the tube to form a separate layer on top. DO
NOT MIX!
7.
Let the tube sit undisturbed for 1-2 minutes.
8.
It's time to see if you have extracted strawberry DNA -- so let's spool! Slide a long, thin, clean glass rod
into your tube until it penetrates the interface between the less polar ethanol layer (top) and the
Adapted from: The GENETICS Project, Department of Genome Sciences, UW, 2012
IHS / IB Biology
hydrophilic water-based strawberry layer below. Then, begin making a circular, stirring motion with it as if
stirring a witch’s brew.
9.
If you are successful, long threads of DNA will precipitate onto the glass rod. Reel in a good-sized clump!
Stop & think: What charge does DNA carry? How well would a charged substance stick to itself? Which chemical in
the buffer neutralizes DNA so that it can spool? How might this same chemical affect proteins associated with DNA?
Clean up

Used baggie & cheesecloth go in the trash, liquids go down the sink, check that your kit is in order, & wash
your hands!

If you’d like to save your strawberry’s DNA, ask your teacher how to do so.
Adapted from: The GENETICS Project, Department of Genome Sciences, UW, 2012