Once upon a time, six eager scientists-in-training and two wonderful fairy-god mentors went where very few other scientists dared to go before… the Black Sage Bench Region of British Columbia. This mystical land was not far from their home but the young scientists had never heard of it before. The Black Sage Bench was home to magical berries (pinot noir grapes) that had the potential of making an extraordinary concoction. This very concoction, a biofuel, could provide lots of information for the chosen few who decided to work hard to obtain it. Many moons passed and many different techniques and protocols were used to finally determine that 14 different amazing little yeast creatures were the magic behind the biofuel concoction. The six scholars were so cheerful but unfortunately, they had no clue what type of yeast they were dealing with!! “Oh, don’t worry…” said the fairy-mentor, “I have a magical tool that can solve this very problem… it’s called a PCR machine!”
Two lucky scholars were chosen to do the PCR to determine which species they had discovered but being a scientist, one of the little students was not content with the explanation that a PCR machine was magic. Oh no, she had to figure out exactly what and how a PCR machine worked!
So what is a PCR machine you ask? You may know the answer already but in case you don’t, this is the perfect spot for you. PCR is an acronym which stands for Polymerase Chain Reaction. Used in many different biological and chemical areas, it produces copies of DNA to generate thousands or even millions of copies of a particular segment. It has so many amazing uses like analyzing genes, phylogeny (studying evolutionary relationships between organisms) and detecting/diagnosing diseases. It is also used to determine genetic fingerprints which are crucial in forensic sciences and paternity testing. However, the little scientists will use it to determine which species of yeast they have obtained.
PCR is able to copy sections of DNA through a heating cycle. The temperature first rises to about 95oC which melts the DNA strand and splits its two sugar-phosphate backbones apart. The temperature then cools so that primers can bind to the 3’ end of each target sequence. With the assistance of free nucleotides and a DNA polymerase taq, the primers direct the synthesis of the strands. The taq polymerase continues so that at the end of the first cycle, there are two partially double stranded DNA molecules. In the second cycle, the temperature shoots back up to 95oC which melts and splits the strands once again. Like in the first cycle, taq, nucleotides and primers create partially double stranded copies of the DNA molecule: but this time the result is 4 molecules. In the third cycle, this process is once again repeated but the result is 8 molecules: 2 that are just the target sequence (that we were initially trying to copy) and 6 that are longer. The cycles continue to proceed just like this and the target molecules are produced exponentially. Before long, there are thousands of copies of the target sequence.