Introduction

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Lab: DNA Extraction from Human Cheek Cells
Introduction
DNA…you hear about it all the time. DNA is used every
day by scientists and lawyers to help in criminal
investigation, paternity suits, and for cloning (to name just
a few instances). Your DNA is your “genetic
fingerprint”—this means that your DNA is like no one
else’s in the world! The procedure that we will use to see
your DNA includes the same basic processes that
researchers use to isolate, analyze, and manipulate DNA in
a laboratory setting (although the DNA isolated here is not
nearly as “pure” as the research lab version).
DNA is in the nucleus of almost every cell in your body.
The length of DNA per cell is about 100,000 times as long
as the cell itself. However, DNA only takes up about 10%
of the cell’s volume. This is because DNA is specially
packaged through a series of events to fit easily in the cell’s
nucleus. The structure of DNA, the double helix, is
wrapped around proteins, folded back onto itself, and
coiled into a compact chromosome.
Individual chromosomes can be studied using
microscopes, but the double helix of a chromosome is so
thin that it only be detected through innovative, high-tech
procedures. Chromosomal DNA from a single cell is not
visible to the naked eye. However, when chromosomal
DNA is extracted from multiple cells, the amassed quantity
can easily be seen and looks like strands of mucous-like,
translucent cotton.
Clumping the DNA...
When DNA is released from a cell it typically breaks up
into short strand fragments. The released DNA--highly
soluble in water because the phosphate group of each
nucleotide carries a negative charge--dissolves in the water.
However, the positively charged sodium ions from the salt
in the extraction solution are attracted to the negatively
charged phosphate groups on the DNA backbone,
effectively neutralizing the DNA's electric charge.
This neutralization allows the DNA molecules to aggregate
with one another. When the alcohol is added, the DNA
clumps together and precipitates at the water/alcohol
interface because the DNA is not soluble in alcohol. The
colored layer of liquid above the cell solution will contain
DNA and proteins. The DNA will then move up to a layer
of alcohol carefully placed above the water. DNA does not
stay dissolved in alcohol, but precipitates out and will
appear as a white stringy mass.
Spooling the DNA on a stick...
Alcohol allows DNA fragments to stick together, or
precipitate, producing a blob of DNA which you can
examine. The DNA precipitate can be captured on a hook
or spooled onto a wooden stick by placing the tool in the
DNA and turning it. If you want to save your DNA, you
can transfer it to a small container filled with alcohol.
Introductory Questions: Before beginning the
experiment, it is important that you realize the role each of
the materials will be playing in the performance of this
experiment. Answer the questions associated with starting
this lab on a separate sheet of paper in complete sentences.
A.
Some basic, but cool, chemistry:
DNA is the largest known molecule. A single unbroken
strand of animal or plant DNA contains millions of atoms.
The DNA from a single (diploid) human cell, if the 46
chromosomes in one cell were connected end-to-end and
straightened, would have a length of approximately 2
meters and a width of approximately 2.4 nanometers.
Releasing the DNA...
In the DNA extraction process, the first step is to release
the DNA from the cells of a living organism. Detergents
and soaps are used to (1) dissolve the lipids in the cell
membranes and nuclear envelope, releasing the DNA, and
(2) break up proteins which may harm the DNA.
(Enzymes may also be used to break up the proteins.)
B.
C.
Detergent is being used to break open both the
cell membrane and nuclear envelope.
1.)
Which types of molecules compose
the majority of your cell membrane?
2.)
How does detergent interact with
these molecules?
3.)
If detergents can break open the
nuclear envelope and cell membrane,
which molecules can you infer are
present in the nuclear envelope.
DNA typically breaks apart when it is placed in
water.
1.)
Which substance will you use to avoid
the fragmentation of DNA?
2.)
Explain why your DNA will not break
apart in water.
How is the alcohol that is being added to your
solution going to ensure your DNA sample is
visible?
Procedure
Ingredients:
Water
Clear dish soap
Food coloring
Table salt
Isopropyl alcohol
And… you!
1. For the table, mix 200 ml drinking water with 2
tsp salt
2. Stir until salt is dissolved. Then transfer 5 tsp of
salt water into a clear cup.
3. Gargle the salt water for 1 minute.
4. Spit the water back into the cup. Now your cheek
cells are suspended in the salt water.
5. Gently stir the salt water with one drop of soap.
(Avoid bubbles as much as possible).
6. In a separate cup, mix 10 ml isopropyl alcohol and
1drop of food coloring.
7. Tilt the salt water cup and gently pour the alcohol
so that it forms a layer on top (about 2 cm thick).
8. Wait about 2.5 minutes. You should see white
clumps and strings forming. That's your DNA!
Discussion Questions- Please complete these questions
on a separate sheet with your introduction questions.
Your answers should still be in complete sentences.
1. Describe how long strands of double-helical DNA
fit into the nucleus of a single cheek cell.
2. If DNA is so thin, how is it that we are able to see
it during this simple lab exercise?
3. What does gargling with salt water do to your
cheek cells?
4. If you had extracted DNA from plant cells instead
of animal cells, what cell barrier would have been
different?
5. Do you think that DNA from plant cells would
look the same as DNA from animal cells (explain
your reasoning)?
6. Why is DNA extraction important and what can it
be used for? Give one example of something a
scientist can do with extracted DNA.
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