Question: How do pcr tests work?

Shona Moore answered on 20 Jan 2022:

PCR works my amplifying genetic material. It stands for polymerase chain reaction. Polymerase is an enzyme that synthesises chains of nucleic acids. What we do when you take a swab for a PCR test is isolate the genetic material, in the case of SARS-CoV-2 we are isolating an RNA virus. PCR works on DNA so we convert the single strand of RNA to double-stranded DNA (a process call reverse transcription which uses an enzyme to convert it). The idea behind PCR is to break the double strand of DNA (denaturation) into a single strand, add something called a primer which is a sticky short section of DNA that attaches to a target region (annealing step). Actually, we add two primers: one at the 3′ and one at the 5′ end. We then add the polymerase along with additional free nucleic acids (called dNTPs) and allow the polymerase to stick the free nucleic acids down, creating a double stranded section after the primer (extension). The process is repeated over and over to amplify multiple pieces of target DNA. In reality each step happens at a different temperature so all that a PCR machine does is to take your reaction mix and change the temperature required for each step. PCR isn’t particularly glamorous! We set the machine to repeat the temperature cycle in order to repeat the reaction and amplify the target. The reaction will fail (and you get a negative PCR) if the primer doesn’t stick to the target (ie the target doesn’t exist) because the dNTPs have nothing to attach to. I find it quite difficult to describe without sketching! A schematic diagram can really help with the concept of denaturation, annealing and extension.