BIOLOGY: Sixth formers get a taste of DNA testing

Sixth form biologists provided DNA samples from their cheek cells to analyse their tasting genes at Newcastle’s Centre for Life. JOSEPH HORNER and KATHERINE HOOPER report

UPPER sixth form biologists travelled to the Centre for Life in Newcastle to explore the world of genetics. The task for the day was to assess whether we had the gene that codes for a certain taste receptor on our tongue.

To do this, we needed to use our own DNA and produce our DNA profiles in the laboratory.

We began by each tasting two pieces of paper, discussing in our groups what they tasted like and rating the strength of each taste out of five. It turned out one was just a plain piece of paper, the other had been soaked in phenylthiocarbamide (PTC), a bitter-tasting chemical found in Brussels sprouts.

Everyone had to provide a DNA sample, which was then multiplied in a process called the polymerase chain reaction (PCR). This sample was then processed by gel electrophoresis, resulting in everyone having a sample that could identify whether they were a taster or a non-taster.

In order to extract our own DNA, we had to remove some of our own cells by biting the insides of our cheeks and swirling salty water around our mouths. We then had to spit it back into a cup, so the cells could be collected. This was rather amusing as some of the group had eaten a packet of Jaffa Cakes just before entering the lab. Once we had isolated our cells, we needed to add chemicals to break down the cell membrane and nuclear envelope. We then used a centrifuge to separate and amplify the DNA from the rest of the solution and added the DNA primers and free nucleotides that are needed for PCR.

PCR consists of three steps; the first takes place at a temperature of 95C to separate the strands of DNA, the PCR machine is then lowered to 55C so the DNA primers can anneal the separated DNA strands. These primers are added so the enzyme DNA polymerase can attach complementary nucleotides to the existing single strands of DNA in the final step of PCR at 72C. This produces two new DNA fragments. The PCR process takes a while to complete as it undergoes multiple cycles, each time doubling the number of DNA fragments.

Once we had used PCR to amplify our DNA samples, they could then be cut up into smaller fragments using restriction endonucleases. These enzymes cut DNA at a specific nucleotide sequence known as a restriction site. After we had digested the DNA, we could separate the fragments using gel electrophoresis. In this process, a charge is passed through a gel medium with wells containing the DNA fragments. The DNA is also tagged with a fluorescent marker. We then waited half an hour for the fragments to move through the gel, the smaller fragments moving the fastest and therefore the furthest.

Whilst we waited for the electrophoresis to be completed, we discussed the possible evolutionary advantages and disadvantages of having the taster gene that can detect PTC, including aliens and the gene making you more or less attractive. Once the half hour was up, we removed the charge from the gel electrophoresis plate and viewed the plates under UV light to see our DNA profile.

From this profile we could determine whether we each had a gene that coded for the tasting of PTC; and if we were a taster, whether we were homozygous or heterozygous for that gene. Finally, we compared our DNA profile with our original observations about the taste of PTC on the paper and there were some surprising results.

The day involved a number of practical and discussion activities, which all helped widen our understanding of the topic, which we are studying for A-level. Overall it was an interesting day and was very useful for our revision.