TEACHER GUIDE:
Diagnosis of Huntington’s Disorder
This teacher guide is provided to give sample answers to questions. Most of the questions are open-ended,
so students may have correct answers that aren't included in this guide. Finally, although the experiment
is set up to yield one correct answer, there are variations in data between students. As long as students
examine their data carefully and can justify their answers based on their data, that's science! Data are
always right and there isn't necessarily a 'right answer'.
Some questions to get you thinking about today’s lab:
What is DNA and what does it do?
deoxyribonucleic acid, makes you who you are, codes for proteins, etc.
What are some characteristics or properties of DNA?
double-helix, double stranded, negatively charged, colorless, uniqueness in DNA patterns in all individual
How can we take advantage of these properties to help us figure out genetic disorders?
The huntingtin gene can be evaluated using gel electrophoresis because allele sizes vary within the human
population. The allele has a trinucleotide repeat within the gene ranging between 9 to 37 in normal
individuals and 37 to 100 in HD individuals. DNA is negatively charged allowing the molecules to move it
in an electric field. Due to differences in allele size, DNA molecules will move further the smaller the piece
of DNA or remain closer to the wells the larger the piece of DNA. Not all genetic disorders can be tested
using DNA gel electrophoresis, for example if a gene has a point mutation, gel electrophoresis will not be
able to detect a size difference and the normal (wild-type) and the mutant will run the same.
Materials
DNA samples Mom, Dad, Son and Daughter
Molecular Weight Marker
agarose
Tris-acetate/EDTA solution (TAE)
micropipette/tips
electrophoresis apparatus
Procedure:
1. Get your electrophoresis apparatus. Make sure the combs are in place (there should be 2 combs) and that
there are stoppers at both ends of the gel space.
2. Pour hot agarose into the gel space until it reaches the top of the gel box. Let the agarose harden, which
should take about 10 minutes. Don’t touch/move your gel until it’s hard. Why not?
If the agarose moves while it's hardening, it will harden unevenly, making it more difficult for the DNA to
move through evenly.
3. When the agarose gel is hard, take out the stoppers and gently remove the comb. Draw a picture of your gel
and label which samples are where before you add DNA to the gel.
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The samples are indistinguishable once they are loaded, so everyone needs to have a drawing of what they
will load where before they load their DNA samples. This drawing will be useful during the analysis, once
the gels are stained.
4. Load your DNA SAMPLES into the wells near the BLACK ELECTRODE. Why near the black
electrode? Be sure to keep track of which samples you loaded in which lanes.
DNA is negatively charged, so to move the DNA into the gel with electricity, the DNA needs to be loaded
on the negative or black side, it will then move towards the red. If it's loaded near the red electrode, it will
migrate off the gel into the TAE.
5. Pour the TAE solution into the side farthest from you samples, so that you gels is completely covered plus a
little more. What do you think the TAE solution is for?
TAE is like saltwater - it conducts electricity, plus it is a buffer to control for pH changes
6. Run that gel!! Plug the electrodes into your gel box (red to red, black to black), being careful not to bump
your gel too much. Plug the power source into an outlet. How can you tell your gel is running?
It bubbles at the electrodes. This is a redox reaction, forming H2 gas at the black electrode and O2 gas at
the positive electrode.
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Analyzing Your DNA Data
HUNTINGTON DISEASE [HD, (CAG)n EXPANSION ]
Huntington disease is caused by expansion of a polymorphic trinucleotide repeat (CAG)n located in the
coding region of the gene for huntingtin. The range of repeat numbers is 9 to 37 in normal individuals
and 37 to 100 in HD patients.
Marker starts with 30nt then, 60, 90nt and120nt. Everything closer to the wells of the 120 nt marker is bigger
than 120 nt.
Marker
dad
daughter
mom
son
120 nt
90 nt
60 nt
30 nt
Use the molecular weight marker information to determine the size of each person’s alleles. Based on the
number of nucleotides in the region of DNA determine the number of trinucleotide repeats.
Both the Mom and the Son have a region of DNA that is larger than 120 nt which is equivalent to 40
amino acids, glycine in the huntingtin protein. More than 37 glycines results in possible onset of the
disease, with an increased possibility the greater the number of repeats. The Mom and the Son have
much greater than 40 repeats and therefore would be expected to exhibit Huntington’s symptoms. The
Dad and the Daughter have 30 repeats or less in their alleles and therefore would not be expected to show
symptoms.
It is up to the students research on possible diseases as to a possible diagnosis for the Daughter’s disorder.
There is no right answer, as long as they have the research to back it up. The Son however should be
diagnosed with HD.
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