If you’ve taken a Biology class, you may have learned about the monumental Polymerase Chain Reaction, or PCR. The purpose is to make many copies of a certain piece of DNA, making it easier to analyze. We mix the genes of interest with primers and nucleotides, set the tubes into a machine, and choose how many cycles to set it through. The cycle involves three phases: denaturation at 94-98 C, where the double stranded DNA is separated into single strands; annealing at 50-68 C, where primers attach to the DNA and identify which specific parts of the DNA the nucleotides should copy; and extension at 72 C, where nucleotides add complementary strands to the single strands. With n cycles, we get 2 raised to the nth power of copies. For example, 2^3 is 2*2*2 = 8, so after 3 cycles, we would have 8 copies of the DNA. Of course, it’s usually far more than that! Think 30 cycles for an impressive number- 2^30 = over 1 billion copies.

After the PCR is complete, we can use gel electrophoresis to confirm that the reaction ran correctly. We run the mix we used for PCR through a gel, with charges on each end. Opposite charges attract- the negatively charged DNA is repelled from the negative electrode and travels down the gel towards the positive electrode. The smaller pieces can run quicker, while larger pieces lag behind. Looking at our gel, we should see bands that show a successful reaction; we can also determine the size of the DNA pieces as the smaller ones will be farther down the gel.

PCR is a pretty fascinating process, right? We have one little microbe to thank for making it so efficient. Remember, the denaturation component of the cycle is very hot- over 90 C! We need a certain enzyme that can handle the heat while continuing the chemical reactions. The enzyme in question, Taq polymerase, was found in Thermus aquaticus. Two scientists, Thomas D. Brock and Hudson Freeze, discovered this species of bacteria in 1969. Based on how well it handles the heat, where do you think it might live? If you’re envisioning a hot spring, you’re absolutely right. The scientists found Thermus aquaticus in the Lower Geyser Basin of Yellowstone National Park in Wyoming.

PCR really comes in handy for many things in science: forensic testing to identify DNA at a crime scene, testing peoples' genes to see which foods are better or worse for them, or genetic manipulation to grow tougher crops, among other projects. Don’t forget medical diagnostic tests- PCR can help figure out if a patient has the flu or Covid. If you pursue a career in genetic or medical research, or rely on PCR for any services, always remember to thank our friend Thermus aquaticus!