From DNA to PCR product

I’m Matt Johnson, a PhD Student in the Genetics Department. As Aamisha explained in the previous section, the first stage in obtaining a genetic result from a blood sample is to obtain high quality genomic DNA. Even with a fresh blood sample, the amount of DNA that can actually be extracted is usually too low to be used to the full genetic testing. So having a method to amplify the specific region that is related to the clinical question is critical to most genetic tests. The next step in the testing pipeline is a reaction known as PCR, Polymerase Chain Reaction.

This uses the genomic DNA as a template and amplifies a small region of it millions of times over, like photocopying copying a page from a book, but using a lot of sheets of paper. The reaction uses a naturally occurring enzyme, polymerase, to make identical copies of the original DNA and small DNA molecules called primers that are specific to the region we want to amplify. It’s called a chain reaction because the molecules created by one cycle are used for the next cycle. And because of this cyclic amplification, hundreds of millions of copies of DNA are created from the few thousand that were in the original sample.

This all takes approximately three hours and is done on a machine called a thermocycler, which can quickly change the temperature of the reaction mix to facilitate the different stages. In the next section, we will learn how PCR works in more detail.