Bitterness Tasting

Reagents

Consumables

Equipment

Abstract

Do raw broccoli or brussels sprouts taste bitter to you? An evolutionary explanation is that bitterness tasting developed to help animals detect toxins or poisons in food. But not everyone perceives bitterness equally.

What are we testing?

In this project, we are analysing a gene called TAS2R38, which affects your ability to taste bitterness.

The TAS2R38 gene contains genetic information for a taste receptor that can detect  chemicals like phenylthiocarbamide, or “PTC”. Bitter foods, such as vegetables like broccoli and brussels sprouts contain molecules that similar to PTC.

There are two forms (or alleles) of this gene: C or T. The C allele is associated with the presence of the receptor, and thus the ability to taste PTC and bitterness. The T allele is associated with an absence of the receptor, and thus an inability to taste PTC and bitterness.

Overview

TAS2R38 gene variations

You can taste bitterness flavours thanks to your tongue receptors. One of these receptors is the Taste Receptor 2 Member 38, which is responsible for detecting PTC. The presence of this receptor is determined by the gene TAS2R38. You will explore two forms of the TAS2R38 gene: The T allele and the C allele.
C allele is associated with the presence of the receptor, which means you could taste PTC and bitterness.
T allele is associated with the absence of the receptor, which means you could not taste PTC and bitterness.

What are the possible results for this experiment?

There are three possible results, since the gene has two variations, and two occurrences of the gene exist – one from each chromosome:

  • Homozygous dominant: Both copies of the gene contain the C allele, which means the person can taste bitterness.
  • Homozygous recessive: Both copies of the gene contain the T allele, which means the person is likely to be unable to taste bitterness.
  • Heterozygous: One copy of the gene contain the T allele, and the other is C allele. The C allele is dominant, so the person likely to be able to taste bitterness.

Protocol

In this project, you will first extract human DNA from saliva. This will take about 20 minutes. After this, you will use PCR to amplify the variations of the TAS2R38 gene. This will take about 90 min, but most of it will be waiting time.
Finally, you will visualise the results using Gel Electrophoresis, which will take about 45 min.
At the end of each section, you can continue right away, or store your samples and continue later.


  1. DNA Extraction

    First, obtain the DNA sample. Use the DNA Extraction from saliva protocol. It will take ca 20 min, at the end of which you should have a clean DNA template sample in a PCR tube.

  2. PCR

    In this step, you will use PCR to amplify part of the TAS2R38 gene. The sequence of gene fragment we are copying includes a C/T mutation, that contributes to taste perception. This C/T mutation is also known as a single nucleotide polymorphism (SNP) with the code rs1726866.

    The experiment uses a method called ARMS PCR. This method uses 4 primers: 2 outer primers that copy the whole fragment, and 2 internal primers, each one specific to either the C or T mutation. This allows amplification of DNA only when the target allele is present in the DNA sample.

    You will need the DNA template sample (1), an empty PCR tube (2), the primer mix for this project (3), the mastermix (4), and PCR grade water (5). The total final volume of your tube will be 20μL.

    First add the mastermix. Set your micropipette to 4μL.

    Using a fresh pipette tip, transfer 4μL of the master mix into the empty PCR tube. Then discard your tip.

    Next add the primer mix. Set your micropipette to 2μL. Using a fresh pipette tip, transfer 2μL of the primer mix into the PCR tube. Then discard your tip.

    Now add the DNA template. Set your micropipette to 4μl.

    Using a fresh pipette tip, transfer 4μl of the DNA template sample from the sample tube into the PCR tube with the mastermix and primer mix. Then discard your tip.

    Finally add PCR grade water to make the total volume up to 20μl. Set your micropipette to 10μl.

    Using a fresh pipette tip, transfer 10μl of PCR grade water into the PCR tube. Then discard your tip.

    Place your PCR tube in the thermocycler block.

    Set up the thermocycler with the following PCR program:

    • 120 sec at 94°C
    • 35 cycles made of 3 steps
      • 30 sec at 94°C
      • 30 sec at 56°C
      • 30 sec at 72°C
    • 120 sec at 72°C

    If you need help operating the Bento Lab thermocycler, check the manual. You can use the PCR  preset (1), then modify (2) the program to the required settings (3) before running the program (4).

    The program will run for ca 2 hours. When it is finished, you can keep the result in the freezer, or use it right away for gel electrophoresis.

  3. Gel Electrophoresis

    Follow the Gel Electrophoresis Protocol to cast a gel and run it with your PCR result, and a 100bp ladder. This should take about 40 min.

  4. Visualising the Gel

    After the gel run has completed, you can visualise your results.

    Continue to wear gloves as you handle the gel.

    Open the orange lid of the gel box, and wipe off the condensation.  

    Gently pour out the buffer, and dispose of the buffer down a drain.

    Drain disposal of TBE running buffers is a standard waste disposal procedure followed by research labs. If you have questions, get in touch with us.

    Place the gel box onto the Bento Lab transilluminator surface. In order to get best visibility, you should do this in a room as dark as possible.

    Turn Bento Lab on, select the Gel Electrophoresis module, and turn on the Transilluminator light.

    Hold the orange filter lid over the gel to visualise the DNA bands. For documentation, use your mobile phone to take a clear picture of the gel. Rather than holding the lid over the gel, you can hold the lid directly in front of your camera lense.

    If the bands are faint, try to reduce the light in the room, e.g. by closing the curtains and turning off the lights.  You can also carefully take the gel out of the gel box and place it directly onto the transilluminator. Wear gloves when doing this, and be careful not to break the gel.

  5. Analysing your results

    Compare the picture of your gel to this example result, which has been run with all variations. Your sample should correspond to one of these variations.

    1 – Ladder – 100 bp DNA Ladder

    2 – Result CT – Control (366 bp), T allele (270 bp), C allele (151 bp)
    This result shows a person with a copy of both the T allele and C allele, so someone who is heterozygous. The ability to taste bitterness is dominant, so the person is likely to be able to taste bitterness.

    3 – CC – Control (366 bp), C allele (151 bp)
    This result shows a person who has both copies of the C allele, so homozygous dominant. This person should be able to taste bitterness.

    4 – TT – Control (366 bp), T allele (270 bp)
    This result shows a person who has both copies of the T allele, so homozygous recessive. This person is likely to be unable to taste bitterness.

    After you have taken good photos of the gel for your documentation, you can dispose of the gel in your regular trash.

    Disposal of agarose gels is a standard waste disposal procedure followed by research labs. If you have questions, get in touch with us.

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