Using a SNP to predict the ability to taste PTC
A single nucleotide polymorphism is a single nucleotide difference between two alleles.
Depending on the nucleotide substitution, the SNP may or may not change the amino acid coded for.
Our sense of taste depends upon the binding of chemicals to receptors in the taste buds of the mouth. The chemical phenylthiocarbamide (PTC) is perceived as bitter by some individuals.
The gene for tasting PTC is called TAS2R38 and is located at 7q34. The two most common alleles are called AVI and PAV (named for the variations in their amino acid sequences). These are the two alleles we will investigate in this experiment. Diseases associated with TAS2R38 include dental caries and nicotine dependence [1].
The PAV allele is the ancestral allele, as it is shared by chimpanzees, lowland gorillas and orangutans, as well as by some monkeys.
People with two copies of the PAV allele will find the PTC test paper to taste distinctly bitter.
People with one copy of the PAV and one of the AVI allele normally find the taste bitter, but will
not have as strong a reaction. People with two copies of the AVI allele will be nontasters.
In the North American white population, approximately 20% are homozygous tasters, 50% are
heterozygous, and 30% are nontasters.
I. DNA isolation using saline mouthwash
Reagents & supplies
10 mL 0.9% saline (0.9
g/100 mL H2O)
Permanent marker Micropipetors (100-1000 μL
& 5-50 μL or 10-100 μL)
100 μL 10% Chelex in
water
Micropipette tips (blue &
yellow box)
Microcentrifuge tube rack
Paper cup 1.5 mL microcentrifuge
tubes
Microcentrifuge
Vortexer 200 L PCR tube Thermalcycler or waterbath
1. Label a 1.5 mL microcentrifuge tube, a 200 L PCR tube and a paper cup with your initials or assigned number.
2. Pour 10 mL of saline into your mouth. Vigorously swish for 30 seconds.
3. Spit the saline into your paper cup.
4. Swirl the cup gently to mix in any cells that have settled to the bottom of the cup.
5. Use micropipet with tip to transfer 1000 μL of solution from the cup to the tube.
6. Centrifuge at full speed for 90 seconds. (Balance the centrifuge!)
7. Pour off the supernatant into your cup, being careful not to disturb the pellet.
8. Resuspend the pellet in the liquid that remains in the tube, using a micropipet set to 30 μL. Be careful to minimize bubbles.
9. Add 100 μL Chelex solution to the PCR tube FIRST. Add 30 μL of your cell suspension.
10. Place your tube in the thermal cycler along with those of the other students. Set the
thermal cycler to 99 °C for 10 minutes. This lyses the cells, releasing the DNA.
11. Shake the tube vigorously (or vortex) for 5 seconds.
12. Place your tube in a balanced microcentrifuge. Use the second rotor. Centrifuge for 90
seconds at maximum speed to pellet the Chelex and cell debris.
13. Using a fresh tip on your micropipetor, transfer 30 μL of the supernatant to a labeled 200
L PCR tube. Be careful to avoid the pellet. FOR LARGE GROUPS: Put your DNA
sample on ice or in a -20 °C freezer until you are ready to continue with Part II. FOR
SMALL GROUPS: Go straight to Part II.
Part II. PCR
Cheek cell DNA from part I 0.2 mL microcentrifuge tube Micropipettors (1-10 μL & 10-
100 μL or 5-50 μL)
PTC Primer/loading dye mix Pipet tips PCR tube rack with ice
Ready-To-Go PCR beads
(already in 200 L tube)
Indelible marker Thermal cycler
1. Label a 0.2 mL PCR tube with Ready-To-Go PCR beads.
2. Add the following to your PCR tube
22.5 μL of the above PCR PTC primer/loading dye mix to the tube. Allow the bead to dissolve (with a good shake) for a minute or so – avoid sticking the pipet
tip into the bead!
2.5 μL of your cheek DNA mix from Part I. Avoid sticking the pipet tip into the bead.
Store your sample on ice until everyone is ready to run the PCR.
3. Put your tube in the thermal cycler.
4. We will run the following PCR program:
Hot start denaturation 2 min 95 °C
35 cycles of:
Denaturation 30 sec. 95 °C
Annealing 45 sec. 64 °C
Extension 45 sec. 72 °C
Final extension (termination) 3 min. 72 °C
Hold at 4 °C infinitely.
5. Store the samples at -20 °C or on ice if time is available to go directly on to Part III.
Part III: restriction digest of PCR products with Hae III
200 L PCR tubes Microcentrifuge tube rack
Pipet Tips Micropipet (1-10 & 10-100 or 5-50 μL)
Hae III restriction enzyme 37 °C water bath
PCR product
1. Label a 200 L PCR tube.
2. Transfer 10 μL of your PCR product to this tube. Add 1 μL Hae III restriction enzyme. Pipet the enzyme carefully, as it is in a viscous fluid.
3. Place this tube in the rack in the 37 °C heat block for one hour.
4. Keep the rest of your PCR reaction frozen or on ice while doing the digest.
5. Store your sample on ice or at -20 °C until ready for part IV.
Part IV: DNA gel electrophoresis
Undigested PCR product Pipet tips (yellow & red
boxes)
Micropipetors (1-10 & 5-
50 or 10-100 μL)
PCR product digested with Hae III Ethidium bromide Electrophoresis
apparatus
TBE buffer (50 mL concentrate +
H2O, qs 1 L – this is enough for 6
gels and 3 electrophoresis tanks)
pBR322/BstNI MW
marker solution
Power supply
Loading buffer (dye)
1. Prepare a 2.5% agarose gel. Place the following in an Erlenmeyer flask (2 gm of agarose in 80 mL of 1X TAE buffer)
2. Microwave for about 30 sec, to bring to a boil, swirl. You should not see any crystals of agarose. If crystals are still visible, microwave some more.
3. Place the dams in the ends of your gel tray[s]. Add the appropriate comb.
4. Add 3 ul of Ethidium bromide in the agarose solution, swril.
5. Pour agarose solution into your gel tray. If there are any bubbles in your gel, use a pipet tip
to move them to the edge of the gel. You’ll have a good gel if about a third of the gel (or
so) is over the comb teeth.
6. When your gel has solidified (it will look cloudy), remove the dams and carefully remove the
comb by pulling it straight up. Place the gel in the gel rig, matching the notch to the tab on
the rig. Cover with enough running buffer (1x TAE) to cover the gel and wells completely.
7. Plan the loading of the gel along with the other students who will be sharing it with you.
If you put the lanes at the “top” or the “bottom”, the wells are numbered starting with 1 on the
left. Include a well for the DNA size marker (15 L pBR322/BstNI) at the far right (last well in
the gel).
8. Load the gel. Work carefully, but not slowly. Add 10 μL of undigested (“U”) PCR product,
and 10 μL of digested (“D”) PCR product to the appropriate wells. Add 15 μL of DNA size marker, as directed by your instructor (see above).
9. Place the lid on the gel rig, connect the electrodes to the power supply and turn on the power
supply to 150 mV and run it for 25-30 minutes, until the dye has traveled about ¾ the length
of the gel.
10. Turn off the power supply. Remove lid from gel box. Carefully remove gel from box.
11. View gel using the UV transilluminator (wear your goggles) and use a camera to take a picture of it.
12. Analyze the gel bands by observing the number of bands in your samples: a tt non- taster
(homozygous recessive) shows a single band in the “D” sample identical to the band in the
“U” sample; a TT taster (homozygous dominant) shows a band in the “U” sample that is closer to the origin and one band in the “D” sample that is located a little farther away from the origin than the band in the “U” sample. Tt tasters (heterozygous) shows one band in the “U” sample and 2 very intense bands in the “D” sample, the first of which is identical in location to the band in the “U” sample. Single bands, BTW, indicate homozygousity; double bands indicate heterozygousity.
13. Dispose of all ethidium bromide contaminated items as directed by your lab instructor.
Obtain PTC and control test papers and determine whether you taste PTC.
Questions:
Did you taste PTC?
Explain your taste response using the band data from the electrophoresis, i.e., connect genotype
with phenotype.
Are you homozygous or heterozygous?
References:
Using a Single-Nucleotide Polymorphism to Predict Bitter-Tasting Ability. Dolan DNA Learning
Center.
Hundley, L.R. 1960. “Taste Test Papers” Carolina Biological Supply Company
Merritt, R. B., Bierwert, L.A., Slatko, B., Weiner, M.P., Ingram, J., Sciarra, K., Weiner, E. 2008.
“Tasting Phylthiocarbamide (PTC): A New Integrative Genetics Lab with an Old Flavor. BioOne
