PTC Tasting ability determined through PCR and Restriction
Amgen Biotechnology Experience:
Supplement Lab 7
Have you ever wondered why some people like broccoli and others do not?
Or some like sweet candy and others like sour?
What determines food preferences?
Science is researching how our genes play a role in our ability to taste or not taste certain chemicals or flavors. Through this lab, you will discover your genotype for the PTC tasting. You will then predict your phenotype and test your prediction by tasting a PTC test strip.
PTC (phenylthiocarbamide) is a chemical that is considered “bitter” in taste. This chemical is not found naturally in foods. Why would our genome need to have a gene that would determine our ability to taste this chemical? Scientists are not completely sure. PTC has a molecular structure that is similar to chemicals found in poisonous plants. One thought is that this gene played a part in the taste receptors for these other poisonous chemicals. There seems to be a correlation between PTC tasting and the dislike of broccoli. Even if you wish that broccoli was a dangerous vegetable, though, it isn’t.
Arthur Fox discovered PTC tasting ability in the 1930’s, when he synthesized phenylthiocarbamide (PTC) in his lab. While doing so, the PTC dust made a lab assistant comment that he could taste bitterness in the air. On the other hand, Fox could not taste the dust in the air. Why could one person taste it and another couldn’t? We now know it is because of their genes. Albert Blakeslee continued this research through taste testing and found that the inability to taste PTC is a recessive trait (one’s genotype is “tt,” with “t” standing for “taste”).
In 2003, when scientists had the ability to determine the sequence of certain genes, they found there were many different bitter taste receptors in the human genome, including one gene associated with PTC bitter tasting called taste receptor 2 member 38, or TAS2R38. This gene is 1,143 nucleotides long and has 3 locations where variations in the amino acid sequence correlate with the PTC tasting and non-tasting. Each of these changes is called a “SNP” or a single nucleotide polymorphism. We will use one of these changes, together with polymerase chain reaction (PCR) and restriction enzyme digestion to predict your ability to taste PTC or not. Purpose of this Lab:
We will use one of the SNPs to predict is you are a PTC “taster” or “non-taster.”
You will isolate DNA from your cheek cells and use PCR to copy a small region of the TAS2R38 gene using. PCR is a way to make millions of copies of our desired section of DNAin just a few hours. The size of our PCR product will be 221 base pairs in length. We will then use a restriction enzyme called HaeIII to perform a restriction digest on this PCR product. A restriction digest is the process of cutting a strand of DNA at a specific restriction site: a small unique DNA sequence recognized and cut by restriction enzymes like HaeIII. HaeIII recognizes and cuts at GGCC. If your PCR product contains GGCC, it will cut the PCR product into two pieces. This result will indicate that your gene has the code for the ability to taste PTC. If your gene doesn’t contain GGCC, HaeIII will not cut. This will predict your inability to taste PTC, and you are homozygous recessive (tt) for this gene. You will compare your uncut PCR product to your restriction digested product using agarose gel electrophoresis.
Remember, you have 2 copies of the TAS2R38: one from your mother and one from your father. Your PCR product might have some copies with GGCC and some without; some of the pieces will cut and some won’t. This would indicate that you are heterozygous with a genotype of Tt. Since “T” is dominant, you are a “taster.” If all of the DNA copies cut into two pieces, you are homozygous dominant (TT) and are also a “taster.”
Day 1: Isolating your DNA