Nov 2022


Polymerase Chain Reaction
Kary Mullis (1944-2019) was an American biochemist who invented the Polymerase Chain Reaction technique. In 1993 he won the Nobel Prize in Chemistry with colleague Michael Smith for the invention of the PCR technique. Analysing DNA requires quite a large amount of DNA. The PCR technique takes a small amount of DNA and copies it into larger quantities over a short period of time.
By applying heat, the DNA molecule’s two strands are separated and the DNA building blocks that have been added are bonded to each strand. With the help of the enzyme DNA polymerase new DNA chains are formed and the process can then be repeated. The majority of PCR methods rely on thermal cycling. Almost all PCR applications employ a heat stable DNA polymerase such as Taq polymerase, an enzyme originally isolated from the thermophilic Thermus aquaticus. These heat resistant enzymes that are a key component in PCR were discovered in the 1960’s as a product of a microbial life form that lived in the superheated waters of Yellowstone’s Mushroom Spring. To follow is some points on what PCR is useful for and what it can’t do:
PCR is intended to identify substances qualitatively, but by its very nature is unsuited for estimating numbers.
The tests, can detect genetic sequences of viruses, but not viruses themselves, cannot determine whether the individual tested is infectious.
PCR does not indicate that the virus is fully intact and infectious.
PCR presence of a virus doesn’t tell us that virus is involved in the disease process, see commensalism.
Like all enzymes, DNA polymerases are also prone to error, which in turn causes mutations in the PCR fragments that are generated.
Another limitation of PCR is that even the smallest amount of contaminating DNA can be amplified resulting in misleading or ambiguous results.
Questions to be asked with regards PCR, at what point does amplification start creating false positives?
Now ask yourself why are you taking a PCR “test”.